Theories Of Aging Summary

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Why do we age? summarize 3 theories or hypotheses that explain why our bodies age.( see attachment for material

In the US and abroad, cosmetic surgery is tremendously popular. Many patients who request cosmetic surgery do so wishing to regain their once youthful appearance. Why do you think Americans are so concerned with the outward signs of aging, most of which are relatively harmless? Is this a positive or a negative social trend? Explain and support your response.

Theories Of Aging Summary
Theories Of Aging Summary

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info aging guides BIOLOGY OF AGING THEORIES OF AGING An introduction to aging science brought to you by the American Federation for Aging Research WHAT IS A THEORY OF AGING? Theories of aging can be divided into two categories: those that answer the question “Why do we age?” and those that address the question “How do we age?” Only a few broad, overarching theories attempt to explain why we and nearly all living organisms age. These theories compete with each other, making it unlikely that more than one of them could be true. Over time, some theories have fallen out of favor as others have become more widely accepted. Other theories, more properly called hypotheses, are smaller in scope and address the ­question, “How do we age?” They attempt to explain the mechanisms that affect how we and other ­species age, and it is likely that a ­number of them are simultaneously true. Testing these hypotheses is the current pursuit of most ­aging ­research. Identification of the mechanisms that affect ­aging could lead to interventions that slow or alter aging. Recent ­research implies that there may be a limited number of these mechanisms, giving scientists hope that their efforts may one day lead to strategies that could help us lead longer, healthier lives. MODIFYING THE COURSE OF AGING A critical issue in aging research is whether aging is affected by one, several, or a multitude of underlying processes. If there are ­hundreds of different biological pathways that affect aging, then odds are slim that science could ever hope to devise a way of slowing down how we age or even understand why aging happens at all. 2 | Infoaging Guide to Theories of Aging Identification of the mechanisms that affect aging could lead to interventions that slow or alter aging. However, evidence seems to be pointing to just a few fundamental processes as the primary culprits in the scenario of aging. The best evidence lies in the existence of single-gene mutations that affect lifespan in experimental animals, as well as a well-known environmental intervention called caloric restriction. Caloric restriction, in which laboratory animals are maintained on nutritionally balanced but sparse diets, containing 30 to 40 ­percent fewer calories than a normal diet, has been shown to reliably ­increase the average and ­maximum lifespans of a range of organisms, including worms, insects, and rodents. It is currently under investigation in primates. By itself, caloric restriction ­retards almost all of the age-related changes mice normally undergo, including the onset of age-related diseases. Single-gene mutations that ­extend lifespan, discovered so far in roundworms, fruit flies, and mice, are also a powerful argument that a finite number of pathways influence aging. Interestingly, the genes all seem to affect one of a few biochemical pathways, such as energy consumption, stress resistance, or regulation of what is called the insulin/IGF-1 neuroendocrine pathway. IGF stands for insulin-like growth factor. These findings offer hope that ­researchers may eventually be able to modify the course of ­aging in humans. However, there is a ­caveat. Animals ­modified to live longer often show ­inherent ­defects. Some mutant ­roundworms have reduced ­fertility and a reduced ability to enter a dormant state. Mutant Ames dwarf mice live a long time but are sterile and inactive. Rodents maintained on calorically restricted diets are thin, cold, stunted, and ­sometimes sterile. It is likely that such animals, although they survive to a ripe old age in the laboratory, would never stand a chance in the wild. AGING IS NOT A PROGRAMMED ASPECT OF DEVELOPMENT Aging is not a programmed aspect of development. It is the deterioration of what might be thought of as a survival program. Not long after Charles Darwin ­published his groundbreaking theory of ­evolution by natural selection in On the ­Origin of Species in 1859, ­scientists began to try to use ­Darwin’s theory to explain ­aging. One of the first was August ­Weismann, who published his hypothesis in 1891. He proposed that aging evolved to benefit ­species or groups by eliminating unfit animals to make room for the next generation. Although this idea was popular for decades, Weismann later rejected it, as do modern biologists. Evidence overwhelmingly shows that natural selection operates to affect the ­reproductive success of individuals, not the overall survival of groups. Further argument against a ­ ging as a programmed aspect of ­development, ordered by a g ­ enetic blueprint, lies in its v­ ariability. Although members of a species ­develop into adults in the same way, even g ­ enetically similar or identical individuals, raised in ­identical c ­ onditions and eating identical food, age differently. Whereas one person (or mouse) may die of heart failure, another may succumb to cancer with his or her heart functioning perfectly. When scientists discovered that changing just one gene in the roundworm, C. elegans, could significantly extend its lifespan, some researchers argued that this showed aging was ­genetically programmed. However, most scientists now believe that overstated the case: just because a gene happens to affect the rate of aging does not mean that it was designed by nature to do so. The majority of scientists now prefer other theories. THE CROSS-LINKING/­ GLYCATION HYPOTHESIS OF AGING The cross-linking hypothesis is based on the observation that with age, our proteins, DNA, and other structural molecules develop inappropriate attachments or cross-links to one another. These ­unnecessary links or bonds decrease the mobility or elasticity of proteins and other molecules. ­Proteins that are damaged or are no ­longer needed are normally broken down by enzymes called proteases. However, the ­presence of cross-linkages inhibits the ­activity of proteases. These ­damaged and unneeded proteins, therefore, stick around and can cause problems. One of the main ways cross-­ linking occurs is through a ­process called glycosylation or ­glycation. Glucose molecules can stick to proteins, then ­transform into brownish molecules called ­advanced ­glycosylation ­endproducts, or AGEs. When both of the sticky ends of AGEs adhere to neighboring proteins, they form ­permanent cross-links that disable the ­proteins’ ­functions. This is the same process that causes food to brown when it is cooked. Some research supports the ­hypothesis that cross-linking contributes to aging. Cross-linking of the skin protein collagen, for example, has proven at least partly responsible for wrinkling and other age-related dermal changes. Cross-linking of proteins in the lens of the eye is also believed to play a role in age-related cataract formation. Researchers speculate that cross-linking of proteins in the walls of arteries or the filtering systems of the kidney account for at least some of the atherosclerosis (hardening of the arteries) and age-related decline in kidney function observed in older adults. Another study conducted at the Bjorksten Institute in Wisconsin treated brain tissue from young animals with known cross-linkinducing compounds. That brain tissue soon looked quite similar to older brain tissue, with its naturally cross-linked brain proteins, adding evidence in support of this theory of aging. Recently, scientists have found evidence that glycation contributes to the formation of beta-amyloid, Infoaging Guide to Theories of Aging | 3 the protein that clumps together in the brains of Alzheimer’s patients. Somewhat indirect experimental evidence in support of the crosslinking theory of aging appears in studies that look at drugs that ­prevent cross-linking, and the ­impact of taking those drugs on the various components of the ­aging process. Studies done in China and in the United Kingdom on the molecule carnosine are ­provocative. Carnosine occurs in very low concentrations in the brain and other tissues. In the­ ­laboratory, carnosine has been shown to delay the senescence or aging of human cells called fibroblasts. Carnosine works by ­preventing cross-linking of proteins. The more recent ­Chinese studies suggest carnosine might be of benefit in delaying the formation of cataracts, in which crosslinking is thought to play a part. Although many scientists agree that cross-linking of proteins, and perhaps the cross-linking of DNA molecules as well, is a component of aging, it is likely only one of several mechanisms that contributes to aging. THE EVOLUTIONARY SENESCENCE THEORY OF AGING The most widely accepted overall theory of aging is the evolutionary senescence theory of aging. Unlike the earlier programmed theory of evolution and aging, which tried to find reasons why evolution might favor aging, evolutionary senescence theory focuses on the failure of natural selection to affect latelife traits. Natural selection, because it operates via reproduction, can have little effect on later life. In the wild, predation and ­accidents ­guarantee that there are always more younger individuals reproducing than older ones. Genes and mutations that have ­harmful effects but appear only after reproduction is over do not affect ­reproductive success and therefore can be passed on to future generations. In 1952, Peter Medawar ­proposed that the inability of natural selection to influence late-life traits could mean that genes with detrimental latelife ­effects could continue to be passed from generation to generation. This theory is called the mutation accumulation theory. Many scientists believe that mitochondrial aging is an important contributor to aging in general. 4 | Infoaging Guide to Theories of Aging A few years later, George ­Williams extrapolated on this idea by formulating the theory of ­“antagonistic pleiotropy.” Antagonistic pleiotropy means that some genes that increase the odds of successful reproduction early in life may have deleterious effects later in life. ­Because the gene’s harmful effects do not appear until after reproduction is over, they cannot be eliminated through natural ­selection. An example of antagonistic pleiotropy in humans is p53, a gene that directs damaged cells to stop reproducing or die. The gene helps prevent cancer in younger people, but may be partly responsible for aging by ­impairing the body’s ability to renew deteriorating tissues. Because of antagonistic pleiotropy, it is likely that tinkering with genes to improve late-life fitness could have a detrimental effect on health at younger ages. Much experimental evidence exists to support the basic premises of the evolutionary senescence theory of aging. For example, the theory predicts that delaying the age of reproduction should delay aging, as it would increase the power of natural selection later in life. Experiments with fruit flies in which younger flies were prevented from mating, ­allowing only older flies to reproduce, ­confirmed this prediction. Aging in the fruit fly population was delayed. ­However, these long-lived flies were less fertile in early life than normal flies, giving support to the idea of antagonistic pleiotropy. In experiments with roundworms given a gene mutation that extended their lifespan, scientists found that these long-lived worms ­exhibited defects, such as reduced ­ability to enter a protective dauer stage (a developmental state in which worm larvae can better ­survive harsh conditions), delayed ­development, and impaired ­reproduction. In the 1970s, Thomas Kirkwood added to the evolutionary biology theory of aging with his ­“disposable soma” theory. He believed that organisms have to balance the demands of maintaining their body cells, or soma, and reproducing. Because an organism invests resources into ­reproduction, over time mutations and other cellular damage accumulate in the soma because the body cannot repair all of it. This idea explains some of the ­disparity in lifespan between different types of organisms. Species that are likely to die due to predation, such as mice, invest more energy in reproduction than in maintaining health because an individual is unlikely to live long anyway. ­Humans, on the other hand, have few predators and can therefore allocate more resources to repairing physical damage since they will be able to reproduce over a longer period of time. Research conducted by Steven Austad in the early 1990s ­provides interesting proof of this idea, namely, that hazardous ­environments favor early reproduction and short lifespans, whereas safer environments favor the ­opposite. Studying Virginia opossums in South Carolina and ­Georgia, he found that animals ­living on a predator-free island aged much more slowly and reproduced later than opossums on the more dangerous mainland. The disposable soma theory may also explain why some organisms, like salmon or certain kinds of spiders, reproduce only once and then die. If the animal is likely to die anyway before the next breeding season, then natural selection would favor allocating all an animal’s resources to reproduction, leaving nothing for somatic maintenance. Although many scientists believe the evolutionary theory of aging needs further refinement, most agree that it is currently the best explanation for why we and other organisms age. THE GENOME MAINTENANCE HYPOTHESIS OF AGING Damage to our DNA happens thousands of times every day in every cell in our body throughout our lives. This damage can be caused by oxidative free radicals, mistakes in replication, or ­outside environmental factors such as radiation or toxins. Mutations or spontaneous changes in the structure of our genes that occur in our egg or sperm cells will be passed on to future generations, if those mutations are not so potentially disruptive as to be fatal to our offspring. Mutations that occur in the rest of the cells of the body will only affect that individual and ­cannot be passed on to future generations. Most of those body cell, or somatic, mutations will be corrected and eliminated, but some will not. Those will accumulate, eventually causing the cells to malfunction and die. This process, it has been suggested, is a crucial component in the aging process. This theory also encompasses a role for mitochondria, the ­cellular powerhouses, as important ­factors in aging. Mitochondria create ­damaging free radicals as a by-product of normal energy production. Somatic mutations in the DNA of the mitochondria ­accumulate with age, increasing free ­radical production, and are associated with an age-related decline in the functioning of ­mitochondria. Many scientists believe that mitochondrial aging is an important contributor to aging in general. Luckily, our bodies have repair mechanisms to take care of much of that damage. In fact, many scientists believe that humans have long lifespans because we are much better at repairing our genome than short-lived animals like mice. This is related to an evolutionary theory of aging called the “disposable soma” theory. Defects in DNA repair seem to be directly related to aging. Evidence exists for the decline in DNA repair and the accumulation of DNA damage in several different types of cells taken from elderly subjects. Elderly patients’ blood and skin cells have less capacity to repair themselves than those from young adults. Indeed, one study that looked Infoaging Guide to Theories of Aging | 5 in white blood cells found DNA ­damage in two to four percent of the cells from young adults, but six times more often in cells from the elderly. These aging white blood cells with their higher level of DNA damage may explain some of the decline in immune function associated with aging. In addition, scientists have linked Werner’s syndrome, a rare disease of premature aging, to mutations in the WRN gene. These ­mutations lead to abnormalities in DNA replication and repair of DNA damage. Poor capacity for DNA repair is also linked to the most prevalent disease of aging, cancer. Exploring the role of DNA damage and repair remains a critical area of aging research. THE NEUROENDOCRINE ­HYPOTHESIS OF AGING The neuroendocrine system ­refers to the complex connections between the brain and nervous system and our endocrine glands, which produce hormones. The hypothalamus, a structure at the base of the brain, stimulates and inhibits the pituitary gland, often called the “master gland,” which in turn regulates the glands of the body (ovaries, testes, ­adrenal glands, thyroid) and how and when they release their hormones into our circulation. As we age, this system becomes less functional, and this can lead to high blood pressure, impaired sugar metabolism, and sleep abnormalities. The effects that the various hormones our different glands produce have on different facets of aging have been studied extensively. For a time, aging researchers working in neuroendocrinology— the study of hormones regulated by the brain—thought that laterlife reduction of hormones, such as the reduction of estrogen that accompanies menopause, was responsible for aging. However, although some late-life functional changes may be linked to reduced hormone levels, experimental evidence in mice from as early as the 1960s and continuing today shows the opposite: reduction in ­hormones can lengthen life. ­Studies in mice whose pituitary glands were removed showed the mice lived longer with a delay in age-related changes. A flood of recent evidence has ­pinpointed this effect to one area: the insulin/IGF-1 hormonal ­pathway. IGF stands for insulinlike growth factor, a substance ­activated by growth hormone. Single-gene mutations in fruit flies and the roundworm C. ­elegans, widely studied by aging ­researchers, have recently been tied to the insulin/IGF-1 ­pathway. In 2002, a study by French ­researchers published electronically in Nature showed a similar effect in mice. In all the laboratory Natural substances within our cells called antioxidants sop up and neutralize dangerous free radicals. But those that escape this cleanup process can damage DNA, proteins, and ­mitochondria. 6 | Infoaging Guide to Theories of Aging organisms studied, mutations that reduce the amount of circulating IGF extend life. In many cases, however, the long-lived mutants have defects that could potentially affect their ability to survive in the wild, possibly making the IGF-1 pathway’s relationship to aging an example of antagonistic pleiotropy. This consistency among ­species makes scientists optimistic that the insulin/IGF-1 pathway may work in a similar fashion in ­humans, and may be an ­excellent target for interventions that could affect aging. Interestingly, this ­evidence flies in the face of popular support for anti-aging treatments involving injections of growth hormone, which increases circulating IGF-1. Rather than prolonging life as some ­companies claim, such treatment may instead do the opposite. A recent study of humans who genetically lack an ability to use growth hormone found that these people were ­protected against cancer and the development of adult-onset ­diabetes. THE OXIDATIVE DAMAGE/ FREE RADICAL HYPOTHESIS OF ­AGING Oxidative free radicals are one of the toxic byproducts of normal cell metabolism. Natural substances within our cells called antioxidants sop up and neutralize these dangerous free radicals. But those that escape this cleanup process can damage DNA, proteins, and mitochondria. This damage, called oxidative damage, accumulates over time. Some fruit fly studies suggest that oxidative damage is one of the direct causes of aging. Proponents of the free-radical hypothesis of aging note that free radicals can cause DNA damage, the cross-linking of proteins, and the formation of age pigments. Oxidative damage contributes to many age-related diseases, such ...
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Why do we age?
Human beings and other animals are biologically designed to change in appearance with
time. When a young child is compared to an adult, the differences are evident. A grown-up adult
is far more advanced in terms of thought and physical appearance. As time goes by, the adult
human being will continue undergoing changes in the body, and after a given duration he or she
will not be able to do some of the things, which they used to do when young. At this stage, the
individual’s sight will become weaker, the ability to hear will be affected, the person may not be
in a position of running and performing other duties he or she used to perform at a younger age.

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