LOS ANGELES -- For 75 years, scientists have documented a curious fact: If rats and mice eat 30%-40% fewer calories than usual, they live 15%-40% longer than is typical for their species. The observation has offered humans hope that our own maximum life-span could one day be extended, permitting people to live well past their 100th birthday.
A new study in monkeys pours cold water on that notion -- while at the same time offering some heartening health news.
Among a colony of rhesus monkeys tracked for more than 20 years, animals whose calories were restricted to 30% below normal lived no longer on average than monkeys whose eating was unrestricted, scientists found. But the diet did offer clear benefits, notably in warding off cancer.
It appears that "we are seeing a separation between what we call 'health span' from 'life-span' -- they are not hand in hand," said Rafael de Cabo, an experimental gerontologist at the National Institute on Aging in Baltimore and senior author of the new study.
The report, published online Wednesday by the journal Nature, suggests that what has proved true for rodents and various other animals may not hold true for primates -- humans included -- at least under the conditions that were studied. It likely will disappoint those hundreds of people in the U.S. who practice a strict regimen of calorie restriction in hopes of postponing their appointment with the Grim Reaper.
But the findings also have some researchers scratching their heads. The results are quite different from a 2009 report of monkeys in a colony in Wisconsin that found a clear survival edge from age-related diseases such as diabetes, cancer and heart disease in calorie-restricted animals. That study also saw a trend toward longer life for monkeys on the diet when all causes of death were considered.
Figuring out whether differences in diet, the animals' genetic makeup or something else caused the results to diverge could offer important clues to the way that calorie restriction -- and aging -- work, de Cabo said.
The new study tracked 121 male and female monkeys at the National Institutes of Health's animal center in Maryland starting in 1987. One group of 35 animals -- the "old onset" group -- were 16 to 23 years old when the study began. (Rhesus monkeys mature around the age of 4 or 5 and live to 27, on average, in captivity). The rest were in a group that included juvenile and adolescent monkeys, as well as younger adults up to the age of 14.
All the monkeys received the same food diet, but the control animals could eat as much as they wanted during daylight hours. The rest were limited to only 70% of what they ate before the experiment began.
In the older group, there was no overall difference in ages of death between calorie-restricted and free-eating animals. That was true when all causes of death were considered together, as well as when deaths from age-related illnesses were calculated separately, the authors found.
Among the younger animals, the scientists didn't find any survival edge for dieting animals either. But they did find reduced rates of diabetes and stark differences in the cancer rates.
So far, no monkey in the calorie-restricted group has been diagnosed with cancer, de Cabo said, whereas six cases have occurred in the controls and are believed to have been the cause of death of five of them. That is in line with rodent studies, which have also found that calorie restriction seems to ward off cancer.
There were other differences, too. Calorie-restricted monkeys weighed less and looked younger than the animals that ate freely. They also appeared "younger" in some metabolic respects, with lower blood levels of triglycerides and glucose in the old group and lower cholesterol levels in males of both age classes.
The fact that diseases of old age can be warded off even if life-span isn't necessarily extended suggests "that health and longevity are not the same thing," said Steven Austad, a biogerontologist at the University of Texas Health Science Center at San Antonio.
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