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In 2014, a study on dietary protein intake and stroke risk found that greater intake was associated with lower stroke risk, and furthermore it was the animal protein that appeared to be particularly protective. Might that help explain why vegetarians were recently found to have a higher stroke rate than meat-eaters?

Animal protein consumption increases the levels of a cancer-promoting growth hormone in the body known as IGF-1, insulin-like growth factor one, which can accelerate the progression of precancerous changes to invasive cancer. High blood concentrations are associated with an increased risk of breast, prostate, colorectal, and lung cancers, potentially explaining the association between dairy milk and prostate cancer risk, for example. But there are also IGF-1 receptors on blood vessels; so, maybe IGF-1 promotes cancer but also brain artery integrity.

People who have strokes appear to have lower blood levels of IGF-1, but it could just be a consequence of the stroke rather than the cause. There weren’t any prospective studies over time —until 2017, and indeed higher IGF-1 levels were linked to lower risk of stroke, but is it cause and effect? In mice the answer appears to be yes, and in a petri dish IGF-1 appears to boost production of elastin, a stretchy protein that helps keep our arteries elastic. Higher levels are associated with less artery stiffness, but acromegaly patients, like Andre the giant, those with excessive levels of growth hormones like IGF-1 do not appear to have lower stroke rates, and a more recent study of dietary protein intake and risk of stroke, looking at a dozen studies of more than a half million people, compared to only 7 studies with a quarter million in the previous analysis, found NO association between dietary protein intake and the risk of stroke. In fact, if anything dietary plant protein intake may actually decrease the risk of stroke.

However, those with high blood pressure who have low IGF-1 levels do appear to be at increased risk of developing atherosclerosis, thickening of the artery walls leading up to the brain, but no such association was found in people with normal blood pressure. So, there may be a cautionary lesson for vegans here. Yes, a whole food plant-based diet to down-regulate IGF-1 activity may slow the human aging process, not to mention reducing the risk of some of the common cancers that plaque the Western world. But, perhaps, the ‘take-home’ message should be that people who undertake to down-regulate IGF-1 activity by cutting down on animal protein intake as a pro-longevity measure should take particular care to control their blood pressure to preserve their cerebrovascular health, the health of their arteries in their brain. In particular, they should keep salt intake relatively low while ensuring an ample intake of potassium to keep their blood pressures down. So, that means avoiding processed foods and avoiding added salt, and in terms of potassium-rich foods, beans, sweet potatoes, and dark green leafy vegetables.

So, might this explain the higher stroke risk found among vegetarians? No. Why? Because dairy and egg whites are animal proteins too. Only vegans have lower IGF-1 levels in both men and women; so, low IGF-1 levels can’t explain why higher rates of stroke were found in vegetarians. So, what is it already? I think the best explanation for the mystery is something called homocysteine, which I’ll cover, next.

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