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Could too much protein put you on the path toward an early grave?
For middle-aged people who consume lots of meat, milk and cheese, the answer could be yes, according to a study published Tuesday in the journal Cell Metabolism.
U.S. and Italian researchers tracked thousands of adults during nearly two decades and found that those who ate a diet high in animal proteins during middle age were four times more likely to die of cancer than contemporaries with low-protein diets — a risk factor, if accurate, comparable to smoking. They also were several times more likely to die of diabetes, researchers said.
"The great majority of Americans could reduce their protein intake," said one of the study's co-authors, Valter Longo, a University of Southern California gerontology professor and director of the school's Longevity Institute. "The best change would be to lower the daily intake of all proteins, but especially animal-derived proteins."
That advice comes with a caveat. Even as researchers warned of the health risks of high-protein diets in middle age, they said eating more protein actually could be a smart move for people older than 65. "At older ages, it may be important to avoid a low-protein diet to allow the maintenance of healthy weight and protection from frailty," another co-author, USC gerontology professor Eileen Crimmins, said in a release detailing the findings.
Exactly how much protein belongs in the average diet has been a topic of perpetual debate, one complicated by popular diets such as Atkins and Paleo, which rely heavily on animal-based proteins to help people shed weight. While such diets might well succeed in that short-term goal, Longo said they could be leading to worse health down the road.
Part of the confusion, he argues, is that researchers too often have treated adulthood as a single period of life, rather than closely examining the many ways in which our bodies change as we grow older. In studying data about protein intake during many years, he says the picture becomes clearer: What's good for you at one age might be harmful at another.
Marion Nestle, a nutrition expert and public-health professor at New York University, said the findings raise as many questions as they answer. She said they don't amount to a convincing argument that too much protein consumption in middle age is directly linked to health problems later in life, while more protein in old age is protective.
In short, she said, lifestyle choices beyond protein consumption could have played a role in the longevity of the people surveyed for the study and helped to determine whether they ended with cancer, diabetes or other afflictions.
"I'm also puzzled by the idea that there is a significant difference between the effects of protein from animal and vegetable sources," Nestle said. "Protein is not, and never has been, an issue in American diets, and the data presented in this study do not convince me to think otherwise."
In the study, researchers defined a "high-protein" diet as one in which at least 20 percent of calories came from protein; a "low-protein" diet was defined as less than 10 percent. They found that even moderate amounts of protein consumption among middle-aged people had detrimental effects over time, a result that held true across ethnic, educational and health backgrounds.
The authors also tested the relationship between protein intake and cancer progression in mice, saying that during a two-month experiment there was lower cancer incidence and significantly smaller average tumor size among mice on a low-protein diet.
Longo said many middle-aged Americans, along with an increasing number of people around the world, are eating twice and sometimes three times as much protein as they need, with too much of that coming from animals rather than plant-based foods such as nuts, seeds and legumes.
He said adults in middle age would be better off adhering to the recommendation of several top health agencies to consume about 0.8 grams of protein per kilogram of body weight each day. Translation: For a 150-pound person, that means about the equivalent of the protein in an 8- or 9-ounce piece of meat or several cups of dry beans.
As an example of an ideal approach to protein consumption, Longo pointed to the inhabitants of the small, southern Italian town of Molochio, home to one of the highest rates of centenarians in the world. Their secret: For much of their lives, many villagers maintained a low-protein, plant-based diet. In their older years, many ended up moving in with their children and eating higher-protein diets more common today.
"There is no harm," Longo said, "in eating the way our grandparents used to eat."