The causes of death -- cancer, heart disease -- also were the same in underfed and normal monkeys, the study said.
For 25 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 just 120 to 133 pounds. The hope was that if the monkeys lived longer, healthier lives by eating a lot less, then maybe people, their evolutionary cousins, would too.
Some scientists, anticipating such benefits, began severely restricting their own diets.
The results of this major, long-awaited study, which began in 1987, are finally in. But it turns out the skinny monkeys did not live any longer than those kept 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 both 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 any longer.
Rafael de Cabo, lead author of the study published online Wednesday in the journal Nature, said he was surprised and disappointed. Like many other researchers, he had expected an outcome similar to that of a 2009 study from the University of Wisconsin that concluded that caloric restriction did extend monkeys' life spans.
But even that study had a question mark hanging over it. Its authors had disregarded about half of the monkey deaths, 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 Wisconsin study "was not nearly as conclusive as it was made out to be."
But other researchers still think that it does, and one of the authors of the new study, Julie Mattison, said there was still a bit of hope.
The study is continuing until the youngest monkeys are 22. While the data pretty much rule out any notion that the low-calorie diet will increase average lifespans, there still is a chance that the study might find that the diet increases the animals' maximum lifespan, she said.
The idea that a low-calorie diet would extend life originated in the 1930s with lab rats. But it was not until the 1980s that the theory took off. Scientists reported that in a range of species, eating less meant living longer. And, in mice at least, it also meant less cancer.
De Cabo, who says he is overweight, advised people that if they want to try a reduced-calorie diet, they should consult a doctor first. He said that he still believes they would be healthier.
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."