"The majority of Americans are eating about twice as much protein 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," at least in middle age, study senior author Dr. Valter Longo, of the University of Southern California, said in the university news release. "But don't get extreme in cutting out protein; you can go from protected to malnourished very quickly."
A second study, this time conducted in mice, found that a high-protein, low-carbohydrate diet shortened the rodents' lifespan.
Overall, the findings provide "convincing evidence that a high-protein diet -- particularly if the proteins are derived from animals -- is nearly as bad as smoking for your health," Longo said in a journal news release. He was the senior author of the study conducted in humans.
The effects of protein consumption on a person's risk of an early death may be partly due to the activation of natural growth hormone and a cellular growth factor called IGF-1, the researchers explained.
"Notably, the activity of these factors, but also body weight, declines naturally with aging, which may explain why older people not only did not benefit but appeared to do worse if they ate a low-protein diet," Longo said.
The researchers believe the studies will boost understanding about links between diet and health.
"We have shown explicitly why it is that calories aren't all the same -- we need to look at where the calories come from and how they interact," Steve Simpson, senior author of the mouse study, said in the journal news release.
"This research has enormous implications for how much food we eat, our body fat, our heart and metabolic health, and ultimately the duration of our lives," said Simpson, a researcher at the University of Sydney in Australia.