Decades of research have linked low-calorie diets with extended survival, but a new report finds that rhesus monkeys on strict diets don’t live longer than their counterparts getting a standard diet.
The findings, reported August 29 in Nature, run counter to a 2009 study from the University of Wisconsin–Madison that showed a clear survival advantage in a calorie-restricted group of similar rhesus monkeys. Scientists suspect that differences in the two studies’ designs might explain the discordant findings, leaving the question of longevity still dangling.
Both research groups will need to wait another decade or more before all the monkeys live out their lives. But the authors of the new study, conducted at a National Institute on Aging laboratory in Baltimore, say their data are unlikely to change, since calculations show that the chance of a survival difference arising in the remaining monkeys is exceedingly low.
“I don’t think one study overturns 75 years of research,” says Steven Austad, a bio-gerontologist at the University of Texas Health Science Center in San Antonio, who isn’t part of either study team. But he notes that most previous calorie-restriction studies have been done in short-lived animals. “It’s always been possible that whatever you used to increase their lives might not work the same in long-lived animals.”
The median life span for a rhesus monkey in captivity is about 27 years, but some can reach age 40, says study coauthor Julie Mattison, a physiologist at the aging institute. The NIA study includes 121 monkeys, divided between calorie-restricted and standard-diet groups. Some monkeys were put on one diet or the other when they were already well into middle age. Others started on the regimen earlier in life, some very young.
Now, 23 years into the study, more than half of the monkeys have died and no survival advantage shows up from calorie restriction, the scientists report. The team considered only aging-related deaths attributable to causes such as heart disease, diabetes and cancer.
The calorie-restricted monkeys in the NIA study get about 25 to 30 percent fewer calories per day than normal, Mattison says. But she acknowledges that the control diet might also slightly underfeed the monkeys — by perhaps 5 to 10 percent fewer calories. “It may very well be that slight calorie restriction in the control animals, plus a nutritionally balanced diet,” limits the survival difference between the groups, Mattison says.
The Wisconsin study control monkeys get more food than the NIA controls — a standard diet plus 20 extra grams of chow per day to eat as they wish. The NIA monkeys, on calorie restriction or not, get a variety of foods, whereas the Wisconsin monkeys all get standardized pellets. While both approaches provide essential nutrients and the same percentages of carbohydrates in the diet, the NIA monkeys get less sugar than the Wisconsin animals do.
“We don’t know what that difference really means yet,” says Ricki Colman, a biological anthropologist at the University of Wisconsin. “Those are the kinds of things we will be looking at when we go through our data.” Austad observes that the NIA control monkeys weighed less and had less diabetes than the Wisconsin controls.
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Looking beyond longevity, the value of calorie restriction — in any species — might lie in overall health, not just racking up years, Colman says.
Mattison agrees, noting that while the new study didn’t show a survival advantage from fewer calories, it did offer hints of a better life. None of the monkeys that were started on calorie restriction early on has developed cancer, she notes, compared with six monkeys in the standard-diet group.
But calorie restriction very early in life may come with risk. Some monkeys started very early on calorie restriction developed diabetes, even though they are far from obese, she says, and some show signs that their immune systems might not be as good as those in the control animals.
“Science thrives when people get a result they don’t expect,” Austad says. “This study is going to provoke a lot of useful discussion.”