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 a resounding yes, according to a new study published in the journal Cell Metabolism.
Researchers tracked thousands of adults for 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 comparable to smoking. They also were several times more likely to die of diabetes, and nearly twice as likely to die in general.
"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 over 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," said another co-author, USC gerontology professor Eileen Crimmins.
Exactly how much protein belongs in the average diet has proven 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 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 over many years, he says the picture becomes clearer: What's good for you at one age might be harmful at another.
In the new 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.
Longo said many middle-aged Americans are eating 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 beans. He said adults in middle age would be better off adhering to the recommendation to consume about 0.8 grams of protein per kilogram of body weight each day — roughly 55 grams for a 150-pound person, or the equivalent of an 8-ounce piece of meat or several cups of dry beans.