New research suggests consuming moderate to high levels of protein could increase one's risk of cancer and early mortality.
A second study found consuming a high-protein, low-carbohydrate diet led to a shorter life span in mice, a Cell Press news release reported.
"We studied simple organisms, mice, and humans and 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," the University of Southern California's Doctor Valter Longo said in the news release.
The researchers looked at 6,831 middle-aged and older adults participating in the NHANES III survey. The team found over 50s who consumed a high-protein diet (about 40 percent of calories coming from protein) were four times as likely die cancer or diabetes and twice as likely to die from other causes over the subsequent 18 years. A moderate-protein diet was linked to a three times greater chance of cancer mortality. Plant-based proteins did not increase mortality risk.
Once the subjects reached the age of 65 this conclusion was reversed; the team found those who ate a high protein diet had a 28% reduced risk of dying from various causes and a 60 percent reduced risk of cancer mortality.
These mortality risks could be a result of growth hormones and the growth factor IGF-1 used in the meat industry.
"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 cancer risk could also be linked to the fact that aging reduces the body's ability to absorb proteins. Certain amino acids that make up proteins can damage DNA and eliminate or reduce cellular protection.
"The advice we are always given is to eat a healthy balanced diet, but what does that mean? We have some idea, but in relation to nutritional composition we don't know terribly well," co-author Prof. David Le Couteur of the University of Sydney's Charles Perkins Centre in Australia said in the news release. "This research represents an important step in finding out."
