Caloric Restriction in Primates

At the NIH Animal Center in Poolesville, Maryland, about 75 rhesus and squirrel monkeys are on diets; they eat 30 percent less than they would normally but get all the necessary nutrients. Another 75 monkeys, the control group, are eating as much as they want. The differences between the two groups, as they reach maturity and begin to age, are expected to provide insights into how caloric restriction influences life span.

The monkeys that arrived at the Poolesville laboratory in 1987 have responded to caloric restriction as expected; their maturation, measured by factors such as skeletal development and onset of puberty, has been delayed by about a year or year and a half. This is comparable to the delays in maturation seen in calorically restricted rodents.

As the monkeys grow into young adulthood and beyond, George Ruth and his colleagues at the NIA's Gerontology Research Center in Baltimore, where the project is coordinated, will be monitoring dozens of signs of aging, ranging from immune response to activity level to anti-oxidant levels to fingernail growth. The measurements will be compared with those of the monkeys in the control group and should provide leads to some of the anti-aging mechanisms at work in caloric restriction.

On a practical level, though, most gerontologists don't expect caloric restriction ever to become a widespread means of extending the human life span. What they hope to learn from studies of caloric restriction, once its mechanisms are understood, is how to improve health and prevent or postpone the diseases of advancing age.

Speculation about how caloric restriction works covers a broad field, reflecting the wide range of effects it has in laboratory animals. Because cutting down on calories slows metabolism, and free radicals are by-products of metabolism, caloric restriction may reduce free-radical damage. And because caloric restriction lowers body temperature slightly, cells may sustain less genetic damage and repair it more readily than at normal body temperature. In addition, scientists speculate that caloric restriction preserves the capacity of cells to proliferate, that it moderates the decline in growth hormone, and that it keeps the immune system functioning at youthful levels.

In fact its effects are so pervasive that some scientists postulate the existence of a single, master gene whose expression is influenced by caloric restriction and which in turn modifies all aging processes. Whether or not this proves correct, continued work with caloric restriction is expected to uncover much more about the mechanisms of aging. See also Behavioral Factors and Aging