Animal protein-rich diet compared to smoking
New research has revved up the debate about eating meat, showing that eating animal protein in middle age may be as bad as smoking.
The study followed 6381 people aged 50 and older for 18 years. It found people aged 50 to 65 who ate a diet rich in animal proteins during middle age were more than four times as likely to die of cancer during the study period than those who ate a low-protein diet.
In an article about the research, the University of Southern California (USC) said that mortality risk factor was comparable to smoking.
In contrast, once people passed the age of 65, those eating a moderate or high protein diet were less susceptible to disease.
The study found middle-aged people who ate lots of proteins from animal sources - including meat, milk and cheese - were also more susceptible to early death in general.
"Protein lovers were 74 per cent more likely to die of any cause within the study period than their more low-protein counterparts," USC said.
"They were also several times more likely to die of diabetes."
But the researchers found 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 carbohydrates or fat consumption, "suggesting that animal protein is the main culprit", USC said.
The study linked protein to regulation of the growth hormone IGF-I, which helps growth but has been linked to cancer susceptibility.
"Almost everyone is going to have a cancer cell or pre-cancer cell in them at some point," study corresponding author Valter Longo said.
"The question is: does it progress?
"Turns out one of the major factors in determining if it does is protein intake."
Longo is a professor of biogerontology at the USC Davis School of Gerontology and director of the USC Longevity Institute.
Some nutrition experts have cautioned it is too early to draw firm conclusions from the research.
The Guardian quoted Peter Emery, head of nutrition and dietetics at King's College London urging general caution over observational studies, particularly when looking at diet, "given the difficulties of disentangling one nutrient or dietary component from another".
"You can get an association that might have some causal linkage or might not," Emery said.
Beef + Lamb New Zealand nutrition manager, Fiona Greig says that "the results of one single study do not change dietary advice or recommendations, particularly when a study only shows an association, not cause. Diabetes and cancer are complex conditions that are not caused by single foods, and the results of one study does not change dietary advice or recommendations. The greatest risk factors in cancer development are obesity, a lack of physical activity, and alcohol, not one specific food."
At the same time, the article is seen as challenging the thinking behind popular protein-heavy diets such as Paleo and Atkins.
The USC article said the study, which has been published in Cell Metabolism, found people who ate even a moderate amount of protein during middle age were still three times more likely to die of cancer than those who ate a low-protein diet.
But things changed around the age of 65, when levels of growth hormone IGF-I dropped off dramatically, leading to potential frailty and muscle loss. For those older people, a high-protein diet was protective.
USC said the study was the first to show a definitive correlation between high protein consumption and mortality risk. The research supported recommendations from several leading health agencies that middle-aged people should eat about 0.8 grams of protein per kilogram of body weight.
The researchers also looked at tumour rates and progression in mice, finding lower cancer incidence and 45 per cent smaller average tumour size among mice on a low-protein diet than those on a high-protein diet at the end of the two-month experiment.