A large long-running study has found that eating a diet rich in animal proteins during middle age makes you four times more likely to die of cancer than a low-protein diet (a mortality risk factor comparable to smoking), 74% more likely to die of any cause within the 20-year study period, and five times more likely to die of diabetes. Even those eating a moderate amount of protein had much higher levels of mortality risk.
However, a high or moderate protein diet is helpful for those over 65, when levels of the growth hormone IGF-I drop off dramatically.
Plant-based proteins didn’t seem to have the same mortality effects as animal proteins.
The 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.
A "high-protein" diet derives at least 20% of calories from protein; a "moderate" protein diet includes 10-19% of calories from protein; a "low-protein" diet includes less than 10% protein.