Scientists have known for decades that lab rats and mice will live far longer than normal if they're fed a super-low-calorie diet, and that's led some people to eat a near-starvation diet in the hopes that it will extend the human life span, too.
But a new study in monkeys suggests they may be disappointed.
The long-awaited results of this study, which started back in 1987, show that rhesus monkeys fed a diet with 30 percent fewer calories than normal did not live unusually long lives.
The monkey study is about as close as it's possible to get to knowing how caloric restriction might affect the life spans of people, says Rafael de Cabo of the National Institute on Aging in Baltimore, noting that humans are so long-lived that a long-term study wouldn't be practical.
He explains that the study involved about 120 monkeys. Some were assigned a regular diet, while the others got 30 percent fewer calories than normal. And if you walk into the lab, he says, it's obvious who is eating the low-calorie meals: "The males are about 25 percent smaller than the control males, so it is an obvious difference in terms of body weights and in terms of overall size of the monkeys."
But what doesn't look different is their life spans. Now that enough animals in each group have died, researchers have been able to do a comparison, which they reported in the journal Nature. It turns out that even though the low-calorie group seemed to enjoy better health, they didn't live longer.
The result seems to dash some hopes that were raised in 2009, when a similar long-term study in monkeys, done in Wisconsin, saw hints that eating less did lead to longer lives.
"Here we have these two studies that reach, you know, broadly different results that differed in relatively minor ways," says Steven Austad, who studies aging at the University of Texas Health Science Center at San Antonio.
The monkeys in the Wisconsin study did eat a different diet — for example, their food had far more sugar. And the Wisconsin animals that weren't on the restricted diet were allowed to munch as much as they wanted — instead of having food doled out in regular meals, to prevent obesity.
Researchers will try to tease out the significance of these differences. But overall, from these two studies, Austad says one thing is clear: "If dietary restriction increases longevity in monkeys at all, it only does so under very specific conditions. It's not very robust."
There are some people following near-starvation diets in an effort to mimic the dramatic results seen in rodents, but not that many, because most people couldn't face that kind of dietary deprivation.
"Don't feel so bad that you can't get yourself to this phenomenally lean, you might say emaciated, body state," says Austad, "because there's not any evidence that that's really going to help you live a lot longer anyway."
But he says there's still no doubt that exercising and avoiding being overweight or obese will keep you healthier for your normal life span.