National Institute on Aging
A pair of 27-year-old monkeys are part of a national study that failed to find a longevity benefit to calorie restrictions. The one on the left consumed 30 percent fewer calories than the monkey on the right.

SALT LAKE CITY — Restricting calories may extend the life of rodents, but it doesn't seem to increase longevity for monkeys, according to surprise findings in a much-anticipated study published this week by the journal Nature. Monkeys do, however, get some health benefits from consuming fewer calories.

Many earlier studies suggested that restricting calories adds years to life in lab-bred rodents. The findings were so compelling, in fact, that many scientists and others have curbed their own caloric intake in anticipation of reaping extra years. Caloric restriction decreases calories by as much as 40 percent, but maintains the nutrients found in a standard diet.

Drug manufacturers have sought drugs that would "mimic the salutary effects of a skimpy diet without triggering severe hunger pangs," according to an article in the Wall Street Journal.

The new research, collected over 30 years (because that's the outer lifespan of monkeys), casts doubt on whether the benefits found for rodents would extend to people, since monkeys and humans are primates and more similar genetically.

"One thing that's becoming clear is that calorie restriction is not a Holy Grail for extending the lifespan of everything that walks on earth," Rafael de Cabo, an experimental gerontologist in the U.S. National Institute on Aging and lead study author, told WSJ.

To test the calorie restriction theory, the NIA scientists restricted by 30 percent the diets fed two sets of monkeys. The first group included monkeys ages 1 to 14, while the other group was made up of older monkeys, 16 to 23. They were compared to similar groups with more normal diets.

The male animals on calorie restriction had lower cholesterol; the females didn't. Cancer incidence appeared to fall with caloric restriction, but heart disease increased slightly. Age-related diseases, however, appeared a bit later in calorie-restricted animals.

A 20-year study begun shortly after the NIA study and reported in 2009 by the University of Wisconsin found that monkeys lived longer if deaths from non-aging related causes weren't counted. But if they were included, no longevity benefit existed.

"Caloric restriction research has a long history," the NIA reported in a release announcing the newer study results. "The first finding came in the 1930s, when investigators observed laboratory rats and mice lived up to 40 percent longer when fed a calorie-restricted diet. Subsequent research has cited calorie restriction as extending lifespan of yeast, worms, flies and some strains of mice. But other studies have not shown a longevity benefit."

It said some strains of mice not only saw no benefit, but had a shorter lifespan when fed fewer calories. "To date, research does not provide evidence that calorie restriction is an appropriate age regulator in humans," NIA said, noting that limited human studies are now taking place.

"People may respond differently to calorie restriction depending on their genetic makeup and the composition of their diets," the Journal article speculated. "Outcomes will also vary depending on whether people are overweight or already lean when they adopt the practice. When done properly, calorie restriction appears to provide some health benefits for people, such as lowering the risk of heart disease."

"These results suggest the complexity of how calorie restriction may work in the body," said NIA director Dr. Richard J. Hodes. "Calorie restriction's effects likely depend on a variety of factors, including environment, nutritional components and genetics."

Both studies raised questions and had faults, Luigi Fontana, a gerontologist at Washington University, told Wired. He noted that the stress of being kept in isolated cages, as monkeys in both studies were, could create enough stress to alter the results.

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“If you’re in a single cage for your whole life, and are a highly intelligent animal like a primate, deprived of contact with other peers, and on top of that you’re calorically restricted — can you imagine the psychological depression issues that will ensue?” Fontana told Wired. “And we know the hypothalamus in the brain is a major regulator of many downstream metabolic factors.”

He also noted that not all calories are the same.

Experts agree that maintaining a healthy weight and exercising do provide health benefits.

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