DAVID GREENE, HOST:

Scientists have known it for a long time, for decades. Lab rats and mice will live far longer than normal if they are fed a super-low-calorie diet. Well, that has led some humans to eat a near-starvation diet in the hope that it will extend the life spans as well. But as NPR's Nell Greenfieldboyce reports, a new study in monkeys suggests this is not so simple.

NELL GREENFIELDBOYCE, BYLINE: Rats and mice normally only live a few years. So even though caloric restriction can lengthen their lives by 30 percent or more, it's still relatively easy to study this effect. It's a lot harder to do that in a long-lived animal - for example, humans.

RAFAEL DE CABO: It we were to try to implement this in humans, we're talking about a study that is going to be, you know, five or six generations of scientists trying to get an answer.

GREENFIELDBOYCE: Rafael de Cabo is with the National Institute on Aging in Maryland. And he belongs to the second generation of scientists working on a more manageable but still long term study in monkeys.

CABO: Doing this study in non-human primate, in this particular case in Rhesus monkeys, is probably the closest species that we can get the study done close to humans.

GREENFIELDBOYCE: This study started 25 years ago, in 1987. It involves about 120 monkeys. Some were assigned a regular diet, while others got 30 percent fewer calories than normal. And if you walk into the lab, it's obvious who is eating the low-calorie meals.

CABO: 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.

GREENFIELDBOYCE: But what doesn't look different is their life span. That's according to a new report in the journal Nature. Now that enough animals in each group have died, researchers have been able to do a comparison. It turns out that even though the low-calorie group seems to enjoy better health, they didn't live longer.

The result seems to dash some hopes that were raised a few years ago, when a similar long-term study in monkeys, done in Wisconsin, saw hints that eating less did lead to longer lives.

STEVEN AUSTAD: Here we have these two studies that reach, you know, broadly different results that differed in relatively minor ways.

GREENFIELDBOYCE: Steven Austad studies aging at the University of Texas Health Science Center at San Antonio. He points out that the animals in the Wisconsin study did eat a different diet. For example, it had far more sugar. He says researchers will try to tease out the significance of these differences. But overall from these two studies he draws this conclusion.

AUSTAD: If dietary restriction increases longevity in monkeys at all, it only does so under very specific conditions. It's not very robust.

GREENFIELDBOYCE: 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 of us couldn't face that kind of deprivation. And Austad says that's fine.

AUSTAD: Don't feel so bad that you can't get yourself to this phenomenally lean, you might say emaciated, body state, because there's not any evidence that that's really going to help you live a lot longer anyway.

GREENFIELDBOYCE: 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.

Nell Greenfieldboyce, NPR News.

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