In a study conducted by the University of Southern California and published in the journal Cell Metabolism, researchers have found that high protein intake is linked to increased cancer, diabetes and overall mortality, carrying a cancer risk similar to that of smoking 20 cigarettes a day.
The researchers examined the links between protein intake and mortality, concluding that respondents aged 50-65 reporting high protein intake had a 75% increase in overall mortality compared to those eating a low-protein diet. They also had a four-fold increase in cancer death risk during the 18 years over which they were studied.
The university’s Dr Valter Longo said: “We 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.”
Leading nutritionist and author Patrick Holford commented on the research: “This finding doesn’t surprise me at all. We’ve known about the cancer-promoting properties of both meat and milk for along time. It is this increase in cancer risk that is largely driving the increase in risk of mortality.”
The study also found that high levels of the growth hormone IGF-1 increased the relationship between mortality and high protein intake and that pderived proteins are associated with lower mortality than animal-derived proteins.
However, talking to The Times, epidemiologist Professor Tim Key from the University of Oxford said the findings weren’t conclusive. He said that “nutritional science is still in its infancy, scientists simply don’t have the evidence to back up many of their claims”.
Key is currently carrying out a UK study of vegetarians, involving some 70,000 subjects (the USC study involved around 6,000 people). He has published findings showing that although these vegetarians consume less animal protein and saturated fat, their rates of premature death are the same as with meat-eaters. In addition, the rates of bowel cancer are the same in the two groups
“In relation to cancer and diet, the only two things that are unequivocal are obesity and alcohol – both cause cancer. There are loads and loads of studies and they are completely consistent. Once you get onto other things, meat, bacon, fibre, the data is just not clear,” said Key.