Barbara C. Hansen, professor of physiology in the School of Medicine at the University of Maryland and director of the university's Obesity and Diabetes Research Center, responded as follows:
"The effects of caloric restriction on life span in mammals have been demonstrated only among rodents. At present, there are no studies that can definitively say whether caloric restriction extends life span in any primate, so it is premature to draw conclusions about the possibility of extending the maximal human life span.
"We are currently researching caloric restriction in nonhuman primates (monkeys). Our monkey study has been going on for 13 years. We began the study on young adult monkeys, equivalent to humans 20 to 25 years old. The animals were placed on a weight-control regimen, designed so they could not put on middle-age weight. We set a monkey's caloric intake for each week based on whether he has gained or lost weight since the previous week.
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"There are two key issues under study here: Does the average monkey live longer, and is the maximal life span of the monkeys extended? For rodents, caloric restriction has been shown to extend both average and maximal life span. For monkeys, we already know that the animals in our group are living longer than average, if you judge by the 50 percent death point (the time at which half of the animals in the group have died). We have also observed general improvements in health and decreases in disease, which are likely to translate into longer lives. For example, restricting the caloric intake of a monkey sufficiently to prevent the onset of middle-age obesity completely prevents type II diabetes, even though these animals are prone to diabetes.
"What we do not know is the effect on maximal life span. Will the longest-lived calorie-restricted monkey survive longer than any other monkey has survived? The monkeys in the University of Maryland study are now at an age equivalent to that of 50- to 60-year-old humans, so we won't know the answer to that question for another 10 to 15 years; monkeys are thought to live well into their thirties under laboratory conditions.
"There are two other, related primate studies under way. Richard Weindruch started a similar one at the University of Wisconsin about five year after our own. George Roth and Mark Lane are running a slightly different program at the National Institute on Aging. Roth and Lane are working with several groups of monkeys; they started some of the monkeys on a program of caloric restriction before the animals reached adulthood. This study will produce the best data on the effects of early caloric restriction. It is not yet clear whether early caloric restriction has a net positive or negative influence on health.
"We do know enough to say that caloric restriction to prevent obesity during adulthood is likely to be very beneficial, to humans as well as to monkeys. Nobody knows whether the health benefits are because of the positive effects of not having excess adipose tissue or because of altered metabolic activity. My colleagues and I just finished a paper showing that caloric restriction increases the efficiency with which the body burns calories. The problem is, we do not know how that change occurs--that is what we are studying right now.
"Incidentally, there is no evidence that excessive leanness is healthy. In fact, most evidence shows that there is an increased mortality associated with excessive leanness. It seems that there is an increasing risk of disease and risk at both ends of the weight range, though probably for different reasons.
"Turning back to the original question, we must consider several issues separate from caloric restriction. The lifestyle of an athlete would normally extend life span because it would involve exercise, a healthy diet, absence of obesity, absence of smoking and absence of alcohol and drug abuse (one would hope!). Such behaviors reduce the risk of various diseases, including cardiovascular disease. The athletic lifestyle won't necessarily cause you to live to 100, but it might extend your life span from, say, 70 to 80.
"The rapid rate at which an athlete burns calories is not associated with either an increase or decrease in life span. There is, in fact, a theory that animals have only a fixed number of calories to burn during their life, but it is pure speculation, intended to explain why calorie-restricted rodents live longer. There is no evidence that an enhanced turnover rate of calories hastens the aging process. Based on our current level of knowledge, we would expect athletes to have longer or at least healthier lives than they would if they were not athletic.
"Exercise, diet and genotype all interact in complicated ways. Despite all our attempts to take our destinies into our own hands, genes seem to play a significant role in determining life span, susceptibility to disease and probably the effects of caloric restriction.