Restricted-Calorie Diet May Not Lead to Longevity

Dieting monkeys don't always live longer, according to a new Nature study.

August 29, 2012, 2:09 PM

Aug. 29, 2012 — -- To Merrill Averill and Paul McGothin, two 60-something marketing executives from Ossining, N.Y., a rumbling tummy equates to the fountain of youth. They practice an extreme calorie restriction because they believe that eating less is the secret to living longer. Now a new study published in the online version of the journal Nature casts doubt on that idea.

In the late 1980s, scientists set out to test the theory that dietary restriction could extend the life span of long-lived primates, as decades of studies had found it did in mice and other lower organisms. If true, this would strongly imply that the same assumptions could be made about humans.

Two independent teams -- one at the U.S. National Institute on Aging in Bethesda, Md., and the other at the Wisconsin National Primate Research Center in Madison, Wis., each placed rhesus monkeys on diets that contained 30 percent fewer calories than normal and have periodically provided updates on the health and longevity of the animals.

As the latest Nature dispatch found, the NIA monkeys fed a calorie-restricted diet didn't live any longer than monkeys on a higher-calorie diet. No matter what they ate, maximum lifespan seems to hover around 40 years of age. Half the monkeys that began the study as youngsters were still alive, but the researchers say, based on survival patterns, they predict the remaining calorie-restrictors and controls will all live to be about the same age.

Monkeys that started the diet in their youth did show a trend toward a delay in the onset of age-associated disease. Interestingly, the strict diet appeared to decrease the risk of cancer and possibly diabetes but slightly increased the risk of cardiovascular disease.

"It is likely that calorie restriction alters cellular pathways that contribute to cancer differently than it does those pathways leading to metabolic dysfunction," said Dr. Julie Mattison, an author of the paper. "Given the experimental design, it is possible that pathways leading to cancer are impacted earlier or to a greater extent than others."

And the dieting monkeys also enjoyed improved health. For example, eating a restricted diet made them slimmer than those in the control group, and if they began the diet in middle age (16-23 years old for monkeys) they had lower blood fat and blood sugar levels compared to the nondieters. Male dieters of all ages had lower cholesterol levels than the controls.

Conflicting Results

These latest findings are at odds with the WNPRC study in which calorie-restricted monkeys have far outlived the controls. Floyd Chilton, a professor of physiology and pharmacology at Wake Forest University Medical Center, said the study design might account for some of the disparities.

For one thing, the Wisconsin monkeys subsisted on a diet that shared many of the same unhealthy aspects of a typical Western diet, such as a high amount of sugar, whereas the NIA primates were fed a much healthier diet and were also given vitamin supplements.

"The NIA monkeys were already eating so healthy to begin with, the calorie reduction may not have provided much more of a health advantage," Chilton said.

Mattison said this could be a limitation. "Certainly quality of the food and the nutrient composition/ratios could factor into the equation. Because calorie restriction is causing a metabolic stress, it is reasonable to speculate that a nutritionally complete and balanced diet would be better for the organism, regardless of the quantity," she said.

Another difference: The NIA monkeys were given two meals a day on a schedule while the Wisconsin monkeys ate whenever they pleased. Both groups were also genetically quite diverse; since each study included a relatively small number of individuals – 70 divided between calorie restriction and control groups – the genetic variations might have further skewed results.

Making Sense

The caloric restriction theory has been around since the 1930s, when Cornell researchers found it extended the lives of lab mice by about 40 percent. Since then, restricted eating has been found to increase the lifespan in a variety of species from worms to dogs. Researchers assume it works because being in a state of slight hunger acts as a mild but persistent stressor that somehow makes an organism more resistant to the effects of aging. Taking in fewer calories also slows metabolism, which may in turn slow down the aging process.

Whatever the mechanisms may be, Chilton says the two primate studies are heroic and should be respected for being the first long-term investigations to provide clues about how humans might respond to eating a sparse diet. Nevertheless, the debate will certainly continue.

"We didn't see the miracle change in longevity we've seen in rodents and other primitive animals, but questions about the effects of calorie restriction on aging are far from answered," he said.

As for Averill and Paul McGothin, they say these recent findings don't shake their faith in calorie restriction in the least. They plan on continuing with the diet and spreading the gospel through their organization, CR Society International.

"At my recent physical exam my doctor told me I am in remarkable shape," McGothin says. "That's all the proof I need."