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 Tuesday in the journal Cell Metabolism.

U.S. and Italian 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,” 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. While such diets might succeed in a short-term weight-loss goal, Longo said they could be leading to worse health down the road.