WASHINGTON — Eat less, live longer? It seems to work for monkeys: A 20-year study found cutting calories by almost a third slowed their aging and fended off death.

This is not about a quick diet to shed a few pounds. Scientists have long known they could increase the lifespan of mice and more primitive creatures — worms, flies — with deep, long-term cuts from normal consumption.

Now comes the first evidence that such reductions delay the diseases of aging in primates, too — rhesus monkeys living at the Wisconsin National Primate Center. Researchers reported their study today in the journal Science.

What about those other primates, humans? Nobody knows yet if people in a world better known for pigging out could stand the deprivation long enough to make a difference, much less how it would affect our more complex bodies. Still, small attempts to tell are under way.

"What we would really like is not so much that people should live longer but that people should live healthier," said Dr. David Finkelstein of the National Institute on Aging. The Wisconsin monkeys seemed to do both.

"The fact that there's less disease in these animals is striking," Finkelstein said.

The tantalizing possibilities of caloric restriction date back to rodent studies in the 1930s. But it's a hot topic today among researchers trying to understand the different processes that make our bodies break down with age. The hope is that some of those processes could be delayed or reversed.

Captive rhesus monkeys have an average lifespan of 27 years, so spotting an effect takes a lot longer than in short-lived mice. The newest study involves 76 monkeys — 30 tracked since 1989 and 46 since 1994. They were normal-sized adults eating a normal diet for a captive monkey, a special vitamin-enriched chow plus some fruit treats.

Then researchers at the University of Wisconsin-Madison assigned half the monkeys to the reduced-calorie diet, cutting their daily intake by 30 percent but ensuring what they did eat was properly nourishing.

So far, 37 percent of the monkeys who kept their regular diet have died of age-related diseases compared with just 13 percent of the calorie-cut monkeys, a nearly threefold difference, the researchers reported. A handful of other monkeys died of unrelated conditions, such as injury, not deemed affected by nutrition.

Death wasn't the only change. The calorie-cut monkeys had less than half the incidence of cancerous tumors or heart disease of the monkeys who ate normally. Brain scans showed less age-related shrinkage in the dieting monkeys. Those animals also retained more muscle, something else that tends to waste with age.