The secret to a disease-free long life may be simple, a new study says: Eat a third less food.

But for most people that is no easy task, so scientists have turned to monkeys to learn what a severely restricted diet can do.

The answer, from a 20-year study at UW-Madison: Monkeys that eat 30 percent fewer calories than normal are three times less likely to develop or die from age-related diseases at any given time than other monkeys.

"This is the strongest evidence to date that caloric restriction can delay disease and improvesurvival," said Richard Weindruch, a professor of medicine at the UW School of Medicine and Public Health and leader of the study.

The findings, published in Thursday's edition of the journal Science, are likely to spur further interest in dietary efforts to slow the biological clock. Other studies in mice, worms and flies have had similar results. But this is the first one in primates.

Weindruch plans to help leaders of a study of people on caloric restriction analyze the extreme dieters' disease risks. Research he has done in mice suggests a more palatable way of prolonging a healthy life: consuming large doses of resveratrol, a chemical in red wine, also available in pills.

Smaller rations, less fat For now, the rhesus monkeys aging gracefully at UW-Madison's Wisconsin National Primate Research Center offer the most detailed proof of the power of caloric restriction in primates.

The study, which started in 1989 with 30 monkeys, expanded five years later to 76 monkeys. Half have been given small portions of food pellets - an average of about 475 calories a day - and half a normal ration, about 640 calories. The smaller meals were enriched with vitamins and minerals to be nutritionally equivalent to the normal meals.

The average life span for rhesus monkeys in captivity is 27 years. Most of the monkeys in the study, which each animal began as an adult, are approaching that age.

As of February 2008, the end of the study period being reported, half as many monkeys on caloric restriction developed heart disease and cancer as the other monkeys.

None of the dieters had diabetes, while 42 percent of the other monkeys got diabetes or signs of the disease. The brains of the low-cal monkeys didn't shrink as much in areas responsible for motor control and executive functions.

Age-related diseases killed 14 of the 38 monkeys on normal diets - but just five of the 38 monkeys on caloric restriction.

Additional monkeys in both groups died for other reasons, such as stomach bloat and injuries. No monkeys were euthanized. Today, 13 monkeys on normal diets remain, outnumbered by 20 monkeys on restricted diets.

Caloric restriction is thought to alter metabolism and keep fat cells from dumping inflammatory molecules into the blood, Weindruch said.

The monkeys on restricted diets weigh 30 percent less and their bodies have about 70 percent less fat. Their faces seem more taut and their coats more plush, said Ricki Colman, a primate center researcher and another author of the study.

The monkeys move around in their cages more easily and grasp treats more quickly, Colman said. And when it's time for their morning feedings, they really get moving, she said.

"They know exactly what the cart sounds like," she said.

'Hunger is your friend'

Whether the monkeys' restricted diets extend their longevity - the next big scientific question - won't be known for about 10 years, Weindruch said.

The study he plans to work on of people on caloric restriction, led by Washington University in St. Louis, includes strong-willed souls such as Tadd Ottman, of Hayward, Calif., near San Francisco. No study participants are from Wisconsin.

Ottman, a 54-year-old software engineer, started eating 1,500 calories a day, instead of the recommended 2,200 calories for a typical sedentary man, six years ago after learning that his cholesterol level was high.

His total cholesterol has dropped from 244 mg/dl to 180 mg/dl, now within the ideal range. At 5 foot 10, he weighs 131 pounds, down from 182 pounds before.

He eats an oatmeal mixture for breakfast and a rice-bean-vegetable medley, sometimes with bits of fish or chicken, for lunch and dinner. He carefully rations "budget blowers," like half a slice of bread, a sip of wine or a spoonful of ice cream.

"Mild hunger is your friend," said Ottman, who reports a high energy level but admits he has a diminished libido and cold hands and feet.

Weindruch said he has tried caloric restriction but couldn't stomach it.

"It's a very difficult thing to do, even for somebody who has been studying it for 34 years," he said.