A high-protein diet during middle age could make you almost twice as likely to die early and four times more likely to die of cancer, a study has suggested.
Researchers from the University of Southern Carolina found those between the age of 50 and 65 who ate large amounts of protein were 74 percent more likely to die of any cause within the study period, reports Sky News today.
They also found those with a diet rich in animal proteins were four times more likely to die of cancer than someone with a low-protein diet - a mortality risk factor comparable to smoking - and several times more likely to die of diabetes.
But the study, which tracked 6,318 adults over the age of 50 for almost 20 years, goes on to suggest a moderate protein intake could be good for those over the age of 65.
Those behind the research, published in journal Cell Metabolism, said protein controls the growth hormone IGF-I, which has been linked to cancer.
Corresponding author Professor Valter Longo said: "Almost everyone is going to have a cancer cell or pre-cancer cell in them at some point. The question is: Does it progress?"
"Turns out one of the major factors in determining if it does is protein intake."
Co-author Eileen Crimmins said IGF-I levels fell dramatically after the age of 65, so a healthy diet at one age may be damaging at another.
"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," she said.
"However, we also propose that at older ages, it may be important to avoid a low-protein diet to allow the maintenance of healthy weight and protection from frailty."
The researchers added that plant-based proteins, such as those from beans, did not seem to have the same mortality effects as animal proteins.
They said cancer and death rates also seemed not to be affected by carbohydrate or fat consumption, suggesting animal protein was the main factor.
Dietician and nutritionist Nicole Berberian told Sky News the conclusions were based on an observational study, which she said could not establish cause and effect.
"This is an observational study, so from that you can never tell whether protein was the cause of the increase in cancer," she said.
"It could be anything. It could be a lifestyle factor in those people they observed, it could be just a random chance."
She added that the study did not identify what types of proteins those being observed were eating, and said some forms - such as processed meat - had already been linked to cancer.