A new study showing that high protein intake during middle age quadrupled the risk of cancer raises big questions about the current paleo diet trend that greenlights meat while lowering carbs as a way to control weight and increase energy.
The study of 6,318 adults found that those who ate a diet rich in meat, milk and cheese were twice as likely to die – and four times more likely to die of cancer – as those who did not.
Lead researcher Valter Longo of the University of Southern California David School of Gerontology said he believes that the key factor in the discrepancy between the two groups is that the IGF-1 growth hormone is being stimulated by animal proteins, but not by other foods. This could foster cancer cell growth, he said, in a statement about the study published March 4 in Cell Metabolism.
“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 is protein intake.”
Longo and his co-researchers said that diets that emphasize meat and reducing carbs may result in lower weight. "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?" Longo said.
Indeed, the study provided an answer to the age-old question aimed at vegetarians: But how do you get enough protein? The answer: that's not a problem, at least in middle age. Plant proteins did not have the same level of dangerous effects as that from animals, according to the study.
The study also looked at carbohydrates and fats, determining that these were not the variables contributing to the higher cancer risk.
Longo and his co-authors, Morgan Levine, Jorge Suarez and Pinchas Cohen of the USC Davis School of Gerontology, found that for adults 50-65, a low-protein diet was the most protective against cancer. In this age group, even those who ate a moderate amount of protein were still three times more likely to die of cancer as those who at a low-protein diet in middle life.
Low-protein diets also were protective against diabetes, a finding that suggests meat and dairy intake plays a significant role in this disease.
“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,” Longo said. “But don’t get extreme in cutting out protein; you can go from protected to malnourished very quickly.”
But there's one group of adults that should be careful to get enough protein. Those over age 65 appeared to benefit from moderate protein in their diets. It guarded against frailty and disease, the researchers reported.
Given that IGF-1 levels drop dramatically after age 65, this finding tracked with other research.
The scientists followed the study group for 18 years.
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