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The rate of living theory of aging
 The rate of living theory of aging is perhaps the oldest explanation of aging. Ancient philosophers believed that we possess a finite amount of some “vital substance.” When that substance is consumed, we die. Philosophers even argued that each person had only a finite, predetermined number of breaths or heartbeats, and that once they were used, death ensued.
In the 20th century, scientists proposed a new twist on this old theory: energy consumption limits longevity. In other words, an organism’s metabolic rate determines its lifespan. This idea was supported by the discovery that reactive oxygen species (free radicals), a byproduct of normal metabolism, can damage cells and contribute to aging. Experiments in cold-blooded organisms showed that their life span was inversely related to the temperature they lived in or how active they were. More recent work with C. elegans, a roundworm, showed that changing just one gene related to metabolism could significantly extend the worm’s lifespan.
On the other hand, some experimental evidence has shown no clear relationship between temperature and longevity. Experiments in fruit flies have shown that temperature either has no effect, or the opposite effect. For example, a 1997 experiment showed that briefly exposing fruit flies to elevated temperature could actually slow aging for several weeks. Scientists now believe that although metabolic rate can affect aging, that doesn’t mean that it always does so. Caloric restriction, the only intervention known to extend life in mammals, does so without reducing the animal’s metabolic rate. In addition, experimentally boosting an animal’s metabolic rate does not always reduce longevity. And even though there is a rough correlation among species between body size, metabolic rate, and longevity, there are many exceptions to this rule. For example, birds typically have a metabolic rate 1.5 2.0 times as high as similar-sized mammals, yet they live on average about three times as long—a pattern inconsistent with the rate of living theory.
Modern scientists have now rejected the rate of living theory as being a valid overall explanation for why we and most other species age. However, oxidative damage is still considered to be one of several mechanisms contributing to the aging process, and numerous aging researchers are pursuing research in this area.
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