Despite the popularity of protein-rich diets like Atkins and Paleo, new research suggests that it may be wise to steer the opposite way, especially when it comes to cancer risk. In the new study, middle-aged people who ate protein-heavy diets had a markedly increased risk of dying from cancer compared to their low-protein counterparts. But, as always, there are caveats: Protein from animal sources – meat and dairy – was what largely produced the risk, whereas plant-derived proteins seemed to be “safer.” And the rules seem to be different for people over the age of 65.
So what are we to do?
"There's a misconception that because we all eat, understanding nutrition is simple,” said study author Valter Longo, director of the Longevity Institute at the University of Southern California. “But the question is not whether a certain diet allows you to do well for three days, but can it help you survive to be 100?"
In the new study, the team followed over 6,300 adults over the age of 50, to see what effect high-, medium-, and low-protein diets had on longevity. A high-protein diet was defined as 20% of one’s daily calories coming from protein, a moderate-protein diet is made up of 10-19% calories from protein, and a low-protein diet consists of less than 10% protein. People in the study ate, on average, 16% protein, with two-thirds coming from animal sources – pretty typical of an American diet, the authors say.
The findings were intriguing: People from ages 50-65 who ate high-protein diets were four times more likely to die of cancer – this is in the ballpark of smoking risk, say the authors – compared to people who ate low-protein diets. Even those who ate moderate-protein diets were three times as likely to die from cancer. And people who ate high-protein diets were 75% more likely to die from any cause, including three times as likely to die from diabetes. The team calculated that reducing protein intake from moderate to low would reduce the risk of death by 21%.
Interestingly, when the source of the protein was taken into consideration, things changed a bit. When animal-derived sources taken out of the mix, the mortality risk was significantly reduced: In other words, plant-based protein did not seem to present the same kind of problem as protein from animal sources.
"The majority of Americans are eating about twice as much proteins 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," said Longo. "But don't get extreme in cutting out protein; you can go from protected to malnourished very quickly."
Why the protein-cancer connection? Protein intake influences the levels of the growth hormone IGF-I, which not only affects the growth of healthy cells, but can also encourage cancer cell growth. In fact, in the current study, the team found that for every 10 ng/ml increase in IGF-I, people who ate high-protein diets were 9% more likely to die from cancer than those on a low-protein diet.
This growth hormone mechanism seems to be the reason that calorie-restricted diets have been shown to increase longevity in certain species, including, possibly, humans.
But again, there are more caveats. IGF-I levels decline over the years, especially after age 65, which is part of the reason that people lose muscle tone and become frailer with age. And the current study bore this out, too: When the team looked at people above the age of 65, people who ate more protein had a reduced risk of death.
The protein issue is therefore complex, and will require some more research.
But the recommendations arising from the current study, says Longo, line up with those from the World Health Organization and the Institute of Medicine. Longo tells me that generally people should stick with “plant based proteins and/or stay as close as possible to 0.36 grams of protein per pound of body weight per day. This is about 54 grams of protein per day for a 150-pound person… However, going lower than that can be detrimental.”
So, it may be wise to watch your protein intake, at least in middle age. And at any age, eating a plant-based diet is probably smart, as study after study shows the near-indisputable health benefits of doing so.
"Almost everyone is going to have a cancer cell or pre-cancer cell in them at some point,” says Longo. “The question is: Does it progress? Turns out one of the major factors in determining if it does is protein intake."
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