"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," 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 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 lead 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 study published Tuesday, 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.
Longo said many middle-aged Americans, along with an increasing number of people worldwide, 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 beans.
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.
Ideally, Longo said, Americans would start following the example of the inhabitants in the small, southern Italian town of Molochilo, home to one of the highest rates of centenarians in the world. Their secret: For much of their lives, a large number of 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. Whether on purpose or by happenstance, they seem to have hit on the ideal recipe for a long life.
"There is no harm in eating the way our grandparents used to eat," Longo said.