Beefing about your diet probably won’t lengthen your life, but a new study suggests that cutting down on beef and other protein-laden foods might.

A group of researchers at the Center for Human Nutrition at Washington University in St. Louis, led by Luigi Fontana and John Holloszy, is investigating how various diets affect people. One of the diets is restricted in calories but still provides full nutrition.

Cutting 25 percent or more calories from the diets of rodents, dogs, worms, flies, yeast and other animals has been shown to prolong life. But no one knows whether restricting calories in people will also make them live longer.

Previous studies from the Washington University researchers have shown that members of a group called the Calorie Restriction Society, who have voluntarily followed a calorie restricted diet for years, have vastly improved cardiovascular health than people of the same age who eat 20 percent to 30 percent more calories.

In their latest study, reported in the October Aging Cell, the scientists found that people who eat a high-nutrition, minimal calorie diet don’t get all the benefits from calorie restriction that rodents do. But restricting proteins along with calories seems to mimic the full effect of calorie restriction seen in other animals.

This recent study focused on how diet affected amounts of insulin-like growth factor, or IGF-1. The growth factor stimulates cells to grow and high levels have been linked to cancer. Lowering levels of the growth factor may be a key step in slowing down aging and prolonging life.

One study that placed people on a calorie restricted diet for a year showed no reduction of IGF-1 in the volunteers’ blood compared with people who ate a healthy diet with more calories. So the researchers examined IGF-1 levels in volunteers from the Calorie Restriction Society, who had been practicing caloric restriction for at least six years. Their IGF-1 levels were not lower either. In previous studies, rodents placed on a calorie-restricted diet did show a drop in IGF-1.

“It was a little surprising,” says Andrzej Bartke, an endocrinologist at Southern Illinois University School of Medicine in Springfield. “IGF-1 reduction is kind of a textbook response to caloric restriction, and in this study it didn’t happen.”

Vegans — people who do not eat meat, milk, eggs or other animal products — did have slightly lower levels of IGF-1, even though their diets are higher in calories. At the same time, the vegans did not have all the cardiovascular benefits that people on calorie restriction do.

Fontana and his colleagues realized that the vegans in the study get only about 10 percent of their calories from protein. People following a standard, healthy diet take in about 16 percent of their calories from protein, while people in the calorie restriction group get nearly a quarter of their calories from protein.

Six members of the Calorie Restriction Society agreed to lower their protein consumption to slightly below the recommended daily intake. After three weeks on the lower-protein diet, IGF-1 levels in the volunteers’ blood dropped 25 percent on average. The result suggests that caloric restriction works differently in people than in rodents and that restricting protein consumption, because it lowers IGF-1, is important to achieve maximal health benefits, Fontana says.

“A lot of people underestimate the importance of protein,” Fontana says. “This study says, ‘pay attention, too many proteins can increase your risk of getting cancer, and it can speed up your aging.” The person eating a typical Western diet consumes 30 to 40 percent more protein than the USDA’s recommended daily intake, Fontana says.

This study is part of a growing body of work suggesting that diet composition may be as important as calorie consumption for controlling aging and metabolic health, Bartke says. Some studies suggest that people with low IGF-1 activity stand a better chance of living to be 100, but the importance of IGF-1 to increasing human life spans through caloric restriction is not as clear in humans as in rodents.

Most mice die of cancer, so rodents probably need to lower IGF-1 in order to fend off the disease and live longer. “In humans, the link to longevity is a little more vague because most people don’t die of cancer,” Bartke says. People die more often of heart disease.

Other researchers disagree with the assumption that lowering IGF-1 levels is the key to how calorie restriction works.

“It’s very premature to say that calorie restriction works through IGF-1,” says Arlan Richardson, a biochemist and molecular biologist at the University of Texas Health Science Center at San Antonio. “Everything points to it working through multiple methods.”

“I don’t think that the protein is as important as reducing the amount that you eat,” Richardson says. “The question is how important is IGF in longevity?”

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