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...
Purchase answer to see full
attachment