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.