Although survival curves may be useful tools for scientists studying life extension techniques in target populations, they are of little value in expressing the effects of a clinical intervention program on a specific individual. For this purpose, an individual life-history curve is far more meaningful.
Such a theoretical Life-history Curve for a "typical" American charts a single individual's expected level of overall physiological functional capacity during his/her lifetime. For this individual, his/her 80-year life span is divided into four approximately equal periods or phases of life: Development, Vitality, Degeneration, and Senescence. At the end of the final phase is death.
The first phase of life is characterized by growth, development and maturation. It begins at birth (actually, at conception) and ends when the individual becomes fully mature. Depending upon the individual, this usually happens between the ages of 15 and 25.
The second phase of life is young adulthood, roughly the period between the ages of 20 and 40. This phase is characterized by a high degree of sensual activity, excellent health, intellectual and emotional growth, physical strength, and general vigor.
The third phase of life is "middle age," the period between approximately 40 and 60 years of age. This phase is characterized by a slow, but progressive, degenerative decline in physiological function. Libido wanes and sensual activity becomes less frequent. At the end of this period women go through menopause. There is a general loss of physical strength due to a loss of muscle mass and an increase in body fat.
Flexibility and mobility decline. Wrinkled skin, drooping breasts, graying hair, balding heads, and bifocal glasses declare to the world that "the bloom is off the rose." In addition, insomnia often becomes a problem – minor, at first, but increasing with age. Finally, minor health problems begin to occur more frequently and last longer. General health and vitality are in decline.
The final phase of life is characterized by rapid decline and disease. This phase begins roughly at age 60. As the progressive degeneration described above continues and accelerates, symptoms of homeostatic imbalance reach clinical thresholds and one or more diseases are diagnosed. Immune function continues to decline and susceptibility to new infections increases.
This phase has been characterized as a constant state of physiological stress, complete with anabolic hormone deficiencies and elevated catabolic hormones that produce a negative nitrogen balance, and inflammatory factors. Medical intervention, at this point, is usually aimed at relieving the symptoms of disease rather than eliminating their causes.
Perhaps the greatest indictment of the current health care delivery system in the United States today is that it focuses almost exclusively upon treating existing disease, thus prolonging the fourth phase of life. An alternative strategy is to focus upon prolongation of that period of an individual's life span associated with a high level of physiological function and free of age-related disease symptoms. The goal is not merely to postpone death but to enhance life. Such a program aims to improve the quality of human life as well as its quantity.
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