Reporting in Science on interim results from a study that began 20 years ago, University of Wisconsin scientists have concluded that rhesus monkeys placed on a calorie-restricted diet live longer than controls receiving a normal diet.

In the study by Ricki Colman and Richard Weindruch, monkeys between the ages of 6 and 14 were randomized to receive a normal diet or one that contained adequate nutritional ingredients but 30% fewer calories.

The svelte monkeys, it turned out, had significantly less risk of developing diabetes, cancer and cardiac disease. And their mortality rate from conditions usually associated with senescence was 13% compared with 37% for those on the normal diet.

“Caloric restriction slows aging in a primate species,” declared the scientists. The results make “it more likely it would apply to humans,” added Weindruch in an interview with the New York Times.

Weindruch estimated the dieting monkeys would live 10-20% longer based on extrapolations from  studies in mice.

Many scientists criticized the study, since a lot of dieting monkeys died of conditions not usually associated with aging (such as endometriosis, complications from anesthesia or gastric bloat). If these deaths were included, the mortality difference between the groups was not significant. 

“The results seem pretty inconclusive at this point,” said Steven Austad, an expert on aging at the University of Texas. “I don’t know why they didn’t wait longer to publish.”

Weindruch wasn’t looking back, however. After all, his monkeys didn’t begin calorie restriction until they were young adults. In mice, even more striking extensions of life occur when dieting begins at birth.

It’s another matter of course, whether rhesus monkeys, if given the option, would choose to adhere to such a diet forever.