Valter Longo, the Edna M. Jones Professor of Biogerontology at the University of Southern California, Davis, School of Gerontology and director of the USC Longevity Institute, said a diet high in meat and diary products has a mortality risk factor comparable to smoking.
"There's a misconception that because we all eat, understanding nutrition is simple. But the question is not whether a certain diet allows you to do well for three days, but can it help you survive to be 100?" Longo said in a statement.
Not only is excessive protein consumption linked to a dramatic rise in cancer mortality, but middle-aged people who eat lots of proteins from animal sources -- including meat, milk and cheese -- are also more susceptible to early death in general, Longo said.
The study, published in Cell Metabolism, found protein-lovers were 74 percent more likely to die of any cause within the study period than their more low-protein counterparts. They were also several times more likely to die of diabetes.
How much protein one should eat has long been a controversial topic, Longo said, but rather than look at adulthood as one monolithic phase of life, as other researchers have done, this study considered biological changes as people age. In other words, what's good for you at one age may be damaging at another, he said.
Protein controls the growth hormone IGF-I, which helps bodies grow but has been linked to cancer susceptibility. Levels of IGF-I drop off dramatically after age 65, leading to potential frailty and muscle loss.
The study showed while high-protein intake during middle age is harmful, it is protective for older adults age 65 and older.
The researchers found plant-based proteins, such as beans, did not seem to have the same mortality effects as animal proteins.
"The majority of Americans are eating about twice as much proteins as they should, and it seems that the best change would be to lower the daily intake of all proteins but especially animal-derived proteins," Longo said. "But don't get extreme in cutting out protein; you can go from protected to malnourished very quickly."
Longo's findings support recommendations from several leading health agencies to consume about 0.8 grams of protein per kilogram of body weight every day in middle age. For example, a 130-pound person should eat about 45 to 50 grams -- about 2 ounces -- of protein a day, with preference for those derived from plants such as legumes, Longo explained.