But Dr. Ravussin and Dr. Hadley cautioned that the study was preliminary, and that it did not prove that calorie restriction could make people healthier or add years to their lives.

"It's an important step along the way," Dr. Hadley said.

Scientists have known for years that when people cut calories and lose weight, the body tries to compensate by slowing its metabolic rate. The slowing is a defensive mechanism to fight weight loss. It was probably preserved by evolution because it saved people from starving to death when food was scarce, but it is the bane of dieters because it means that the more weight they lose, the harder it is to keep reducing.

Several explanations exist for why a strict diet, low in calories but high in nutrients, may slow aging. Many scientists think that an important factor in aging is DNA damage caused by free radicals, highly reactive oxygen-containing molecules formed during normal metabolism. Eating less leads to a slower metabolism and fewer free radicals.

Another possibility is that being deficient in calories turns certain genes on and off, with a net effect of slowing the clock.

In rhesus monkeys, calorie restriction has had pronounced effects. A University of Wisconsin team led by Richard Weindruch has been studying 76 monkeys for more than a decade, half on low-calorie diets and half in a control group that eats normally.

The low-calorie animals weigh about 30 percent less and have 70 percent less body fat than the controls, as well as lower insulin levels. The calorie-restricted monkeys have had two cases of cancer, compared with five in the controls. The controls have had twice the death rate from aging-related diseases like heart failure and diabetes. About 90 percent of the monkeys on low-calorie diets are still alive, compared with only about 70 percent of the controls.

Dr. Ravussin's study included men and women, ages 27 to 49, who were overweight but not obese. Some were just a bit heavy, but others were 30 pounds overweight.