Calorie Restriction May Not Extend Life

In a 23-year study, scientists found that significantly cutting calories didn’t extend the lives of rhesus monkeys. The result differs from previous work that linked calorie restriction to longer life in primates.

Calorie restriction research has a long history. In the 1930s, investigators observed that some lab rodents lived up to 40% longer when fed a calorie-restricted diet. Since then, scientists have found that calorie restriction extended the lifespan of yeast, worms, flies and mice. But not all studies agreed. Some even found that certain mice died younger if they were on a calorie-restricted diet.

To look into the question, NIH's National Institute on Aging (NIA) has been conducting a study of monkeys since 1987. Some have been eating a normal diet, while others have been eating a diet with 30% fewer calories but the same nutrients.

Monkeys who ate less didn't seem to live any longer, the scientists reported in Nature on September 13, 2012. This is a surprising contrast with another NIH-funded study published in 2009. That study, conducted at the University of Wisconsin-Madison, found that monkeys on a calorie-restricted diet lived longer than those on a standard diet.

The Wisconsin monkeys who ate the standard diet got diabetes, arthritis, diverticulosis and cardiovascular problems at a younger age than monkeys on the calorie-restricted diet. Although similar patterns appeared in the new NIA study, the differences weren't large enough to prove they weren't due to chance.

The NIA study did observe differences depending on whether calorie restriction began when monkeys were young (under 14 years) or old (over age 16). For instance, those who began calorie restriction when young were less likely to get cancer than monkeys who ate normally. Those who began when old showed improvement in several measures of health, including blood triglyceride and glucose levels. These latter benefits weren't seen, however, in monkeys who started the diet at a young age. The Wisconsin monkeys all started the restricted diet when they were 7 to 14 years old.

One possible explanation for the difference in the 2 studies is the monkeys’ diets. NIA's food had a natural ingredient base, while Wisconsin opted for a purified diet. As a result, the specific protein, oil and carbohydrate mixes all differed. The natural ingredients might also include trace dietary chemicals and minerals that aren't in the purified foods.

Genetics are another difference. The Wisconsin monkeys came only from an Indian colony, while the NIA monkeys were more genetically diverse. The Wisconsin monkeys also ate more than the NIA monkeys and weighed more.

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

“We've learned more by having 2 concurrent and independent studies of calorie restriction in monkeys than would have been possible by just the NIA or Wisconsin study alone,” says Dr. Felipe Sierra, director of NIA’s Division of Aging Biology. “While the 2 studies share many of the same findings, the differences will be particularly important for helping us better understand this aging intervention.”