Copyright © 1999. University Press of Mississippi. All rights reserved. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Copyright © 1999. University Press of Mississippi. All rights reserved. Understanding Childhood Obesity Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Understanding Health and Sickness Series Miriam Bloom, Ph.D. Copyright © 1999. University Press of Mississippi. All rights reserved. General Editor Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Understanding Childhood Obesity Copyright © 1999. University Press of Mississippi. All rights reserved. J. Clinton Smith, M.D., M.P.H. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Copyright © 1999 by the University Press of Mississippi All rights reserved Manufactured in the United States of America 02 01 00 99 4 3 2 1 The paper in this book meets the guidelines for permanence and durability of the Committee on Production Guidelines for Book Longevity of the Council on Library Resources. Illustrations by Regan Causey Tuder Library of Congress Cataloging-in-Publication Data Smith, J. Clinton, 1939– Understanding childhood obesity / J. Clinton Smith. p. cm.—(Understanding health and sickness series) Includes bibliographical references and index. ISBN 1-57806-133-4 (cloth : alk. paper).—ISBN 1-57806-134-2 (pbk. : alk. paper) 1. Obesity in children—Prevention. 2. Obesity in children—Psychological aspects. 3. Children—Nutrition. 4. Behavior therapy for children. I. Title. II. Series. RJ399.C6S63 1999 Copyright © 1999. University Press of Mississippi. All rights reserved. 618.92′ 398—dc21 98-44680 CIP British Library Cataloging-in-Publication Data available Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Contents Acknowledgments Introduction 1. Why Is Obesity an Important Health Problem in America? 2. Who Is Obese, and How Do We Know? 3. How Our Bodies Obtain Energy 4. Obesity: A Disorder of Energy 5. Some Factors That May Determine Obesity 6. What Can Be Done to Prevent Childhood Obesity? 7. If Prevention Doesn’t Work 8. The Great Beyond: New Frontiers in the Treatment of Obesity Notes Glossary References Copyright © 1999. University Press of Mississippi. All rights reserved. Index Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Acknowledgments I am grateful to Jim Joransen, David Braden, Makram Ebeid, and Charlie Gaymes, my colleagues in the Division of Cardiology of the Department of Pediatrics at the University of Mississippi Medical Center, for allowing me time to write this book. Only through their support has the Pediatric Preventive Cardiology Clinic become a reality. I am also appreciative of the physicians, nurses, and dietitians who have made referrals to our clinics. Without the support of the Mississippi State Department of Health, our clinics could not have met in various communities across the state, and families would have been unable to obtain assistance. I would also like to thank the editor of the Understanding Health and Sickness Series, Dr. Miriam Bloom, for her patient efforts. In spite of her many other activities in the community and around the state, she always found time to scrutinize drafts of this book. Her suggestions were consistently on target, and she helped me translate the scientific jargon into everyday language (I take full responsibility for any lapses in this regard). Two other individuals were extremely helpful in the book’s preparation. Dr. Richard Troiano of the National Center for Health Statistics graciously provided me with unpublished data from the National Health and Nutrition Examination Survey III concerning overweight and obesity among children. Dr. Harold White, Emeritus Professor of Biochemistry at the University of Mississippi Medical Center, reviewed the material on digestion and energy and made invaluable comments and suggestions. Copyright © 1999. University Press of Mississippi. All rights reserved. Most of all, I am indebted to Lois, my wife of thirty years, for her unconditional love and her even temperament. She always makes it easy for me to do what I need to do. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Introduction Hardly a week goes by that a magazine or tabloid newspaper doesn’t feature an article about someone who is overweight, who is trying to lose weight, or who should be trying to lose weight. “How-to” books about weight loss are available in great number. We know about celebrities such as Oprah Winfrey, Elizabeth Taylor, David Letterman, and Tommy Lasorda who have successfully dealt with their own weight problems. Others, including swimmer Lynne Cox, musicians Kate Smith, Fats Domino, and Chubby Checker, singer Cass Elliot, Chicago Bear “Refrigerator” Perry, and most professional football linemen, have used excess weight to their advantage. Thanks to overweight opera singers, “It’s not over ’til the fat lady sings” is a national aphorism. Americans seem obsessed with weight loss. We talk about how much weight we want to lose or how much we have lost as frequently as we talk about the weather. Our weight and what we’re trying to do about it is always a timely subject. And our conversations about weight usually have something to do with our appearance—the way we look to other people. Copyright © 1999. University Press of Mississippi. All rights reserved. Self-image is unquestionably important. But there is a far more serious aspect of being overweight that we don’t talk about much, and that is how this condition can affect our health and longevity. Consider the following facts: First, about 500,000 Americans die each year from diseases of the heart, especially coronary artery disease, or disease of the blood vessels supplying blood to the heart. This disease doesn’t just develop overnight. It is a slow, degenerative process that can begin in childhood. Adults who are obese, who have high blood pressure or abnormal blood cholesterol levels, who use tobacco, and who engage in little or no physical activity appear to be at high risk for this degenerative process (Eckel et al. 1998). Yet mounting evidence indicates that if children who have risk factors can be identified and appropriate corrective action taken, there might be less chance that they will have heart disease when they become adults (Bao et al. 1997). Second, in the years 1988–94, about 35 percent of American adults over 20 years of age (nearly 60 million people) were obese, up from 25 percent in the years 1976–80. The news concerning children was also startling: the proportion of overweight children (6-to 11-yearolds) climbed from 20 percent in the period 1976–80 to 27 percent between 1988 and 1994; the proportion of overweight adolescents (12-to 17-year-olds) rose from 16 percent to 27 percent over the same time. Approximately 5 million children and adolescents are now classified as being obese. Many, but not all, obese youngsters become obese adults, and many obese adults can trace their excess weight to faulty nutrition and physical activity patterns established during childhood. A glaring incongruity exists in Americans’ ideas of health care. On the one hand, we have great expectations: we know that when we become ill or injured, we will receive the very best care available in the world, and that our doctors and hospitals will be paid by our insurance programs. If we survive a heart attack, we go to state-of-the-art coronary care units, have Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. coronary artery bypass grafts, take drugs that help keep coronary arteries open, and receive rehabilitation services for damaged hearts. Astonishing, but costly, technological advances in health care since 1950 have made such interventions possible, and we have welcomed them and now take them for granted. The incongruity is that we don’t seem to realize that it is within our power as human beings to prevent, or at least delay, some diseases. Many heart attacks, strokes, cancers, and other diseases and conditions would not occur if we were effective in preventing obesity. My purpose in writing this book is to suggest that childhood is the best time to prevent obesity and its later consequences. The book is intended to assist families, teachers, health providers, and other friends of children in understanding better why some children become obese, how being obese can result in health problems in childhood and in adulthood, and what can be done to help youngsters become healthy adults. I have drawn on the wisdom and contributions of professionals in many different fields who are serious students of the problem of obesity, including basic science researchers, dietitians, exercise physiologists, psychologists, and physicians. I hope that readers will use the book to expand their knowledge of this critical public health issue. I begin with a discussion of the major ways in which obesity is an important health problem and why we need to do something at both the individual and the population levels. The first chapter also explains some of the consequences of obesity in children and in adults. Copyright © 1999. University Press of Mississippi. All rights reserved. In the second chapter I explain the process of classifying a child’s weight and height as “normal” or “abnormal,” how a diagnosis of obesity is made in children, and what the difficulties are in comparing different populations of children. I also identify groups of children and adolescents who are most likely to become obese adults. Chapters 3, 4, and 5 are concerned with how energy is taken into the body and used and with how energy imbalance leads to obesity. In chapter 3 I review how food is broken down into molecules in the digestive tract and how these molecules are absorbed into the bloodstream, enter the cells of the body, and are then either used as immediate energy sources or stored for future energy needs. In chapter 4, I explain how energy from food is supplied and used. I also introduce the energy balance equation, which is the basis for measuring energy intake and expenditure. Some of the possible reasons for obesity in childhood are presented in chapter 5, including evidence from both behavioral and metabolic research. The chapter focuses on the importance of genetic and environmental factors associated with obesity. Chapter 6 examines the question “Can obesity be prevented in American children?” I try to emphasize the difference between hoping that prevention can take place because of successful demonstration projects and realistic expectations within the context of the powerful environmental influences affecting children’s daily eating and exercise habits. I cite examples of the ways in which parents, government, schools, and health professionals have attempted to prevent obesity, and comment on the likelihood that prevention efforts will be expanded. Techniques used in treatment of obese children are discussed in chapter 7. Several approaches are defined, and the traditional elements of treatment, such as nutrition education, decreased calorie and fat intake, increased physical activity, and behavior modification are Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. detailed. Examples of aggressive medical and surgical management, including a discussion of drugs to treat obesity, are presented. I also introduce the reader to the “continuous care and problem solving model” of treatment, and explain how this works in our pediatric treatment clinics at the University of Mississippi Medical Center. Finally, chapter 8 contains a summary of exciting new developments that have taken place in obesity research in recent years, including the discovery of leptin and the leptin receptor and the promising genetics research that may finally explain why obesity develops in some individuals but not in others. I have cited references only to sources other than material that can be found in standard textbooks of basic science and clinical medicine. Copyright © 1999. University Press of Mississippi. All rights reserved. For me, the richest source of information has been the hundreds of obese children and their families in Mississippi who have sought assistance in our clinics. In getting to know them, learning about their family and school environments, and identifying with their struggles to change their eating and physical activity patterns, I have come to realize how little is known about the origins and treatment of obesity in childhood. Without knowing those children, I would have had little incentive to write this book. They continue to be superb teachers. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Copyright © 1999. University Press of Mississippi. All rights reserved. Understanding Childhood Obesity Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. 1. Why Is Obesity an Important Health Problem in America? Leave gormandizing. Know the grave doth gape For thee thrice wider than for other men. William Shakespeare, Henry IV, Part II (quoted in Bray 1985) Twentieth-century Americans have enjoyed a standard of living unparalleled in history. We are well nourished, we live in attractive and affordable houses, we have clean water and milk and good sewage systems, and we are largely free from the scourges suffered by so much of the rest of the world, such as malaria, yellow fever, rheumatic fever, blindness due to parasites, and malnutrition. We are a well-educated nation. Many of us work 40-hour weeks, have guaranteed vacations, and are free to enjoy the fruits of our labor before we grow too old. Our health care system has provided us with powerful drugs and technologies that cure illnesses and prolong lives. Copyright © 1999. University Press of Mississippi. All rights reserved. Yet we are not entirely well. The health problems dominating the early part of this century have been replaced by a new morbidity—diseases brought on by our living habits. Like other industrialized countries, we now have high rates of heart disease, cancer, strokes, and obesity. Nearly 60 million American adults between the ages of 20 and 75 years—1 in 3—are obese. One of every 4 U. S. adults smokes cigarettes regularly, a habit usually acquired in adolescence and linked to several fatal conditions. Since most of us are now city dwellers, we can’t conveniently walk to work or to visit our neighbors. And, thanks to labor-saving technologies, we have discovered how to work without sweating. We reward ourselves at the end of the day with big meals and evenings of watching television. We are sports enthusiasts and spectators, but frequently avoid physical activity ourselves. A fourth of adult Americans have high blood pressure or high blood cholesterol. Many don’t realize it because they haven’t taken the time to find out, while others know but are unwilling or unable to take available medications. The fact is that many Americans want to have good health but don’t want to pay the personal price to achieve it. We undertake to live what we see as “the good life,” and, if something goes wrong, we rely on our doctors and hospitals and technology and drugs to fix us up and get us going again. And since we don’t want to worry about the expense, we say “just bill my insurance company.” Those of us in the health professions do a pretty good job of treating disease, but we’re not always doing so well at getting the message out that a lot of death and disability can be prevented. Government and health insurance companies give lip service to the idea of prevention, but education in this area is infrequently compensated, and it has not been a high priority among health practitioners. Managed health care may eventually modify this deficiency. Many of us have not yet connected obesity to possible poor health. Obesity is a chronic condition—not an acute, urgent, and headline-grabbing disease like AIDS or meningitis—and Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. is therefore likely to receive less attention. Its effects are insidious, its origins are complex and poorly understood, and its treatment is often discouraging. But obesity can be prevented if the American people will that it be so (National Institutes of Health 1985; Lew 1985; Pi-Sunyer 1991; Dietz et al. 1993; Alpert et al. 1993). This chapter explores a number of ways in which obesity is a major health problem among Americans. Some of the complications of obesity, such as heart disease, begin in childhood, but do not become apparent until adulthood. If we are to understand why some adults are healthy and others are not, we must first understand how habits that we acquire early in life can affect our health later. Obesity and Overall Mortality Obese people do not live as long as other people. Life insurance company studies done early in the 20th century showed that, as the weight of individuals increases above an optimal level, the probability of dying increases, too. In 1979 the American Cancer Society confirmed that finding in a 12-year study of 750,000 people which took into consideration their state of health and whether they smoked. The study also found that men and women who were 5-15 percent below average weight were likely to live longest. The most common causes of death among men in the American Cancer Society study were diseased coronary arteries, stroke, and digestive diseases. The same was true in women, except that diabetes was also common. Obese men and women were also more likely to die of cancer than were those who were not obese. Let’s find out why obesity is an important health problem among Americans. Which organs or systems can it affect, and what happens then? Copyright © 1999. University Press of Mississippi. All rights reserved. Effect of Obesity on the Heart and Blood Vessels Obesity is strongly associated with diseases of the heart and blood vessels in several ways. First, obese people appear to be more likely to develop disease of the coronary arteries, which are the vessels that supply the heart muscle with blood. Blood flow in those arteries can be blocked due to a complex process called atherosclerosis, which is more common in obese than in nonobese adults. Blockage of coronary arteries can cause part of the heart muscle to die (a myocardial infarction, or heart attack). After a heart attack, the heart may not be able to pump adequate amounts of blood to other vital organs, such as the brain, lungs, and kidneys, and death or disability may result. Second, obesity also can directly affect the heart muscle, independent of its effect on the coronary arteries. This condition is called obesity cardiomyopathy. Third, heart disease can occur when obesity causes abnormal functioning of the lungs. Finally, obese people are very likely to have high blood pressure (hypertension), which can damage both the coronary arteries and the heart muscle. Since coronary artery disease is responsible for the deaths of about 500,000 people in the Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. United States annually (roughly 25 percent of the 2 million or so Americans dying each year) and causes 1.4 million nonfatal heart attacks each year, let’s begin with a discussion of this condition. Many studies have shown that coronary artery disease occurs more often in people having high blood pressure and abnormal blood cholesterol levels, who exercise little or not at all, who smoke cigarettes, who have diabetes, and who are obese. One way in which obesity seems to lead to coronary artery disease is first to cause some of the other problems just mentioned, such as abnormal blood cholesterol levels or high blood pressure. In this sense, obesity is an indirect cause of coronary artery disease, meaning that its effects on the heart and blood vessels take place because other abnormal conditions have already developed. But obesity also appears to have an effect on the heart and blood vessels that does not depend on the development of intermediate conditions (Higgins et al. 1987). For example, excess body fat can be stored either viscerally (in the abdomen, surrounding the liver and intestines) or peripherally (in the upper arms, the thighs, and the buttocks). Although the amount of visceral fat is more difficult to measure than peripheral fat, studies have shown that the association of visceral fat with coronary artery disease is independent of other risk factors, such as high blood pressure or diabetes. Why excess visceral fat is so much more strongly associated with coronary artery disease than excess peripheral fat is not well understood. Does coronary artery disease ever occur in children? Only rarely. However, the process begins in early childhood; it is now well established that deposits of fat and fibrous tissue are present in the blood vessels of children as young as 4 or 5 years old. While a heart attack caused by this process would be unusual in a child or adolescent, it would not be in a 30- or 40-year-old. The important thing is that, even though the process of atherosclerosis may be inevitable, the rate at which it occurs probably can be slowed down, and the death and disability that it causes can be delayed. Copyright © 1999. University Press of Mississippi. All rights reserved. Another Way in Which Obesity Can Affect the Heart Body fat is a living tissue. It doesn’t just sit there and do nothing! It has to obtain oxygen and to get rid of waste material, and therefore has to have a blood supply. As we put on extra fat, we develop extra blood vessels, red blood cells, and plasma to carry oxygen and nutrition to it. The amount of blood in our circulatory system increases, and, since the heart has to pump all the blood that comes into it, the two pumping chambers of the heart (ventricles) may dilate to handle the extra volume, with their walls becoming thicker (hypertrophic) to pump the blood out with extra force (fig. 1.1). Hypertrophy of the heart muscle means that extra muscle tissue is added to the heart, in the same way that weight lifters put on extra muscle tissue. Excessive dilation and hypertrophy can eventually weaken the ability of the heart to pump blood. And if the heart eventually fails to pump out all the blood that comes to it, heart failure occurs. Heart failure due to obesity is called obesity cardiomyopathy. Can obesity-related heart failure occur in children? Yes, but it happens only rarely, and mainly in children who are extremely obese. But even in children who are only moderately Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. obese, increased blood volume can affect the size of the heart chambers and the thickness of their walls. FIG. 1.1. Cross section of the left ventricle of the heart, comparing dilation and hypertrophy with the normal state. (A) Normal cavity size and wall thickness. (B) Dilation of the ventricular cavity, with thinning of the ventricular wall. (C) Hypertrophy (thickening) of the ventricular wall, with normal or decreased ventricular cavity size. Copyright © 1999. University Press of Mississippi. All rights reserved. Effect of Obesity on the Respiratory System One of the most common consequences of obesity in children and adults is obstructive sleep apnea, caused by excessive fatty tissue in the back of the throat blocking air flow from the nose to the lungs during sleep. Relaxation of the throat and neck muscles during sleep allows excess fat in the walls of the throat to protrude into the airway, causing partial or total obstruction of air flow to the lungs. Loud snoring may be the only sign of partial obstruction. But if the airway becomes totally obstructed, normal breathing may be interrupted for as long as 30 or more seconds. This causes the person to awaken and change positions to overcome the obstruction. The quality of sleep and rest is poor, causing daytime sleepiness, which can, of course, result in a poor attention span and poor school or work performance. Sleep apnea may be life threatening in many children and adults, and surgery to remove the obstructive tissue is often required. In addition, chronic airway obstruction from any cause, including excessively large adenoids, often causes hypoxemia, a condition in which the oxygen in the circulatory system is low. This can eventually result in high pressure in the blood vessels that carry blood to the lungs, and cause thickening (hypertrophy) of the walls of the right ventricle. Heart failure can result. A condition closely related to obstructive sleep apnea is frequently seen in severely obese people and is called the Pickwickian syndrome, after Joe the fat boy in Charles Dickens’s novel The Pickwick Papers. Its technical name is obesity hypoventilation syndrome, which simply means that, because of obesity, not as much air enters the lungs as ordinarily would. This can happen when the amount of fat stored in the abdomen and chest is so great that the chest and diaphragm cannot move in a normal manner. It’s like wearing a very tight girdle or waistband around the chest and abdomen. The affected individual has to work much harder to breathe, and needs more oxygen for this extra work. However, less oxygen is available since Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. the lungs cannot expand normally. In addition, when any part of the lung does not fully expand, the blood cannot get its full supply of oxygen. The net effect is hypoxemia. As in obstructive sleep apnea, sleep quality may be poor and daytime sleepiness is common. Usually, low blood oxygen levels stimulate the breathing centers in the brain to speed up the breathing rate. But over time, the brain centers can become less responsive to low oxygen levels, and hypoxemia worsens. Chronic hypoxemia due to the obesity hypoventilation syndrome can result in heart failure by the same mechanisms as those found in obstructive sleep apnea. Both these conditions, of course, can occur in the same individual. Obesity and High Blood Pressure Obese people, regardless of age, are more likely to develop high blood pressure (hypertension) than nonobese people. American adults (defined as 20–75 years old) who are obese are 3 times more likely than nonobese adults to develop this condition. Even those whose blood pressure is not high enough to treat with medication often have “high normal” pressures. Why obesity is associated with hypertension is not completely understood, but it probably has something to do with increased blood volume (discussed above) and increased resistance of blood vessels to blood flow. In any case, untreated hypertension places a strain on the left side of the heart, causing it to become hypertrophic. In time, the heart can fail. Untreated hypertension also can cause narrowing and obstruction of the blood vessels supplying the heart, resulting in heart attacks. Remember that hypertension can begin in childhood. Although many obese children have hypertension, it rarely causes heart failure or stroke at that stage of life. But if high blood pressure persists into adulthood and is not treated, the consequences can be serious. Copyright © 1999. University Press of Mississippi. All rights reserved. Effect of Obesity on Blood Lipids Lipid is the scientific term for a substance that is soluble (can be dissolved) in fat. Lipids are essential to life; they perform many valuable roles in maintaining good health. Lipids come from our diets, or can be manufactured by the liver from dietary carbohydrates (starches, sugars). Other lipids that we obtain from the food we eat and make in our bodies are known as sterols. Cholesterol is the most important example of a sterol in humans. The two kinds of lipids that are most important to the health of obese people are cholesterol and triglycerides (defined in chapter 3). We can be born with abnormal levels of certain blood lipids, which can cause atherosclerosis. On the other hand, some people can acquire abnormal blood lipid levels if they become obese, smoke cigarettes, are sedentary, or have excessive dietary fat intake. The risk of having abnormal blood cholesterol levels is about 1½ times higher in obese than in nonobese adults. The first line of treatment in obese children or adults with abnormal lipid levels is weight loss. Obesity and Type II (Adult-onset, Non-insulin-dependent) Diabetes Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. In the United States, about 15.7 million people, or 5.9 percent of the population, have diabetes, a disease characterized by abnormally high blood sugar (glucose) levels. One-third of these are undiagnosed. Type II diabetes is the form that commonly occurs in adulthood, as opposed to type I (juvenile) diabetes, which has its onset in childhood and currently affects about 120,000 children under the age of 20 years. Type II diabetes accounts for 90–95 percent of all diagnosed diabetes, and can usually be controlled by diet, exercise, oral medications, and, in some cases, insulin (National Institute of Diabetes and Digestive and Kidney Diseases Home Page 1997). Diabetes has ranked among the 10 leading causes of death among Americans since 1932 and currently results in about 36,000 deaths annually. The risk of diabetes increases with both age and obesity: people who are 50 percent above their optimal weight are 5 times as likely to be diabetic. Of great concern is a recent report that adult-onset diabetes is being found with increasing frequency in obese adolescents (Pinhas-Hamiel et al. 1996). People with either type I or type II diabetes are likely to develop coronary artery disease, abnormal blood lipid levels, high blood pressure, strokes, blindness, gangrene of their feet or legs, and kidney failure. Children born to diabetic mothers are more likely to have birth defects and die during infancy. Our discussion thus far has concentrated on the life-threatening conditions that are frequently found among obese individuals. Other consequences of obesity, which are not necessarily as drastic, nevertheless can result in chronic illness and disability. Copyright © 1999. University Press of Mississippi. All rights reserved. Psychosocial Consequences of Obesity Although many obese children and adults seem to be perfectly happy with their weights, others may have poor images of themselves and become socially withdrawn. Some experience serious depression and may require psychotherapy or medication. Parents know the heartbreak of having their obese children teased by other children, or being told (often by adults) that they don’t have the self-discipline to lose weight, or that their families don’t care enough about them and their health to correct the problem. Youngsters who are obese and want to lose weight often give “being able to do the things that other kids are doing” as their reason; they want to go where their peers are going, wear the same kinds of clothes, be invited out on dates, and form the normal everyday relationships that their friends have. If they do not develop those relationships, some may grow up lacking social skills, and they may have difficulty in being accepted by colleges, in finding and keeping a job, and in forming the intimacy and trust necessary for marriage. The negative attitudes of nonobese children toward those who are obese or otherwise “different” are formed early in life. In one survey, children as young as 6 years of age rated obese children as “less likable” than those who aren’t, and rated obese kids even more negatively than children having facial disfigurement or missing limbs. Obese children or adults may overeat, feel bad about doing so, but then eat again to feel better. This is referred to as binge eating. On the other hand, obese girls and young women often become preoccupied with their weight and may go to the opposite extreme, refusing to eat or eating only very small amounts of food, a condition known as anorexia, or forcing themselves to regurgitate after eating, which is called bulimia. Anorexia and bulimia can be Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. resistant to treatment, and can result in death from starvation. Many obese young people as well as adults seem perfectly happy with themselves, appearing to be confident, sociable, and well liked by their peers. They have accepted their obesity as a fact of life. They may be just as talented, well educated, and socially mature as anyone else. Organizations such as the National Association to Advance Fat Acceptance enable obese individuals to communicate with each other and share ideas. Radiance is a magazine for large women. In addition, various cultural and social groups may have different perceptions of what constitutes the ideal body image. For example, African American families, especially the grandparents, may often perceive obesity to be a sign of robustness and leanness an indication of disease or poor health. Effects of Obesity on Other Body Systems Obese children are likely to develop tibia vara deformity, also known as tibia vara, or Blount’s disease, a condition in which the growth plate of the tibia (the larger bone in the lower leg) develops abnormally. This can result in severe bowing of the legs, which limits a person’s ability to run, jump, or even walk. A second problem frequently seen in obese children is slipped capital femoral epiphysis. In this rare but extremely serious condition, the growth plate of the head of the femur (the single large bone of the upper leg which joins the pelvis bone) becomes detached from the main body of the femur, causing pain and inability to walk. This condition can interfere with the blood supply to the hip, causing severe damage to the hip joint, and can result in permanent disability. About 75 percent of children with either of the above conditions are obese. Copyright © 1999. University Press of Mississippi. All rights reserved. Obese adults are particularly prone to develop osteoarthritis, which is a painful swelling of some of the joints and which may limit movement, as well as to sometimes-disabling low back pain. Obese children are frequently taller than their peers and appear to be larger than normal, not only because of excessive body fat, but also because they have more lean body mass (more muscle tissue and bigger bones). Obese girls may experience early onset of menstruation, and are likely to develop secondary sexual characteristics, such as breast development and the appearance of pubic and other body hair, earlier than their normal-weight counterparts. On the other hand, they are more likely to have menstrual periods which are irregular in occurrence and length and to have masculine characteristics such as coarse skin and facial hair. They may be less fertile than normal as they mature, a condition frequently referred to as polycystic ovary disease,1 which can be both a consequence and a cause of obesity. Obese males frequently have decreased sperm production, as well as lower levels of testosterone, the primary male hormone. Many obese males have been found to have increased levels of some female hormones, which may cause excessive breast size. Some children and adults develop fatty livers, a condition referred to as hepatic steatosis, which is associated with abnormally high liver function tests. The long-term effects of this condition in children are not well understood, but concern has been expressed that some cases Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. of fatty liver can be associated with cirrhosis in later life. Gallstones occur 3 or 4 times more frequently in obese than in nonobese persons, especially those who are attempting to lose weight. Children as well as adults can be affected. Some obese adults (especially women), as well as a few obese children, develop chronic headaches because of increased pressure around the brain and spinal cord. This symptom is similar to one occurring in people with brain tumors. The condition is referred to as pseudotumor cerebri, meaning that symptoms are similar to those accompanying brain tumors, but do not occur because of brain tumors. The condition usually resolves with weight loss. Obese women appear to have higher rates of uterine, cervical, ovarian, breast, and gallbladder cancers (Huang et al. 1997; Ballard-Barbash et al. 1996). The reported association between obesity and cancer of the colon and rectum is unclear (Shike 1996). Cancer associated with obesity has not been reported in childhood. Costs of Diseases Associated with Obesity in the United States Nearly 80 percent of patients with type II diabetes are obese, and much of the estimated $19 billion direct costs of health care due to this disease is attributable to obesity. Nearly 70 percent of diagnosed cases of diseases of the heart and blood vessels are related to obesity; obesity accounts for $22.2 billion spent annually on heart disease, or 19 percent of the total cost of diagnosis and treatment. The annual cost of diagnosing and treating high blood pressure is about $1.5 billion. High blood pressure affects about one-fourth of American adults, and obesity doubles a person’s chances of having this condition. Copyright © 1999. University Press of Mississippi. All rights reserved. About $2.4 billion, or 30 percent of the total amount spent annually on gallbladder disease and surgery, is related to obesity. Americans spend about $33 billion annually, including money for diets and exercise products, in their efforts to lose weight. Little wonder that we call it the weight loss “industry” (National Institute of Diabetes and Digestive and Kidney Diseases Home Page 1997). If obesity had been prevented, the United States could have saved about $45.8 billion in 1990, or 6.8 percent of health care expenditures that year. Moreover, employers would have saved about $4 billion in 1990 if the 52.9 million days of lost productivity had not occurred (Wolf et al. 1994). Summary I have reviewed some of the ways in which obesity can negatively affect a person’s health and productivity. If, as you read, you experienced anxiety about possible complications of obesity in yourself or in your child, that is understandable, since these difficulties are not pleasant to think about. Remember that not every obese child or adult experiences all the possible consequences. My hope is that this information will stimulate you to examine Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Copyright © 1999. University Press of Mississippi. All rights reserved. carefully how daily living habits may affect your and your family’s health. The best time to undertake the prevention of obesity and its consequences is while children are young. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. 2. Who Is Obese, and How Do We Know? A rose is a rose is a rose is a rose. Gertrude Stein We learned in the last chapter that obesity can have serious unwanted effects on a person’s physical and mental health. This chapter attempts to answer the following questions: How is obesity defined, and how do we decide who is and who isn’t obese? What proportion of American adults and children are obese? Has this proportion changed any over the past few years? Are obese children more likely than nonobese children to become obese adults? Were obese adults also obese when they were children? (These last two questions are not exactly the same.) Are people who share certain characteristics such as age, sex, race, or economic status more likely than others to become obese, and why is it important to know this? Copyright © 1999. University Press of Mississippi. All rights reserved. Let’s begin by defining the term percentile, which most of us see occasionally but may not understand. A percentile is a number that divides a range of numbers (a dataset) so that a given percentage lies below this number. We can take height as an example. Suppose we carefully measure the heights of a thousand 10-year-old boys. We find that the shortest boy is 48 inches (4 feet) tall, and the tallest is 60 inches (5 feet) tall. A boy’s height is at the 50th percentile if 50 percent of the thousand boys have heights that are less than his. Another boy’s height is at the 10th percentile if 10 percent of all the boys’ heights are less than his. In the case of height and weight, we can use graphs that display percentiles for boys and girls from birth to 18 years of age, which makes correct calculation of an individual’s height and weight percentile very easy (fig. 2.1). Scientists and physicians often arbitrarily refer to measurements greater than the 95th percentile or less than the 5th percentile as being outside the range of “normal.” This applies not only to body measurements, but also to laboratory tests, metabolic rates, caloric intake, and many other measurements which may relate to obesity. But we could set the “cut point” for whatever we are measuring at the 85th, 75th, or whatever percentile we wish, as long as we clearly explain to others exactly what we are doing. A child having a weight, height, or other characteristic above the 95th or below the 5th percentile does not mean that he or she is “abnormal”; it simply means that the child is at the highest or lowest end of the range of what is being measured. Indeed, no matter what is being measured, there will usually be people in the 5th percentile (5 percent have lower values) and the 95th percentile (95 percent have lower values). The work of the physician or scientist is to decide whether persons having such measurements are at greater risk for a disease or condition than are those whose measurements are between the 5th and the 95th percentiles. By understanding percentiles, we can more easily comprehend the significance of the numbers related to obesity. We will learn more about percentiles when we examine how the percentages of obesity among American adults and children have been increasing in recent years. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Copyright © 1999. University Press of Mississippi. All rights reserved. How do we define “obesity”? The most obvious answer is “excessive body fat.” But the question actually involves several answers more complex than that, since we would have to have a good way to measure body fat in people who don’t look like they have too much of it. The ideal method of defining obesity from a medical perspective would be to match a child’s or young adult’s weight with undesirable outcomes—such as heart attacks—experienced by the person later on in life. Such information is being collected in the United States and elsewhere, but we will have to wait several more years for conclusive information to emerge. FIG. 2.1. Typical growth chart used to assess the height and weight of a child over a period of years. In this case, a boy aged 7 years weighed 25 kg (55 pounds), and was 125 cm (49.5 inches) tall, placing him at the 75th percentile for both weight and height. Three years later, at age 10, the same boy weighed 55 kg (120 pounds), and was 141 cm (55.5 inches tall). This placed him well above the 95th percentile for weight, but still at the 75th percentile for height. He should be evaluated for obesity. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Usually, when parents suspect that a child is obese, they take him or her to a physician. The physician may simply look at the child and, based on experience, agree that the child is obese. But not all obese children are so easy to diagnose. And some kids who may appear to be obese really aren’t. They may be relatively taller than other kids their age, or their appearance may be due to an unusually large muscle mass rather than to excessive fat. So a better method is to weigh the child, measure the height, and plot these values on a common percentile growth chart. If both height and weight are close to the same percentile (for example, the 75th percentile or the 10th percentile) and there is no evidence of abnormal growth, we usually say that a child is within the normal range for both of these measurements, since weight percentile (the “ideal” weight) is usually fairly close to the height percentile in normal children. On the other hand, if a child’s height is at the 25th percentile but weight is considerably above the 95th percentile, then this child’s weight is more than would be expected for his or her height, and the child may be obese. Copyright © 1999. University Press of Mississippi. All rights reserved. There’s another way to diagnose and express obesity. Let’s say a child’s weight is 88 pounds, which is 33 pounds greater than his or her expected ideal weight of 55 pounds. By simply dividing the measured weight by the ideal weight, we can then say that a child’s weight exceeds ideal weight for height by a certain percentage. In this example, dividing 88 pounds by 55 pounds gives a value of 160 percent. We can either say that this child’s weight is 160 percent of expected weight for height, or that weight is 60 percent greater than expected weight for height. Children whose weight is 20 percent or more above ideal weight for height are defined by convention as being obese. Yet another method which bases a diagnosis of obesity on a child’s height is the body mass index, or BMI. There are several different body mass indexes, but the one most commonly used to assess obesity is easy to use and to calculate, is reproducible, and is a generally reliable index of weight for height. The following formula is used in its calculation: Weight (in kilograms) divided by the square of the height in meters.2 For example, in the case of a child who is 122 centimeters tall and who weighs 48 kilograms, the BMI is 48 kilograms divided by (1.22 meters)2, or 32. Body mass index can also be calculated with the following formula, which uses weight in pounds and height in inches: Weight (in pounds) divided by the square of the height (in inches) multiplied by 704.5. Like height and weight, BMI increases with age, and percentile charts are available for plotting. Most authorities accept BMIs greater than the 85th percentile as being indicative of obesity in children. Many growing children, however, may have weights that fluctuate both above and below this cut point, depending upon their age, and it would be a mistake to label such children as obese and initiate treatment. Children whose BMI exceeds the 95th percentile, as in the above example, are much more likely to be obese, assuming that body fat measurements are also increased.3 A disadvantage of the BMI is that it varies with frame size and also with leg length. Frame size is assessed by measurement of the distances between certain bony prominences, such as elbow breadth. Another disadvantage of the BMI is that is often hard to explain to parents and others. In the remainder of this book, I use the term “overweight” to refer to the percentage of Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. children, adolescents, or adults whose BMI equals or exceeds the 85th percentile for their reference standard age and sex and the term “obese” to refer to those whose BMI equals or exceeds the 95th percentile for their reference standard age and sex. The reference standard for adults is the sex-specific 85th percentile values of body mass index for men and women aged 20 through 29 years of age from the National Health and Nutrition Examination Survey II data (1976–80). The reference standard for children and adolescents is the sexspecific 85th or 95th percentile values from National Health Examination Surveys II (1963– 65) and III (1966–70). A final method commonly used in clinics to define obesity is to measure the thickness of the skin and fatty tissue with a special instrument called skinfold calipers. In children, this measurement is most often taken at a point halfway down the back of the upper arm, and is called the triceps skinfold measurement. Although similar measurements are taken at various sites in adults, reliable standards exist only for the triceps measurement in children. A child whose triceps skinfold measurement is at or above the 85th percentile is diagnosed as being obese, since values above this percentile closely correlate with total body fat. Measuring the thickness of the triceps skinfold is not a perfect technique, because the results are poorly reproducible. Quite often neither the same nor different clinicians obtain the same measurements on the same child. Despite this limitation, the measurement has been used in many studies to help distinguish obese from nonobese individuals, and it is most helpful when used in conjunction with the BMI. Copyright © 1999. University Press of Mississippi. All rights reserved. Scientists can use several other methods to more accurately determine a person’s body fat. We will discuss only one of these in detail, and simply mention some others. The most accurate way to measure body fat would be to cut into a person and carefully separate and measure all the fat. The limitations of that method are apparent. We must rely on indirect methods instead. An indirect method of estimating body fat is to calculate the percent body fat by estimating body density. The density of a substance refers to how compact it is, and is usually expressed in the metric system as grams (weight) per milliliter (volume), or gm/ml. Body density can be calculated if body weight and the body volume are known. Weight is easy enough to measure. Body volume can be estimated by submerging an individual in water (Katch et al. 1967) and weighing the water displaced. Actually, the water doesn’t have to be weighed, since we know that the weight of the displaced water, in grams, is equivalent to its volume, in milliliters. Another way to estimate body volume is to weigh it out of water (in air), and then weigh it again while it is submerged (Siri 1956). Remember Archimedes’ principle? Archimedes taught us that the amount of weight a body loses in water is equal to the weight of the water it displaces. This means that we can figure out the volume of water a body displaces by simply weighing the individual both in air and while he or she is underwater. We then can calculate the body density by dividing the weight in air by the difference in air and water weights, and we do this in the metric system (kilograms per liter or grams/milliliter). Knowing a person’s body density enables us to calculate the percent body fat.4 For interested readers, I have explained in further detail how this test is performed.5 Most Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. very young children would not cooperate for this test, although standards exist for children as young as 7 years of age. Another drawback is that this method estimates total body fat, including the fat in the nervous system (brain, spinal cord, and nerves) and not just the fat that determines whether we are obese or not. Bioelectrical impedance (Guo et al. 1989) is frequently used in clinical settings to estimate body fat. The equipment for measurement is portable and inexpensive. The method works because fat tissue contains very little water or dissolved salts (electrolytes) and is therefore a poor conductor of electricity, while other body tissues do contain water and electrolytes and are good conductors of electricity. If certain assumptions are made about the cross-sectional area and the length of the conducting tissue, total body water can be estimated. In general, the higher the percent body water, the higher the lean (fat-free) body mass. The principle of the technology is as follows. A harmless low-voltage current is passed across the body. Resistance to the current, read on the instrument, is determined by relative amounts of fat mass and fat-free mass. Mathematical equations are used to convert percent body water into an estimate of body fat mass and body lean mass, which are incorporated into easy-to-use charts. Bioelectrical impedance is not accurate in severely obese people. Recent data indicate that this technology is no better than triceps skinfold measurements in estimating percent body fat in children. Other methods used to estimate body fat are found primarily in research laboratories and are expensive. They include total body water, total body potassium, total body electrical conductivity, and photon absorptiometry. Both computer-assisted tomography (the CAT scan) and magnetic resonance imaging (MRI) are used to estimate visceral fat. Copyright © 1999. University Press of Mississippi. All rights reserved. In summary, the methods most commonly used to assess childhood obesity in clinical and large epidemiologic studies include height, weight, body mass index (BMI), and skinfold (usually triceps in children) measurements. More sophisticated and expensive methods are available and are used primarily in laboratory research situations. How Obese Are Americans? Let’s try now to get an idea of how big the problem of obesity is in the United States. After all, if only 1 or 2 percent of the American public are obese, then the number of health problems due to obesity may be relatively small when compared to a chronic condition such as asthma, for example. On the other hand, if half of all Americans are obese and if obesity is associated with the health problems cited in chapter 1, I think most people would agree that we have cause for worry. We also need to know whether the problem remains virtually the same from year to year, or whether it is getting better or worse. We can relate changes in the number of obese Americans to matters such as total food and fat consumption, exercise habits, hours spent watching television, or cultural characteristics, and decide whether these factors might qualify as causes of obesity. There are basically two kinds of studies used to estimate how many people within a certain population (for example, Americans, Canadians, or New Yorkers) have a disease or a Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. condition such as obesity. The first is a longitudinal study. A relatively small number of individuals join a study at a particular time. At entry, people might be classified as lean, medium, or obese. Over time, those who were lean may become heavier, or they may remain in the same category. Persons who were obese at entry may remain obese, or they may become medium or even lean. And persons in the “medium” group may remain there, or they may become lean or obese. After a few years, investigators are able to get a good idea of how many people become obese and stay obese. These kinds of studies can be expensive because of the complex logistics of obtaining multiple measurements on the same people over a long period. A second type of study more often used to estimate the prevalence of obesity in a population is the cross-sectional study. Here, a “snapshot” is taken of representative samples of a population. Naturally, this snapshot cannot be taken of all participants instantaneously. Time is required for those conducting the study to invite people to have measurements performed, to perform the measurements, and to inquire about eating and exercise habits. Consequently, a cross-sectional study may last for two or more years. The best-known cross-sectional studies having to do with obesity are those performed by the United States National Center for Health Statistics. The names and dates of the study, along with the age range of the population studied, are given in table 2.1. The advantages of these cross-sectional studies are that the methods used in each are comparable, the participating individuals are selected from widely different communities, and the findings are largely representative of the entire population as well as of certain subgroups of the population, such as sex, age, and racial or ethnic groups. A disadvantage is that a different sample of the population is selected each time measurements are made. And these studies are also expensive, because they include not only weight and height data, but also laboratory data and information on diets, smoking habits, and other risk factors that affect the health of Americans. Copyright © 1999. University Press of Mississippi. All rights reserved. Table 2.1 U.S. national surveys used for evaluation of trends in overweight and obesity. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. Copyright © 1999. University Press of Mississippi. All rights reserved. With this background, let’s examine some of the information about excess weight in American adults, children, and adolescents obtained from the studies listed in the table. Tables 2-2 and 2-3 summarize the proportions of overweight or obese children, adults, and adolescents in selected studies (Centers for Disease Control 1997; Kuczmarski et al. 1994; Troiano et al. 1995; Ogden et al. 1997). As you review these tables, remember that persons whose body mass indices equal or exceed the 85th percentile for age and sex of a population reference standard are referred to as “overweight,” while those whose body mass indices equal or exceed the 95th percentile for age and sex of that population reference standard are classified as being “obese.” The population reference standards are identified in the information accompanying tables 2.2 and 2.3. Some of the findings of these studies are startling: 1. Nearly 35 percent of American adults 20 years of age or greater were overweight in the years 1988–94, up from 24.3 percent in 1960–61. This represented an overall increase of 44 percent since 1960–61! Table 2.2 Changes in percent of overweight* U.S. adults aged 20 years or more, by sex and race or ethnicity; from NHES I, 1960–61, and NHANES III, 1988–94. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. *“Overweight” is defined as a body mass index of 27.8 or greater for men and 27.3 or greater for women, which are the sexspecific 85th percentile values of BMI for men and women aged 20 through 29 years of age from NHANES II data (1976– 80). The term “overweight” is used in this study since only body mass index was used and may have included some individuals having greater-than-usual muscle mass. Figures exclude pregnant women. Please refer to the text of chapter 2 for definitions of body mass index. **Hispanics were not identified as a separate group in studies before NHANES III (1988–94), so comparisons with earlier years cannot be made for this ethnic group. Hispanics may have been included in every category in the 1963–65 period. Beginning in 1988–94, the categories “white” and “black” exclude Hispanics. Copyright © 1999. University Press of Mississippi. All rights reserved. Table 2.3 Percent of overweight* U.S. children (aged 6–11 years) and adolescents (aged 12– 17 years), by sex and race/ethnicity**; from NHES II and III, 1963–70, and NHANES III, 1988–94. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. *In this table, overweight is defined as body mass index at or above sex- and age-specific 85th or 95th percentile BMI cut points calculated at 6-month age levels. The reference standard for children and adolescents is the sex-specific 85th or 95th percentile values from the National Health Examination Survey II (1963–65) and the National Health Examination Survey III (1966–70). Copyright © 1999. University Press of Mississippi. All rights reserved. **Hispanics were not identified as a separate group in studies before NHANES III (1988–94), so comparisons with earlier years cannot be made for this ethnic group. Hispanics may have been included in every category in the 1963–65 period. Beginning in 1988–94, the categories “white” and “black” exclude Hispanics. 2. Higher percentages of African American women than men were overweight in both time periods (41.6 vs. 22.1 and 52.3 vs. 33.3, respectively), and higher percentages of Mexican American women than men in 1988–94 (50.1 vs. 36.4). Yet the percentages of overweight white men and women were approximately the same, 33.7 and 33.5 percent, respectively. 3. The age and sex of a person are related to his or her being overweight. During 1988–91, 20.2 percent of men aged 20–29 years were overweight, but over twice this proportion, 42.1 percent, of men aged 50–59 years were overweight (data not shown). 4. The trend was even more dramatic among women during this same time period: 20.2 percent in the 20–29 year age group were overweight, compared to 52.0 percent in women aged 50–59 years. 5. Another disturbing finding is that, while the percent of overweight adults changed very little between 1960 and 1980, a 37 percent increase—from 25.4 percent up to 34.9 percent— occurred between the two time periods 1976–80 and 1988–94 (Kuczmarski et al. 1994). Significant increases took place in adults of all racial and ethnic groups. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. 6. More of our children are becoming overweight. The percentage (BMI at or exceeding the 85th percentile reference standard of 1963–70) among children (age 6–11 years) and adolescents (age 12–17 years) in 1988–94 was about 27 percent. Nearly 14 percent of all children and more than 11 percent of adolescents were classified as “obese,” an increase from 5.2 percent of children and 5.2 percent of adolescents in 1963–70. 7. Overweight among African American girls and among Mexican American children and adolescents of both sexes tended to be greater than among whites, but these differences were not statistically significant. 8. The percent of black boys and white boys, children as well as adolescents, whose BMIs were at or higher than the age-and-sex specific reference standard percentile increased dramatically between the two time periods 1963–70 and 1988–94—by 12.1 to 15.3 points. Points for white female children increased by 8.5 and for white female adolescents by 10.4. But for African American female children and adolescents, the group increased by 19.6 and 18.0 points, respectively, reflecting the racial differences seen in older adult women. The same trends held true in the comparing of female children and adolescents whose body mass indices equalled or exceeded the 95th percentile, the cut point for definite obesity. Copyright © 1999. University Press of Mississippi. All rights reserved. What do all these numbers mean? They mean that, since the early 1960s, the number of American children, adolescents, and adults who have become overweight or obese has increased dramatically. Of special concern is the evidence that African Americans and Mexican Americans are getting heavier at faster rates than white Americans. Why this is so is unknown, but what is suggested is that cultural factors may play a major role in the development of obesity. Other ethnic groups, such as some Pima, Navajo, and Cherokee Americans, also have high rates of excessive weight. The fact that our children are becoming heavier is a clear signal that preventive efforts should begin in early childhood. Most of the increases in percentages of overweight children, adolescents, and adults occurred after 1976–80 (data not shown). Clearly, something has happened since that period to cause these changes, and the evidence suggests that the environment, not our genes, is to blame. We know that excessive weight is strongly associated with a person’s age, sex, and race. What else could be happening? Are we, as a nation, consuming more food or simply more fat? Have we become increasingly sedentary, perhaps because we spend more time in front of the television set or computer screen than before, and because we spend less time in vigorous exercise? We know that obesity is strongly associated with certain demographic factors. For example, individuals in the Northeast and the Midwest are heavier than those on the West Coast, and city dwellers are generally heavier than those living in rural locations. Obesity is less common in people who have graduated from college than in those who have not finished high school. Members of low-income families, especially adult women, are more likely to be obese than their middle- or upper-income counterparts; persons of low income are also more likely to have limited educations and to be members of an ethnic group in which thinness is perceived as a sign of poor health. There are many possible reasons why overweight has increased, and it has been hard to separate out exactly which factor is the most important. Many investigators have attempted to answer the questions of whether obese children are Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. more likely than nonobese children to become obese adults and whether obese adults were also obese when they were children. Different measurements can be used: body mass index, triceps skinfold thickness, subscapular skinfold thickness, and weight for height (Garn et al. 1985; Serdula et al. 1993; Guo et al. 1994). These different methods of investigation make strict comparisons impossible. However, it turns out that use of different methods is a strength in trying to answer the questions above, since nearly all the studies come to similar conclusions: 1. In various studies, it has been found that about 26–63 percent of obese children (age 0.5– 14 years) become obese adults, depending upon how investigators defined obesity, the age of the child at entry into the study, and the length of time that participants were followed. Put another way, obese children are at 2–6.5 times greater risk for becoming obese adults than nonobese children. The older the obese child is, the more likely he or she is to become an obese adult. 2. Recent information (Whitaker et al. 1997) indicates that if one or both parents are obese, the risk of a child under the age of 10 years becoming an obese adult more than doubles, regardless of whether the child is obese. Whether this effect is due to environmental or genetic factors or both is not known. 3. Not all obese adults were obese children. Both lean- and medium-weight people may become obese because of changes in their living habits: they get less exercise as they grow older, and they may eat more high-fat foods. Depending on the study and techniques used for measurement, it has been found that as many as 44 percent of obese adults were obese as children. Copyright © 1999. University Press of Mississippi. All rights reserved. Summary We have learned in this chapter that obesity can be defined in many ways, having to do with whether a person develops a disease because of excessive weight; whether he or she looks heavier than usual; whether certain body measurements, such as weight, height, the relationship between the two, and skinfold measurements are outside the “normal” range for age; and whether percent body fat determined by water displacement or other methods is excessive. We have learned also that obesity, however defined, is a worsening public health problem in the United States, since about 35 percent of adults and 27 percent of children recently were found to have body mass indices equalling or exceeding the 85th age- and sex-specific reference standard percentile. Finally, we have learned that obesity is a complex condition, possibly associated with a person’s age, sex, race, or ethnic group, and that obese children have a high likelihood of becoming obese adults. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. 3. How Our Bodies Obtain Energy A buena hambre no hay mal pan. (There is no such thing as bad bread when you have a good appetite.) Gabriel Garcïa Márquez Understanding how children and adults become obese begins with knowing how our bodies normally digest, absorb, use, and store food for future needs. The key concept is energy. Our bodies must have energy for carrying out fundamental life activities, such as thinking, breathing, eating, growing, moving around, and reproducing. In this chapter, we will analyze the steps necessary to obtain energy from the food we eat. Acquiring Energy: The Essentials Copyright © 1999. University Press of Mississippi. All rights reserved. Acquiring the energy to sustain life and to perform work and recreational exercise is a complex process. There are two central concepts that we need to become familiar with in order to understand this process. The first is that energy, the fuel needed by our bodies so that they can “run,” comes from the food we eat. This food energy is converted to a form the body can use by the breaking down of dietary carbohydrates and fats, primarily, to water and carbon dioxide. Much of this energy is lost as heat. We will learn the basic concepts of energy utilization in this chapter. The second central concept is that the whole process of the body’s use of energy, including digestion, absorption, and storage of excess intake, is directed toward achieving glucose homeostasis. This means that glucose, the main sugar used as fuel by our cells, is kept at a relatively constant level in the blood at all times. The reason that glucose homeostasis is important is that glucose is usually the only substance used by our nervous system (the brain, spinal cord, and nerves) for energy. Other body cells may use protein and fat for fuel, but the nervous system uses only glucose, except in special circumstances such as prolonged starvation. If our nervous systems do not have glucose to use as fuel, then other parts of our bodies, such as the circulatory, respiratory, digestive, and excretory systems simply fail, since their proper functioning depends in large part on an intact nervous system. Knowing that blood glucose levels have to stay within a narrow range is very important to an understanding of how energy is normally obtained and used and of how obesity can occur. Three terms will help us understand how our bodies take in and release energy. Anabolism refers to the process by which large molecules are synthesized from small molecules by the cells of the body. Catabolism means just the opposite: large molecules are converted, or broken down, to smaller molecules. And metabolism (which means “change” in Greek) includes both anabolism and catabolism: it is the entire process of converting energy from one form to another so that the body can use it. We will learn several examples of each of these. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. The Digestive Process All energy is derived from the sun. Plants and algae capture the sun’s energy, which they store as carbohydrates, and animals ultimately get their energy from plants, which they store as carbohydrates, proteins, and fats. In humans, obtaining energy begins with the consuming and digesting of plants and animals and their products, better known as food. We all know what consumption of food is, but what exactly is digestion? Digestion is the sum of all the processes by which the food that we eat is converted into a form that can be absorbed and used for energy. Since digestion is the first step in acquiring energy, let’s review what our digestive systems—mouth and throat, esophagus, stomach, small and large intestines, liver, gallbladder, and pancreas—do and how they do it. Remember that our digestive systems actually communicate with our brains through a special part of the nervous system, the autonomic nervous system,6 which signals the brain as to when we need food and when we have had enough. Our nervous systems therefore are of immense importance to the digestive process. In this chapter, we will concentrate on the digestion and absorption of macronutrients —the carbohydrates, proteins, and fats that make up the bulk of the food we eat—although micronutrients, such as vitamins and minerals, as well as water, are also critically important to life. Copyright © 1999. University Press of Mississippi. All rights reserved. How Is Digestion Normally Controlled? When we swallow, food is mixed and propelled further down the digestive tract when smooth muscle in the walls of the esophagus, stomach, and intestines contracts. Smooth muscle is supplied by nerves from the autonomic nervous system. These nerves are able to sense the nature of the food or liquid in the stomach and intestines (such as how concentrated the food is and how much acid it contains) and whether smooth muscle is being stretched because of the quantity of food. This information is sent to the brain in the form of nerve signals. Upon receiving these signals, the brain then instructs the autonomic nerves to stimulate smooth muscle to contract or relax and to stimulate the release of digestive enzymes (proteins secreted by cells which make chemical changes possible in other substances, but which are not affected themselves by the process) from glands contained in the walls of the digestive tract. All three of these—smooth muscles, autonomic nerves, and glands—work in concert to provide us with two very important signals: hunger and satiety. We are all familiar with hunger. Without this deep-seated, automatic stimulus to look for and eat food, animal life as we know it would not exist. If we have not eaten for several hours, our stomachs begin to contract rhythmically, often producing what are commonly called “hunger pangs.” Hunger is controlled to a great extent by the brain: stimulation of a specific area of the brain known as the lateral hypothalamus causes animals (including human beings) to eat. On the other hand, damage to this area can cause an animal to lose all desire for food, resulting in starvation. Satiety, the opposite of hunger, is the feeling of satisfaction, or the absence of hunger, that we experience after a filling meal. As with hunger, the brain is important in producing satiety: Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. stimulation of a second area of the hypothalamus, the ventromedial area, results in satiety. For example, even if an animal has had food withheld for several hours, if this area of the hypothalamus is stimulated while food is being offered, it will not eat. However, if the ventromedial area is damaged or destroyed, the person or animal cannot be satiated, but instead develops a voracious appetite, and, usually, obesity results. We will learn more about exciting new developments in the study of hunger and satiety in chapter 8. What Does the Digestive Process Accomplish? We won’t list every step necessary in the digestion of food; let’s just summarize how the process works. When we chew food, we cut, shred, and pulverize it, and our saliva moistens and lubricates it. After it is swallowed, food is further moistened by mucus in the stomach, and hydrochloric acid is added, which aids digestion of protein (and kills germs). The pancreas secretes bicarbonate into the small intestine, which neutralizes the acid, and enzymes secreted by the stomach, the pancreas, and the small intestine cause further breakdown of the food. Food that can’t be absorbed, such as the cellulose found in much of the plant fiber that we eat, is eliminated in the feces. Exactly what happens to the three major macronutrients—carbohydrates, proteins, and fats —during the digestive process? Carbohydrates, including starches and sugars, are found in foods such as bread, pasta, cereal, milk, and fruits. To be absorbed, they must be broken down into their simple component sugars—glucose, fructose, and galactose—by enzymes in saliva and by intestinal and pancreatic secretions. Copyright © 1999. University Press of Mississippi. All rights reserved. Proteins, which are supplied in foods such as meats, fish, cheese, milk, and beans, are usually broken down by several enzymes secreted by the stomach, the pancreas, and the small intestine into their constituent amino acids for absorption. Lipids, including meat fat, cooking oils, and the fats in nuts, cheese, and a host of other foods, are usually eaten in the form of triglycerides. They are acted upon in the intestine by bile salts from the liver and gallbladder, which help make them soluble in water, and by pancreatic enzymes, which further degrade them to their component parts, glycerol and fatty acids; these are easily absorbed. Unless the macronutrients are completely digested, they cannot be properly absorbed by the body’s cells and used to produce energy. To understand obesity, we have to understand the products of digestion and what happens to these products when they are absorbed. The pancreas and certain specialized cells in the walls of the stomach and small intestine also secrete hormones into the bloodstream. Hormones are chemicals produced in one kind of tissue that regulate function in another kind of tissue; they travel via the bloodstream. Insulin is a hormone secreted by the pancreas. One of its functions is to help glucose get into cells. Gastrin is a hormone produced by the stomach that enters the bloodstream and regulates the secretion of acid and certain digestive enzymes. And cholecystokinin is yet another hormone released by cells in the small intestine that makes the gallbladder contract and squeeze its contents (bile) into the small intestine. Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. The Three Phases of Digestion The first phase in digestion is the cephalic (the Greek kephale means “head”), and refers to the fact that digestion is largely under the control and influence of the brain. This is also called the conscious or sensual phase, because all of our five senses can play an active role in the digestive process. The second and third phases are the gastric and the intestinal, so named because most of the action occurs in the stomach and the intestine, respectively. Copyright © 1999. University Press of Mississippi. All rights reserved. Cephalic (Sensual) Phase We are all familiar with this phase of digestion. When we have not eaten for several hours, the smell of coffee brewing or fresh cinnamon rolls baking can focus our attention on eating. Our sense of sight also may compel us to seek food: television ads for pizza or hamburgers, recipes in magazines, and the presentation of a meal (how it is served, and how it looks on the table) are well-known examples. Children may think of food when they hear their parents describing a great pasta dish consumed at a restaurant the night before, or when they hear or smell popcorn popping. How food tastes and feels—whether it is seasoned according to our individual preferences, and whether it is soft or tough and chewy— may determine how much food we eat or whether we eat it at all. Finally, our past associations with the smells and tastes of certain foods and the environments in which they were eaten (such as the madeleines so brilliantly described by Marcel Proust in his novel Remembrance of Things Past) are very important: children may find a birthday party with hot dogs, potato salad, music, and games so enjoyable that hot dogs become their favorite food. A romantic evening meal with candlelight and wine may determine an individual’s preference for a particular food for a lifetime. Real estate agents long ago came to appreciate the fact that the smell of bread baking in the kitchen can help sell a house! On the other hand, we may lose our appetites when food is mentioned or served in certain emotional or unpleasant situations, or when meals are repetitious. We may be less hungry if we have just seen a film showing children who forage for food in garbage dumps, or if foul odors are being emitted from a poultry processing plant close to the restaurant we’re eating in, or if we swallow a loose dental crown with our food. Having to eat military field rations for several weeks on end may not satisfy our hunger at all, and may permanently kill our taste for canned meats. The central role played by our nervous systems in the digestive process, whether at a conscious level or an automatic one, is extremely important. Environmental impulses (sight, sound, smell, taste, and touch) that stimulate our five senses are transmitted by nerves to specific receptors in the brain, cells whose only function is to receive and process these nerve signals. The receptors then send impulses to the nerve cells in the digestive tract responsible for the nervous control of digestion, which activate, participate in, or terminate the digestive process. The cephalic phase of digestion is of particular importance in treating obesity, as we shall see later. There is another important and automatic aspect of the cephalic phase of energy acquisition, and that is our “biological clocks,” also known as circadian rhythms (from the Latin circa, meaning “about,” and dies, meaning “day”). Circadian rhythms influence many body functions, Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. such as our wake-sleep cycles, body temperatures, excretion of certain substances into the urine, and secretion of some hormones into the blood. For example, body temperature is generally about one degree Celsius higher when we are awake than when we are asleep, and the secretion of growth hormone by the pituitary gland is at its peak during a normal day when we are sleeping. Circadian cycles operate internally in each individual, but depend on external (environmental) cues to set the actual hours of the rhythm. For example, our wake-sleep cycles are set largely by the natural light-dark cycle, the most important external time cue. Another important cue is meal timing. In the United States, most people are accustomed to eating three meals a day: breakfast, lunch, and dinner. Our daily schedules usually determine what time we consume these meals each day, and we learn to associate the eating experience with that particular time. If mealtime is earlier or later than usual, some of our body functions that depend on circadian rhythms can be temporarily disturbed and cause irritability or fatigue, as in the state known as “jet lag.” For a little girl whose biologic clock has taught her that school lunch is always served at 11:48 A.M., a delay of 45 minutes can be catastrophic! Gastric and Intestinal Phases The digestive process ends a short time after food has finally been broken down into its absorbable components, and absorption then becomes the paramount activity. The switching of this process on and off belongs to the gastric and intestinal phases of digestion. They are discussed together because they are very similar. Copyright © 1999. University Press of Mississippi. All rights reserved. What is (or is not) in the lumen (the cavity) of our stomachs and intestines largely controls the digestive process: both the amount of food in our digestive tracts and the composition of that food stimulate secretion of hydrochloric acid by the stomach, secretion of digestive enzymes by the small intestine and the pancreas, and excretion of bile by the liver and gallbladder. In this sense, the appearance or disappearance of food in the digestive system activates or deactivates the digestive process. You have probably already realized that the three phases of digestion—cephalic, gastric, and intestinal—can occur simultaneously during a meal and continue while food is being absorbed. In summary, we now know how we sense hunger or satiety, how food is digested, and what controls the digestive process. To begin to answer the question “How does a person become obese?”, we have to follow the digested food further along in the metabolic process. The Absorption, Use, and Storage of Energy The absorption of food and its use by or storage in the body are regulated by mechanisms different from those responsible for the breakdown of food to absorbable molecules. Over a long period of time, humans have adapted to frequent variations in two major factors in our environments: temperature and food availability. We can live within a wide range of temperatures, and we can live without food for a relatively long interval. Of course, adjusting to these environmental changes is not always easy or immediate or a conscious effort, but the biologic capability for survival is nevertheless present. This concept of how we are able to survive without food is critically important to an understanding of how and why obesity may Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. develop. For a moment, set aside the idea that obesity is a condition that can cause serious health complications, and think of excess body fat as a way to survive when food is scarce. When food is plentiful, we have more than enough energy for our bodies to function under resting conditions (breathing, circulating blood, digesting food), and also for conditions requiring greater utilization of energy, such as manual labor and recreational activities. Food energy which is not used immediately after absorption is not simply discarded by our bodies; it is stored in several forms, one of which is fat. Then, when food is scarce, stored energy is used as a source of energy, and we are able to buy time until food becomes available again. In this sense, obesity (storage of excess calories as fat in the body) is not at all an abnormal condition, but is an adaptive mechanism that has developed over thousands and thousands of years. Let’s learn how our bodies obtain and use energy under two conditions: first, directly and immediately from the food that we have recently eaten, and then from stored energy provided by food that was eaten earlier. How Does the Body Use Food as an Energy Source? Copyright © 1999. University Press of Mississippi. All rights reserved. What happens to glucose, amino acids, and fats after they enter the bloodstream? First, recall that dietary carbohydrates are broken down to glucose, fructose, and galactose. I will refer to all three from now on as glucose, since fructose and galactose end up being metabolized much like glucose. Glucose enters the epithelial cells lining our small intestines, and is absorbed into the bloodstream and transported to the nervous system, liver, skeletal muscle, adipose (fat) tissue, and most other cells of the body. With the assistance of insulin, glucose enters cells and provides nearly all the energy required by the body for three or four hours after a meal is ingested. This is especially important to our nervous systems: without a constant and steady supply of glucose for nervous tissue, we become irritable and may have convulsions, become unconscious, and even die. If the glucose levels become excessively high, then glucose may be excreted by the kidneys into urine and lost as an energy source. Thus, blood glucose levels are tightly regulated by the body, as we shall soon learn. Glucose can be transformed for storage in different ways by the liver and skeletal muscles. In the liver, glucose can be changed in one of two ways: it can be stored as glycogen, a large molecule composed of many glucose molecules linked together to be used later as an energy source, or a small amount of glucose can be converted to triglycerides. Most of the triglycerides formed in the liver from glucose are not stored there, but combine with specialized proteins called apoproteins to become water-soluble lipoproteins. These fatprotein complexes are then carried by the bloodstream to fat tissue anywhere in the body. On reaching the fat cell, they are acted upon by lipoprotein lipase, an enzyme made by fat cells and located in the walls of capillaries. This critically important enzyme once again reduces the triglycerides in lipoproteins to their component parts, glycerol and fatty acids. Most of the fatty Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. acid molecules enter the fat cell to combine after activation with phosphorous-containing glucose molecules called glycerophosphate, forming triglycerides for storage. What remains of the lipoproteins moves on in the circulation to be processed by specialized receptors on the surface of liver cells. Glycerol molecules released from lipoproteins by lipoprotein lipase are converted by body tissues to glucose, which can be used as an energy source. In skeletal muscle, which is any muscle attached to a bone, glucose can be used as a source of immediate energy, or it can be stored as glycogen. Whether in liver or in skeletal muscle, glycogen may be thought of as a small “bank” into which glucose is deposited and held until it is broken down by specific enzymes when it is needed for energy. Proteins are broken down during the digestive process to their constituent amino acids, which enter the epithelial cells and are absorbed into the bloodstream. Amino acids are taken up by most cells of the body and used for new protein synthesis. Once the limits of protein synthesis have been reached in cells, additional amino acids are degraded by complex metabolic transformations to be used for energy, or they can be transformed to fatty acids for storage as triglycerides. Dietary proteins provide very little of total dietary energy under usual conditions. Copyright © 1999. University Press of Mississippi. All rights reserved. Dietary fats (mostly triglycerides) are reduced in the intestinal tract to glycerol and free fatty acids, and are also absorbed by intestinal epithelial cells. Both glycerol and fatty acids can be used for energy by almost all cells but not by the brain. Glycerol and fatty acids can also be reconstituted in epithelial cells to triglycerides, which coalesce into large molecules that bind with apoproteins to form water-soluble lipoproteins known as chylomicrons. Chylomicrons enter lymph channels which transport them to the bloodstream, then proceed directly to fat cells for storage in the same manner that lipoproteins manufactured by the liver are stored, as discussed above. In summary, dietary glucose not used as an immediate energy source is stored as liver or muscle glycogen, or it can be converted to lipoproteins for storage in adipose tissue. Amino acids are taken up by body cells and used to make new protein; the small amounts not used can be converted either to glucose or to free fatty acids when blood glucose and glycogen begin to be used up. Dietary fat is broken down to glycerol and fatty acids, molecules that can be absorbed and used for energy, or, if not, then once again reconstituted to triglycerides and transported as chylomicrons to fat cells for eventual storage as triglycerides. Fat tissue is thus a dynamic body tissue, since the constant exchange of fatty acids renews stored triglycerides approximately every two to three weeks. How Does the Body Use Energy That Has Been Stored in Tissues? We have now completed our discussion of what becomes of the carbohydrates, proteins, and fat in a meal that is normally digested and absorbed and of how these processes are regulated. We can use a common situation to summarize how energy stored as glycogen, protein, or fat is utilized. Let’s assume that you had dinner last night, slept the traditional eight hours, and had only a Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. cup of black coffee for breakfast. When you arrived at work at eight o’clock, you discovered that your colleague who had been preparing a major report for your company’s board of directors would not be at work because of illness, and you were being asked to complete the report and present it at four o’clock that day. You dropped everything to work diligently throughout the day, never stopping to eat. The report was ready by the deadline, you made the presentation for your company, and you answered questions until seven o’clock. You had eaten nothing for the entire day, but you nevertheless had sufficient energy to think about the form the report would take, select necessary information from the computer’s database, prepare slides and handouts, make sure that the conference room was clean, and obtain necessary slide projectors. Where did the energy to do all this come from? Within three to four hours after dinner the night before, your body had entered the postabsorptive zone, meaning that it had to obtain energy from nondietary sources, since no food was being provided. In other words, you had to cash in on one of the three energy banks —glycogen, fat, or protein—because your body had to maintain a normal glucose level at all times in order for your brain to function. This constant level of glucose was maintained by a process known as gluconeogenesis (gluco=glucose; neo=new; genesis=creation). “New” glucose was actually created in the liver from other substances in the body. Copyright © 1999. University Press of Mississippi. All rights reserved. The first source of glucose occurred by the process of glycogenolysis (lysis=breaking apart). Remember the glycogen that was formed from glucose and stored in the liver and in skeletal muscle? It’s not a lot, but liver glycogen became the first resource called upon to maintain normal blood levels of glucose during your fasting state. However, since there was only enough liver glycogen to supply glucose for about four hours, and since some of this was used for the energy of breathing while you slept, the glycogen stored in skeletal muscle also had to be used during your busy day. In addition, the formation of fat virtually ceased during your fast, and existing adipose tissue triglycerides began to be catabolized to glycerol and fatty acids. The released glycerol, which constitutes about 12 percent of the weight of triglycerides, served as an important resource for new glucose formation. Fatty acids released by triglyceride catabolism were transported in the bloodstream bound to a plasma protein known as albumin, and were oxidized as a major longterm energy source. Fatty acids were also used in preference to glucose as a source of energy in some body tissues. But the major source of new “building material” for glucose came from skeletal muscle, which was catabolized to form amino acids, the most important of which in your case was alanine (the amino acid mobilized in the greatest amounts when someone is fasting). Further catabolism of amino acids in muscle cells supplied most of the carbon needed for synthesizing new glucose during your fast. Had the fast continued for a few weeks, protein loss would have been very likely to have detrimental effects on your health. The glucose obtained from the three above sources provided only about one-third to onehalf the energy that you needed during your unexpected fast. So how did you get the energy you needed? Your body made a remarkable adjustment to keep your glucose level within a narrow range: the cells of the body actually decreased their dependence on glucose. That way, most of Smith, C. (1999). Understanding childhood obesity. Retrieved from http://ebookcentral.proquest.com Created from lynnu on 2017-10-16 19:21:49. the glucose could be used by the nervous system, which, as you recall, uses only glucose for energy under normal circumstances. Your blood glucose levels were lower than normal during your fast, and the secretion of insulin was decreased, but the secretion of glucagon was increased, as will be explained in the next section. Furthermore, during fasting the formation of active thyroid hormone was reduced, which lowered the energy requirements of your body by as much as 25 percent. How the Use of Stored Fat as an Energy Resource Is Regulated Since we are interested in obesity, let’s briefly examine the ways by which the body regulates the catabolism of adipose tissue fat. Because insulin is such an important player in fat metabolism, as well as in glucose metabolism, we will discuss it first. Insulin is produced by highly specialized cells in the pancreas called beta cells, and is the most important regulator of energy production and utilization in the body. Insulin is normally released from t...
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