Decades ago, in the 1930s, researchers working with lab rats made an interesting discovery. Animals that had been deprived of food seemed to live longer than rodents that were fed to satisfaction, raising the intriguing idea that maybe near-starvation was a good, rather than bad thing, for health.
Follow up studies, particularly in yeast, confirmed the trend and some forward-thinking scientists even began restricting their caloric intake in the hopes of seeing some extra years. But the latest research conducted on close human cousins, rhesus monkeys, shows that the connection may not be as solid as once hoped.
Published in the journal Nature, the results suggest that dramatically cutting back on daily calories — by 30% — does not help monkeys to live longer than those who ate normally. The restriction did help older monkeys to lower their levels of triglycerides, a risk factor for heart disease, but otherwise conferred no significant health or longevity benefit.
Caloric restriction may have its evolutionary roots as a survival mechanism, allowing species to survive on scraps when food is scarce in order to continue to reproduce. But that restriction only has lasting positive effects if the overall diet is a balanced one, which may not always be the case in conditions of famine. (That also explains why anorexia is so unhealthy: people who starve themselves become malnourished). It’s possible the strategy developed as a way to protect species from consuming toxic plants or foods, when it wasn’t always obvious which sources were verboten.
The study, begun in 1987 and one of the longest running trials to investigate the effects of caloric restriction, contradicts the only prior research conducted with rhesus monkeys, which found the opposite effect, highlighting the complex relationship between caloric processing and metabolic functions that contribute to aging and health.
For example, the study’s lead author, Rafael de Cabo of the National Institute on Aging, notes that the effects of caloric restriction on the immune system may not be all good: some studies show slower wound healing and increased risk for infectious disease. In young animals, restricting calories also reduces fertility.
It’s not entirely clear why the two monkey studies had such varying results. Ricki Colman, a co-author of the first monkey study and an associate scientist at the Wisconsin National Primate Research Center, believes that the differences may reflect variance in the diets given to the animals in the two studies. “They may be modeling different things,” she says, explaining that in her study, the control animals were allowed to eat freely while in the new research, both controls and those on the restricted diet were limited to specific maximum amounts. Her control animals, she says, may reflect more of a typical American diet, while the controls in the new research are more like people who already eat healthy amounts. Colman’s Wisconsin study diet also contained far more sugar— 29% of calories, compared to 4% in the NIA trial. In fact, 40% of control animals in the Wisconsin study developed diabetes, but none of the restricted monkeys did, despite their sugary meals.
“If I told you that eating less would prevent diabetes, I don’t think you’d be surprised,” says Dr. Steven Austad of the University of Texas Health Science Center in San Antonio, who wrote a commentary on the new study for Nature. He’s spoken to groups of people who have already put themselves on caloric restriction, hoping for longer life. Rodents on such diets live to the human equivalent of 120-130. But this research suggests “you’re not going to live to 130,” he concludes.
Indeed, the NIA study implies that the benefits of simply eating a balanced, healthy diet may provide as much life extension as dietary restrictions can produce. The meals received by both groups of animals in the study were carefully balanced for nutrient content and even shifted with the seasons as the monkeys’ natural diet in the wild would.
Even with the findings, however, some experts are still holing out hope that restricting calories may prove beneficial for certain health outcomes, specifically in combating cancer and heart disease. The question, of course, is how much restriction can you get away with in order to still get these benefits — and whether a drug could replicate caloric restriction without the self-discipline needed to enforce it. One such possibility, based on a compound found in red wine called resveratrol, is already being tested but its safety and effectiveness in humans has not yet been demonstrated. Austad himself is studying another drug that has shown life extension benefits in rodents called rapamycin, which is already approved to suppress immune rejection in patients receiving organ transplants.
To understand why the NIA and Wisconsin groups got such different results, they plan to collaborate to fully analyze the data generated by the two trials. “We consider our two studies to be complementary, not competitive,” says Colman, “We have plans to work together to directly compare information from our two studies.” The result, they hope, may be some version of the Fountain of Youth.
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Maia Szalavitz is a health writer at TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.