When asked to define aging, almost everyone will do so in terms of the passage of time. However, biogerontologists, the scientists who study aging in living organisms, now believe that aging may have little or nothing to do with time per se. One only needs to consider the facts that a roundworm is "old" at 7 days, a mouse at 36 months, a dog at 10 years, and a horse at 20 years, whereas humans at these same chronological ages are in their growth and development stage.
Aging is a biochemical phenomenon influenced by both genetics (both between and within species) and the environment. Although there is still much debate about the fundamental biology of aging, most scientists share the view that aging manifests itself as a progressive decline in physiological function associated with an array of degenerative processes characteristic of old age.
Within a single species, the symptoms of aging may occur earlier or later in life depending both on genetic factors and environmental factors affecting individuals.
Therefore, as a practical matter, aging can be viewed as a measure of biological function rather than chronological age. Given the above, it is clear that biological aging may occur more quickly or more slowly than chronological aging depending upon a great many interrelated factors that affect the general health status of each individual. It is not being an old person that is undesirable, it is functioning like an old person that is undesirable.
Indeed, there are certain progeric diseases in which biological aging seems to progress at an accelerated rate. People afflicted with such diseases suffer a broad spectrum of the symptoms normally found in people twice their chronological age. Thus, one might say that the goal of life extension is, "increasing chronological aging by decreasing biological aging."
The National Institute on Aging (NIA) was created as the official agency of the United States Government (National Institutes of Health, U.S. Public Health Service) that funds and regulates biomedical aging research in this country. The NIA has officially recognized twelve different theories of aging, each attempting to explain the aging process.
Others have cataloged over 25 different theories, many of which overlap and all of which describe some aspect of the aging process as determined by the scientific community.
Whatever the ultimate nature of aging may be, senescence manifests itself as certain progressive degenerative processes resulting from some combination of biochemical, hormonal and neural imbalances. Many of these imbalances are subject to empirical examination and manipulation. Thus, they may be preventable and/or reversible.
Many characteristics of aging in multicellular organisms are clearly under genetic control at some level in the organism, as deduced from the stable, characteristic patterns of aging and the stability of the maximum life span in most species from generation to generation. However, the extent to which aging changes within the cells of higher organisms are preprogrammed for "intrinsic" senescence remains an open question.
If the putative mechanism for the control of aging involves neural and endocrine factors, then the time course of aging should be subject to extensive "extrinsic" manipulation. In other words, if we can understand how biochemical, hormonal, and neural imbalances produce degenerative disease states, then we may be able to successfully intervene.
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