Biological theories of aging. (The Biology of Aging)

What we experience as human aging is the synthesis of biological, psychological, and social factors. This article will focus on the biological theories of aging and, since entire volumes have been devoted to the subject, will provide a brief overview and a discussion of a few of the current theories. For detailed discussion, the reader is referred to more extensive reviews listed at the end of the article.

GENERAL CONSIDERATIONS

When I attended my first gerontologic conference in 1971, most of the discussion, which was quite heated, revolved around which theory of aging was correct. Perhaps in that early phase of the biological study of aging, one might accept the possibility that all of aging could be explained by a single cause or theory. However, the more I am exposed to the wonderful complexity of biological aging, the more obvious it becomes that no single theory can explain all its phenomena. What we observe as biologic aging is the sum total of many independent causes, some operating at the level of individual molecules, others at the level of individual cells, and still others at the level of tissues, organs, and whole organisms. Therefore, when I outline the various theories of aging it should be kept in mind that, if they are operational, they act in concert to produce biologic aging.

The number of theories in a field usually inversely reflects the state of knowledge in that field. For example, in the early days of the field of immunology, there were many theories to explain the diversity of antibody molecules that respond to various bodily infections. With increasing knowledge, two major theories emerged to explain this diversity. With the advent of molecular genetics, it was finally possible to distinguish between these two explanations, and we now know in great detail how so many different antibodies are produced by the body; theories are thus no longer needed. Since the field of biologic aging research is still in a relatively early stage, it is not surprising that dozens of biological theories have been proposed to explain aging phenomena. In the future, as the new tools of molecular genetics are applied to unravel the mysteries of aging, we may eliminate many of these theories and someday be able to explain the enormously complex biological events that aging comprises.

Theories of biological aging can be divided into those that propose that aging is determined by the programmed actions of specific inherited genes (Programmed or Genetic Theories) and those that propose that random assaults from both our external environment and our body's internal environment accumulate with time and result in aging (Stochastic Theories). As I indicated above, it is probable that aging involves many …