LEARNING OBJECTIVES
• How do rate-of-living theories explain aging?
• What are the major hypotheses in cellular theories of aging?
• How do programmed-cell-death theories propose that we age?
• How do the basic developmental forces interact in biological and physiological aging?
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efore he started selling his Lean Mean Grilling Machine, George Foreman was a champion boxer. In fact, he became, at age 44, the oldest boxer ever to win the heavyweight championship. Foreman’s success in the boxing ring came after a 10-year period when he did not fight and despite the belief that his career was finished.
Why is it that some people, like George Foreman and Dara Torres, manage to stay competitive in their sports into middle age and others of us experience significant physical decline? For that matter, why do we age at all? After all, some creatures, such as lobsters, do not age as humans do. (As far as scientists can tell, lobsters never show measurable signs of aging, such as changes in metabolism or declines in strength or health.) Scientists and philosophers have pondered the question of why people grow old and die for millennia. Their answers have spurred researchers to create a collection of theories based on basic biological and physiological processes. The search has included many hypotheses, such as metabolic rates and brain sizes, that haven’t proved accurate. But as scientists continue unlocking the keys to our genetic code, hope is rising that we may eventually have an answer. To date, though, none of the more than 300 existing theories provides a complete explanation of all the normative changes humans experience (Vintildea & Miguel, 2007).
Before we explore some of the partial explanations from scientific research, complete the Discovering Development exercise. Compare your results of this exercise with some of the theories described next. What similarities and differences did you uncover?
Rate-of-Living Theories
One theory of aging that makes apparent common sense postulates that organisms have only so much energy to expend in a lifetime. (Couch potatoes might like this theory, and may use it as a reason why they are not physically active.) If we examine the metabolic rate of certain animals and look for correlations with their life spans, we find some support for this idea. For example, insects’ life spans can be increased by not allowing them to fly, and some mammals live longer when they are induced to hibernate (Cristofalo et al., 1999).
Additionally, there are some data showing that the number of calories animals and people eat is related to longevity. Reducing caloric intake in rodents and rhesus monkeys lowers the risk of premature death, slows down a wide range of normative age-related changes, and in some cases results in longer life spans than do normal diets (Hayflick, 1996; Roth et al., 1995). On the human front, Okinawans, who consume only 60% of the calories in a normal Japanese diet, have 40 times as many centenarians (people who are at least 100 years old) per capita as there are in the rest of Japan. Moreover, the Okinawan incidence of cardiovascular disease, diabetes, and cancer is half that in the rest of Japan (Monczunski, 1991).
But does metabolism relate directly to longevity across species? Apparently not (de Magalhaes, Costa, & Church, 2007). Although the age at which a mammal becomes mature is related to longevity, a detailed review of data from many species does not support the view that metabolism is related to length of life. So saving your energy won’t directly result in living longer.