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The Reproductive Cell-Cycle Theory of Aging

AN UPDATE

Rather than seeing aging as a loss of functionality as we get older, the Reproductive Cell-Cycle Theory of Aging defines aging as any change in an organism over time, as evidenced by the fact that if all chemical reactions in the body were stopped, no change, and thus no aging, would occur.  The most important change in an organism through time is the chemical reactions that result in a single cell developing into a multicellular organism.  Whatever controls these chemical reactions that regulate cell growth, development, and death, is believed to control aging. The article explains that these cellular changes are directed by reproductive hormones, as part of the hypothalamic-pituitary-gonadal axis (HPG axis).  Receptors for reproductive hormones (such as estrogen and testosterone) have been found to be in all tissues of the body. The HPG axis normally promotes growth and development early in life to achieve reproduction.  Hormones levels then begin to change in men around age 30 and in women when they reach menopause, around age 50.  When the hormones become unbalanced, cellular growth becomes dysfunctional, and cell death can occur, both of which can initiate senescence, the accumulated damage to cells, tissues, and organs that occurs with the passage of time and is associated with aging.

The update article provides an overview of recent new evidence supporting this theory.  Disease studies show that women who reach menopause later have less heart disease, less dementia, and less osteoporosis, supporting the theory that the longer the HPG axis is in balance, the less likely one is to develop age-related diseases.  The most compelling supportive evidence is from studies of Hormone Replacement Therapy (HRT).  Research with women and men undertaking HRT has shown that taking sex hormones that are biologically identical to human hormones delays the onset, decreases the incidence of, and can reverse the course of age related illnesses such as heart disease, Alzheimer’s, osteoporosis, and some types of cancer.  However, it should be noted that only biological hormones have these effects.  The use of non-human synthetic hormones has been shown to increase the risk of these and other diseases. 

Further studies in support of the theory have shown that suppressing the HPG axis increases lifespan.  This can occur when humans experience either caloric restriction, cold, or exercise stress.  From an evolutionary standpoint, this helps increase the survival of our species.  When the environment is hostile, the HPG axis is restrained and fertility decreases, thereby slowing aging and saving our reproductive capacity for a later time when the environment is better suited to raising offspring. 

The authors argue that because of the accumulating evidence supporting this theory of aging, scientific efforts would be well spent on exploring methods to restore balance in the HPG axis as we get older.

Source:  Atwood, C. S., & Bowen, R. L. (2011). The reproductive-cell cycle theory of aging: An update. Experimental Gerontology, 46(2-3), 100-107.  http://dx.doi.org/10.1016/j.