A new theory about the foods that can extend life is taking shape, and it’s sure to be a controversial one. Two studies out this week, one in mice and another primarily in people, suggest that eating relatively little protein and lots of carbohydrates—the opposite of what’s urged by many human diet plans, including the popular Atkins Diet—extends life and fortifies health. The research challenges other common wisdom, too. The authors of both studies believe that calorie restriction, a drastic diet that helps mice and other species live much longer than normal, may work not because it slashes calorie intake, but mostly because it cuts down on protein. They also speculate that the low-protein/high-carbohydrate balance that appears to extend life in the two studies, published in Cell Metabolism, could clarify why slightly plump people live longer on average than skinny ones—something epidemiologists have been hard-pressed to explain. “If these two studies are really correct, what people in general are trying to do” to get and stay thin “might be completely wrong in terms of maintaining health and even longevity,” says Shin-ichiro Imai, a molecular biologist at Washington University in St. Louis who studies aging. The “if” is a big one, however. The interplay between diet and health is extraordinarily complicated, and protein diets in particular have sparked confusion. On the one hand, some researchers have found a correlation between consuming lots of protein and heart disease. On the other, high-protein diets have been linked to improved metabolic profiles, such as lower blood glucose levels. Teasing out the role of protein versus total calories or carbohydrates is no easy task. “There are a whole lot of variables here,” says Cynthia Kenyon, who studies the biology of aging at the University of California, San Francisco.
An Australian group led by nutritional physiologist Stephen Simpson and biogerontologist David Le Couteur at the University of Sydney tried to clear up some of the confusion by assigning 858 mice to one of 25 diets with different mixes of protein, carbs, fats, and fiber. All were allowed to eat as much as they wanted. The mice whose diets included 5% to 15% protein and 40% to 60% carbohydrates lived the longest, up to 150 weeks compared with 100 weeks for those on a diet of about 50% protein. By comparison, Americans on average take in about 16% of their calories from protein. The animals on the low-protein/high-carb plan also had lower blood pressure, better glucose tolerance, and healthier cholesterol. (Levels of fat in their diet didn’t seem to make much difference.) Mice that ate lots of protein were skinnier—just as people on high-protein diets tend to be. But for these mice, slender translated to ill health and earlier death. This finding, Le Couteur says, supports the “concept of healthy obesity,” which has been raised by epidemiology studies of slightly overweight people. He suggests that if their diet is higher in carbohydrates and lower in protein than usual, that might be responsible. The second study, led by gerontology researcher Valter Longo and graduate student Morgan Levine at the University of Southern California in Los Angeles, focused on data from 6381 adults over 50 years old who were interviewed once about their diet as part of NHANES, a national survey of health and nutrition. Longo’s team used death records to conclude that those under 65 whose self-reported diets they classified as high-protein—the participants said at least 20% of their calories came from protein—were at much higher risk of illness and death than a group who took in 10% or fewer of their calories from protein. The high-protein eaters were more than four times as likely to die from cancer over the 18 years after they were surveyed, and 75% more likely to die of any cause.
Those leaping to grab a breadstick instead of sausage should note, however, that as the NHANES cohort aged, protein became more important. In the over-65 crowd, those who ate lots of protein survived longer, on average, than those who ate less. Geriatricians, Longo says, have long extolled the value of protein in older people, and “they’re right.” Longo and Imai speculate that elderly individuals may be less likely to absorb the protein they take in, so need more of it. The findings do fit with some molecular clues. Cutting protein intake is known to reduce levels of a growth factor called IGF-1, and lower IGF-1 levels are linked to longer lifespan and reductions in the risk of cancer and diabetes. Limiting protein intake also reduces levels of a protein called mTOR, and lower mTOR extends life in mice. The Australians saw the mTOR effect in their animals. Longo’s team, testing stored samples from the survey participants, saw that higher IGF-1 levels correlated with more dietary protein. “There’s certainly some truth to this relationship” between protein consumption and lifespan, says Matt Kaeberlein, a molecular biologist at the University of Washington, Seattle, who studies longevity. “But it’s probably overly simplistic to say that everyone should go on a low-protein diet at this point.” Among the many caveats, for example, is that the mouse study used a single strain, though different strains can have different reactions to diets such as calorie restriction. Kaeberlein also thinks it’s unlikely that reduced protein alone explains the dramatic impact of calorie restriction on lifespan. Le Couteur wants to find out for sure: He and his colleagues are planning a study that pits a low-protein/high-carb diet head-to-head with restricted calories, to see which mice live the longest.
March 18, 2014
http://news.sciencemag.org/biology/2014/03/low-protein-diet-may-extend lifespan Original web page at ScienceNow