Study: Low-calorie diets don't necessarily make you live longer, if you're a monkey

Reassuring news if you're a monkey with an appetite: New research suggests that those extra bananas won't necessarily shorten your lifespan.

A previous study had found otherwise. Research begun in 1989 at the Wisconsin National Primate Research Center in Madison, Wisc., found that rhesus macaques placed on a diet with 30 percent fewer calories than is considered normal (for a monkey) tended to escape age-related monkey illnesses. Those investigators found that 13 percent of the perpetually hungry monkeys died from age-related causes, compared with 37 percent of the well-fed control group.

The results of the Wisconsin study, which was published in 2009, are consonant with earlier research, dating from the 1930s, indicating that other animals, from mice to roundworms, placed on calorie-restricted diets tended to be less susceptible to aging. 

Among human primates, these studies have spurred a cottage industry of 'longevity diets'. There has never been a long-term clinical study of gastronomically mingy humans, so we don't know whether or not it actually extends human life.  

And now new data published in Nature this week complicates the picture even more. A 25-year study of rhesus monkeys at the National Institute on Aging in Bethesda, Md., found no increase in the lifespans of the monkeys that were placed on restricted diets.

There are big uncertainties with both studies. An article on Nature's website notes that the monkey meal plans for both groups in the Wisconsin study were less healthy than those in the Maryland study, and that the monkeys in the control group in the Wisconsin study were allowed to eat as much as they wanted, which, as anyone who lives within waddling distance of an all-you-can-eat buffet, is not exactly a recipe for healthy eating.

Furthermore, as Wired's Brandon Keim writes, monkeys in both studies were kept in isolated cages, which can be psychologically devastating for such intelligent, social animals, with profound implications for the animals' physical well-being.  

“It’s not that one group did right and the other group wrong," University of Texas gerontologist Steven Austad told Wired. "They all did a fine job with the experiments. This just shows that fine details matter,” said Austad. “People shouldn’t say, ‘Let’s drop [calorie restriction].’ They should say, ‘Let’s figure it out.’”

So as things stand right now, there's no wrong way to eat, if you're a rhesus. 

What does this mean for humans? At this point nobody knows whether or not a calorie-restricted diet tends to extend the human lifespan. But it's safe to say that being hungry all the time will certainly make your life feel longer.