Eating a diet rich in animal proteins during middle age can make you four times more likely to die of cancer - almost as deadly as smoking, a new study has warned.
Researchers from the University of Southern California studied 6,318 adults over the age of 50 and found that protein-lovers were 74 per cent 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, researchers said.
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, researchers said.
This is the first study to show a definitive correlation between high protein consumption and mortality risk.
The study shows that while high protein intake during middle age is very harmful, it is protective for older adults: those over 65 who ate a moderate - or high-protein diet were less susceptible to disease.
"The research shows that a low-protein diet in middle age is useful for preventing cancer and overall mortality, through a process that involves regulating IGF-I and possibly insulin levels," said co-author Eileen Crimmins.
Crucially, the researchers found that plant-based proteins, such as those from beans, did not seem to have the same mortality effects as animal proteins.
Rates of cancer and death also did not seem to be affected by controlling for carbohydrate or fat consumption, suggesting that animal protein is the main culprit.
Corresponding author Valter Longo said the findings support recommendations from several leading health agencies to consume about 0.8gm of protein per kg of body weight every day in middle age.
The researchers define a "high-protein" diet as deriving at least 20 per cent of calories from protein, including both plant-based and animal-based protein.
A "moderate" protein diet includes 10-19 per cent of calories from protein, and a "low-protein" diet includes less than 10 per cent protein.
People who ate a moderate amount of protein were still three times more likely to die of cancer than those who ate a low-protein diet in middle age, the study found.
Overall, even the small change of decreasing protein intake from moderate levels to low levels reduced likelihood of early death by 21 per cent.
For a randomly selected smaller portion of the sample - 2,253 people - levels of the growth hormone IGF-I were recorded directly.
The results show that for every 10 ng/ml increase in IGF-I, those on a high-protein diet were 9 per cent more likely to die from cancer than those on a low-protein diet.
The study was published in the journal Cell Metabolism.