Eating a lot of meat and cheese is just as bad as smoking in terms of the risk of dying from cancer, researchers have said.
According to a study published in Cell Metabolism, middle-aged people with a diet high in animal proteins are four times as likely to die from cancer as their low-animal protein counterparts – equal to the risk of smoking.
Findings of the study also showed people who eat a lot of meat and cheese are twice as likely to die in general as those with a low-protein diet.
Researchers from the USC Davis School of Gerontology looked at 6,318 adults over the age of 50 for almost 20 years.
Findings showed people who ate a lot of protein were 74% more likely to die of any cause during the study than their low-protein counterparts. They were also several times more likely to die from diabetes.
Rather than examining what the optimal amount of protein is, the researchers looked at how biology and dietary requirements change as we age – they found that while high protein in middle age is detrimental to health, you need more animal proteins after the age of 65.
The researchers looked at the growth hormone IGF-I, which is controlled by protein. While it helps bodies to grow, it has also been linked to cancer susceptibility.
Study co-author Eileen Crimmins said: "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.
"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."
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The researchers discovered that plant-based proteins do not have the same effect on mortality as animal proteins, suggesting this is the cause of death: "The majority of Americans are eating about twice as much protein as they should, and it seems that the best change would be to lower the daily intake of all proteins but especially animal-derived proteins," corresponding author Valter Longo said.
"But don't get extreme in cutting out protein; you can go from protected to malnourished very quickly."
Researchers found that ideally, people should eat 0.8gms of protein per kilogram of body weight every day in middle age, with a preference for proteins derived from plants.
During middle age, researchers found even people with a moderate amount of animal proteins in their diet were still three times more likely to die from cancer than those with a low-protein diet.
"Almost everyone is going to have a cancer cell or pre-cancer cell in them at some point. The question is: Does it progress?" Longo said. "Turns out one of the major factors in determining if it does is protein intake."