For more than 20 years, the rhesus monkeys were kept semistarved, lean and hungry. The males' weights were so low they were the equivalent of a 6-foot-tall man who tipped the scales at 120 to 133 pounds. The hope was that if the monkeys lived longer, healthier lives by eating a lot less, maybe people, their evolutionary cousins, would too.

Some scientists, anticipating such benefits, began severely restricting their diets.

The results of this major, long-awaited study, which began in 1987, are in. But the study did not bring the vindication calorie-restriction enthusiasts had anticipated. It turns out the skinny monkeys did not live longer than those at more normal weights. Some lab-test results improved, but only in monkeys that were put on the diet when they were old. The causes of death — cancer, heart disease — were the same in the underfed and the normally fed monkeys.

Lab-test results showed lower levels of cholesterol and blood sugar in the male monkeys that started eating 30 percent fewer calories in old age, but not in the females. Males and females that started dieting when they were old had lower levels of triglycerides, which are linked to heart-disease risk.

Monkeys put on the diet when they were young or middle-aged did not get the same benefits, although they had less cancer. But the bottom line was that the monkeys that ate less did not live longer than those that ate normally.

Rafael de Cabo, lead author of the diet study, published online Wednesday in the journal Nature, said he was surprised and disappointed that the underfed monkeys did not live longer. Like many other researchers on aging, he had expected an outcome similar to that of a 2009 study from the University of Wisconsin that concluded caloric restriction did extend monkeys' life spans.

But that study had a question mark hanging over it. Its authors had disregarded about half of the deaths among the monkeys they studied, saying they were not related to aging. If they had included all of the deaths, there was no extension of life span in the Wisconsin study, either.

"This shows the importance of replication in science," said Steven Austad, interim director of the Barshop Institute for Longevity and Aging Studies at the University of Texas Health Science Center in San Antonio. Austad, who was not involved with either study, said that the University of Wisconsin study "was not nearly as conclusive as it was made out to be" and that the new study casts further doubt on the belief that caloric restriction extends life.

Some researchers still think that it does, and one of the authors of the new study, Julie Mattison, said there was still hope. The study is continuing until the youngest monkeys are 22 years old. While the data likely rule out any notion that the low-calorie diet will increase average life spans, there is a chance the study might find that the diet increases the animals' maximum life span, she said.

The diet did offer clear benefits, de Cabo said, adding: It appears that "we are seeing a separation between what we call 'health span' from 'life span' — they are not hand in hand."

The National Institute on Aging study involved 121 monkeys, 49 of which are still alive, housed at a facility in Poolesville, Md. Those that got the low-calorie diet did not act famished, de Cabo said.

Some scientists still have faith in the low-calorie diets. Richard Weindruch, a director of the Wisconsin study, said he was "a hopeless caloric-restriction romantic," but added that he was not very good at restricting his own calories. He said he might start trying harder, though.

"I'm only 62," he said. "It isn't too late."

Material from the Los Angeles Times is included in this report.