Biologic theories

Biologic theories of aging attempt to explain why the physical changes of aging occur. Researchers try to identify which biologic factors have the greatest influence on longevity. It is known that all members of a species suffer a gradual, progressive loss of function over time because of their biologic structure. Many of the biologic theories of aging overlap because most assume that the changes that cause aging occur at a cellular level. Each theory attempts to describe the processes of aging by examining various changes in cell structures or function.

Some biologic theories look at aging from a genetic perspective. The programmed theory proposes that every person has a “biologic clock” that starts ticking at the time of conception. In this theory, each individual has a genetic “program” specifying an unknown but predetermined number of cell divisions. As the program plays out, the person experiences predictable changes such as atrophy of the thymus, menopause, skin changes, and graying of the hair. A closely related theory is the run-out-of-program theory, which proposes that every person has a limited amount of genetic material that will run out over time, and the rate of living theory, which proposes that individuals have a finite number of breaths or heartbeats that are used up over time. The gene theory proposes the existence of one or more harmful genes that activate over time, resulting in the typical changes seen with aging and limiting the life span of the individual.

The molecular theories propose that aging is controlled by genetic materials that are encoded to predetermine growth and decline. The error theory proposes that errors in ribonucleic acid protein synthesis cause errors to occur in cells in the body, resulting in a progressive decline in biologic function. The somatic mutation theory is similar but proposes that aging results from deoxyribonucleic acid (DNA) damage caused by exposure to chemicals or radiation and that this damage causes chromosomal abnormalities that lead to disease or loss of function later in life.

Cellular theories propose that aging is a process that occurs because of cell damage. When enough cells are damaged, overall functioning of the body is decreased. The free radical theory provides one explanation for cell damage. Free radicals are unstable molecules produced by the body during the normal processes of respiration and metabolism or following exposure to radiation and pollution. These free radicals are suspected to cause damage to the cells, DNA, and the immune system. Excessive accumulation of free radicals in the body is purported to cause or contribute to the physiologic changes of aging and a variety of diseases such as arthritis, circulatory diseases, diabetes, and atherosclerosis. One free radical, named lipofuscin, has been identified to cause a buildup of fatty pigment granules that cause age spots in older adults. Individuals who support this theory propose that the number of free radicals can be reduced by the use of antioxidants such as vitamins A, C, and E, carotenoids, zinc, selenium, and phytochemicals.

One variation of this theory is the crosslink or connective tissue theory, which proposes that cell molecules from DNA and connective tissue interact with free radicals to cause bonds that decrease the ability of tissue to replace itself. This results in the skin changes typically attributed to aging such as dryness, wrinkles, and loss of elasticity. Another variation, the Clinker theory, combines the somatic mutation, free radical, and crosslink theories to suggest that chemicals produced by metabolism accumulate in normal cells and cause damage to body organs such as the muscles, heart, nerves, and brain.

The wear-and-tear theory presumes that the body is similar to a machine, which loses function when its parts wear out. As people age, their cells, tissues, and organs are damaged by internal or external stressors. When enough damage occurs to the body’s parts, overall functioning decreases. This theory also proposes that good health maintenance practices will reduce the rate of wear and tear, resulting in longer and better body function.

The neuroendocrine theory focuses on the complicated chemical interactions set off by the hypothalamus of the brain. Stimulation or inhibition of various endocrine glands by the hypothalamus initiates the release of the various hormones from the pituitary and other glands, which, in turn, regulate bodily functions, including growth, reproduction, and metabolism. With age, the hypothalamus appears to be less precise in regulating endocrine function, leading to age-related changes such as decreased muscle mass, increased body fat, and changes in reproductive function. It is proposed that hormone supplements may be designed to delay or control age-related changes.