Researchers from the University of Southern California studied more than 6,300 adults aged 50 or over to find out what affect high protein diets have on them. 

They found that "eating a diet rich in animal proteins during middle age makes you four times more likely to die of cancer than someone with a low-protein diet — a mortality risk factor comparable to smoking." 

The study found that they were 74% more likely to die for any reason over their more low-protein counterparts. They were also more likely to die of diabetes.

Researchers found that not only is excessive protein consumption linked to a dramatic rise in cancer mortality, but also that "middle-aged people who eat lots of proteins from other animal sources, like meat, milk and cheese, are also more susceptible to die early generally."

They found that even moderate amounts of protein had detrimental effects during middle age. 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.

Overall, even decreasing your intake of protein by a small amount to low levels reduced likelihood of early death by 21%.

In an unexpected finding, the trend appears to reverse for the over-65's, when high protein diets are beneficial and can help to combat frailty and muscle loss. In other words, they found that what is good for you at one age may be damaging at another.

Additional findings

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.