Sunday, December 11, 2005

aging, biological theory of

aging, biological theory of
Researchers are testing two major sets of theories to explain the biological causes of human aging. One set of theories rests on the idea of "wear-and-tear" of the body, and envisions a situation in which the body accumulates a series of "environmental insults" over time. According to this view, the body can normally repair itself during youth, but its capacity to do so eventually becomes worn down under the steady bombardment of environmental attack.
The "somatic mutation" theory represents one branch of this thinking, and it holds the background radiation from various sources in the environment-not necessarily manmade-produces mutations and genetic damage in our cells, which, in turn, cause our cells to fail to properly reproduce themselves. Research, however, has shown that human, animal, and even insect patterns of aging are much too varied and rapid to be explained by externally caused cell mutation. Another variant of the wear-and-tear theory is that faulty protein molecules in human genes may sometimes contain errors that build up over time and eventually cause an "error catastrophe" that results in death. Although research has shown that faulty proteins do build up in cells and tissues, there is no evidence yet that such proteins result from genetic errors or end in an "error catastrophe. "
A second set of theories perceives aging as a continuous process and a normal development that may be programmed in human genes before birth. One branch of this theory suggests that the endocrine organs, which produce hormones and control nerve impulses, decline in function as people grow older. A particularly popular and successfully researched version of this theory is that the hypothalamus (a specialized cluster of cells in the central part of the brain that joins with other glands to regulate growth, sexual development, the menstrual cycle, the onset of menopause, etc.) and the pituitary glands function as master timekeepers of the body. Over time neuroendocrine tissues in human bodies lose the capacity to respond to signals from the control mechanisms, and the result is that people age.
Another line of thinking in this branch of theory is that the genetic constitution, which faithfully regulates the reproduction of genes within cells, undergoes internally caused changes (mutations) that produce errors. Although the cells manage to repair these errors throughout life, their capacity to do so begins to fail as the years go on. Again, the result is aging.
Yet a third approach arising out of the concept of normal aging relates to the working of the immune system. About 20 years ago researchers determined conclusively that the effectiveness of the immune system declines with the years. The consequence is that people become increasingly vulnerable to disease, whether it be from acute infections like pneumonia or from slower-working malignancies like cancer. Specialists on the immune system also observe that this system loses the power to distinguish between hostile invaders of the body and the body itself. While still providing some protection against outside invaders, the immune system becomes an attacker of the body, a process known as "autoimmunity," or immunity against oneself. Some researchers, however, question the validity of the autoimmune theory on the ground that organisms with immune systems far less complex than those of humans also experience aging.
Many people may be familiar with the term collagen, which introduces another of the ideas about why people age. According to this approach, there are molecules in the body that form stable links with other molecules, and these in turn eventually change the molecules that create them. The result is a loss of elasticity in body tissues. Because the changes in process especially affect the skin, it is the cross-links that carry the main blame for the wrinkles that normally mark people during aging. Loss of elasticity in blood vessels may also contribute to high blood pressure.
Finally, there is a theory that holds that "free radicals" (a free radical is a small molecule with an unpaired electron) escape during the normal cellular process of burning food (metabolism), and that these free radicals cause damage that builds up as life goes on. Although human bodies produce enzymes that usually destroy free radicals, these unwanted products slowly accumulate rather like a buildup of rust or carbon in an automobile engine. Ultimately they cause human bodies to break down.
One of the interesting byproducts of this last theory is the finding that restriction of calorie intake can increase the life span of several species of laboratory animals by as much as 50 percent. Were humans to experience such an increase it would be equivalent to raising the possible biological length of their lives to over 180 years. The well-known researcher Dr. Roy Walford of the University of California is carrying on a unique test of this theory by experimenting on himself. He is consuming a carefully controlled low-calorie diet that may provide striking confirmation of the theory if Walford lives to the extended human life span of 120 years.
Although research is still far from determining the whole truth about the causes of aging, there are a few clearly established findings. One is that certain specific environmental factors under human control do influence the process of biological aging. For example, it is well documented that exposure to the sun speeds up the aging of the skin, and also that the sun worship so widely practiced by Americans contributes to cancer of the skin. It has also been well documented that a high-cholesterol diet may contribute to high blood pressure by building up fatty deposits in the arteries and veins. Environmental factors under human control, therefore, contribute to ill health, and it is increasingly recognized that ill health, rather than the passage of years, underlies the physical changes that people most often refer to when they speak of aging.
Research also suggests that sensescence, the process of growing old, may be built into human cells. Some years ago Dr. Leonard Hayflick performed a simple but elegant experiment where he maintained human cells alive in a laboratory culture medium. He found that the cells continued to reproduce themselves and to proliferate, but that eventually this process slowed down and stopped, resulting in the death of the cells. Accordingly, it appears that human cells, and consequently our entire bodies, have a limited lifetime.
People may never be able to find the key to the biology of human aging, and even if they do, they may never be able to prevent, halt, or reverse the aging process completely. Research findings, however, clearly show that individuals who maintain intelligent and healthful lifestyles have a greater chance of living longer and achieving a better quality of life. They actually modify the seemingly unalterable course of human mortality simply by their own choice to live with moderation.
Cristofalo, V. J. Atherogenesis. Frankfurt, New York, London, Tokyo: Springer-Verlag, 1986.
Weindruch, R., and Walford, R. L. Retardation of Aging by Dietary Restriction. New York: Raven, 1987.

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