For a country in which roughly 200 million people are overweight or obese, scientists have discouraging news: Even those who maintain a healthy weight probably should be eating less.

Evidence has been mounting that the practice of caloric restriction — essentially, going on a permanent diet — greatly reduces the risk of age-related diseases and even postpones death. It has been shown to extend the lives of yeast, worms, flies, spiders, fish, mice and rats.

Now, in a study funded by the National Institutes of Health and released today, many of the same benefits have been demonstrated in primates, the best evidence yet that caloric restriction would help people.

The findings, published in the journal Science, tracked rhesus monkeys that were on a reduced-calorie regimen for as long as 20 years. The animals’ risk of dying from cancer, heart disease and diabetes fell by more than two-thirds.

The study comes as some validation to the cadre of Americans who profess to practice caloric restriction in their daily lives. It was also welcomed by scientists who study the biological mechanisms of aging and longevity.

“It adds to the evidence piling up that caloric restriction, independent of thinness, is a healthy way to stay alive and healthy longer,” said Susan Roberts of the Human Nutrition Research Center on Aging at Tufts University, who wasn’t involved in the study. “Less diseases in old age has to be something most everyone wants.”

Dr. Luigi Fontana, a medical professor at Washington University in St. Louis, examined people who have been practicing caloric restriction for an average of 6 1/2 years and found their heart function was equivalent to those of people 16 years younger.

“Mild caloric restriction is beneficial to everybody,” Fontana said.

Although the regimen sounds grueling, it is hardly a starvation diet, experts said.

It typically begins with an assessment to determine how many calories an individual needs to consume to maintain a healthy weight. Then that number is shaved by 10 percent to 30 percent.

People on caloric restriction can eat three meals a day. A typical menu includes cereal with fruit and nuts for breakfast, a big salad for lunch and dinner featuring lean meat and reasonable portion sizes. There’s also room for a couple of snacks and even a small dessert from time to time.

Caloric restriction has produced consistent health benefits for animals.

In the new study, scientists tracked 76 adult rhesus monkeys from the Wisconsin National Primate Research Center starting in 1989. Half the animals were fed a typical diet of lab chow, and the rest got a version with a higher concentration of vitamins and minerals to make up for the 30 percent reduction in chow quantity.

Over the course of the study, the monkeys that ate the regular diet were three times more likely to die of an age-related disease than their counterparts on caloric restriction. Fourteen deaths in the control group were attributable to age-related diseases, compared with five such deaths among the animals that ate 30 percent fewer calories, according to the study.

The rates of cardiovascular disease and pre-cancerous cell growths were twice as high in the control group compared with the reduced-calorie group.

The researchers also noted that while five of the control monkeys became diabetic and 11 were classified as pre-diabetic, all the calorie-restricted animals remained diabetes-free.

In all, the monkeys on caloric restriction “appear to be biologically younger than the normally fed animals,” the researchers wrote in their report.

Scientists aren’t sure why eating less slows the aging process, but theories abound.

There’s evidence from mice that caloric restriction induces the body to activate fewer genes related to inflammation, which many scientists suspect plays a key role in aging.

Another theory holds that starved organisms hunker down in maintenance mode, shutting down activities such as reproduction that wear on the body.

Or perhaps caloric restriction reduces body temperature, thus limiting production of dangerous free radicals that gradually break down the body by damaging tissues and DNA.

“It’s all speculation,” said Dr. Sergei Romashkan of the National Institute on Aging, who is overseeing a clinical trial on caloric restriction in people.