August 26

Vegetarians and Stroke Risk Factors—Vitamin B12 & Homocysteine?


Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

Leonardo da Vinci had a stroke. Might his vegetarian diet been to blame? “His stroke… may have been related to an increase in homocysteine level because of the long duration of his vegetarian diet.” A suboptimal intake of vitamin B12, which is common in those eating plant-based diets unless they take B12 supplements or regularly eat B12-fortified foods, can lead to an increased level of homocysteine in the blood, which is accepted as an important risk factor for stroke. “Accepted” may be overstating it; there’s still controversy surrounding the connection between homocysteine and stroke risk. But, those with higher homocysteine levels do seem to have more atherosclerosis in the carotid arteries that lead up to the brain compared to those with single digit homocysteine levels, and do seem to be at higher risk for clotting strokes in observational studies, and more recently bleeding strokes, as well as increased risk of dying from cardiovascular disease, as well as all causes put together.

Even more convincing is the genetic data. About 10% of the population has a gene that increases homocysteine levels about 2 points, and they do appear to have significantly higher odds of having a stroke. Most convincing would be randomized double-blind placebo-controlled trials to prove homocysteine lowering with B vitamins can lower strokes, and that indeed appears to be the case for clotting strokes, strokes with homocysteine-lowering interventions more than 5 times as likely to reduce stroke compared with placebo.

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Ironically, one of the arguments against the role of homocysteine in strokes is that assuming that vegetarians have lower vitamin B12 levels then the incidence of stroke should be increased among vegetarians, but supposedly this wasn’t the case, but it had never been studied… until, now, and vegetarians do appear to be at higher risk and no wonder, as about a quarter of the vegetarians and nearly three-quarters of the vegans studied were B12 depleted or deficient, resulting in extraordinarily high homocysteine levels.

Why so much B12 deficiency? Because only a small minority were taking a dedicated B12 supplement. And unlike the U.S., B12 fortification of organic foods isn’t allowed; so, while U.S. soymilk and other products may be fortified with B12, UK products may not. We don’t see the same problem among U.S. vegans in the Adventist study, presumed to be because of the B12 fortification of commonly eaten foods. It may be no coincidence that the only study I was able to find that showed a significantly lower stroke mortality risk among vegetarians was an Adventist study.

But start eating strictly plant based without B12 fortified foods or supplements, and B12 deficiency can develop, but that was only for those not eating sufficient foods fortified with B12. Those eating plant-based not being careful about getting a regular reliable source of B12 have lower B12 levels and consequently higher homocysteine levels.

The only way to prove vitamin B12 deficiency is a risk factor for cardiovascular disease in vegetarians is to: put it to the test. When researchers measured the amount of atherosclerosis in the carotid arteries, the main arteries supplying the brain, between vegetarians and nonvegetarians, no significant difference was found. They both look just as bad even though vegetarians tend to have better risk factors such as cholesterol and blood pressure. They suggest B12 deficiency is playing a role, but how do they know? Some measures of artery function weren’t any better either. Again, they surmise vitamin B12 deficiency overwhelming the natural plant-based benefits. Yes, the beneficial effects of vegetarian diets on cholesterol and blood sugars need to be advocated, but the necessity to correct vitamin B12 deficiency in vegetarian diets cannot be overstated.

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Sometimes vegetarians did even worse. Worse artery wall thickness. Worse artery wall function, raising concern, more than a decade before the new stroke study, about the vascular health of vegetarians. Yes, their B12 was low, yes, their homocysteine was high suggesting that vitamin B12 deficiency in vegetarians might have adverse effects on their vascular health, but what you’d need is an interventional study, where you give them B12 and see if that fixes it and… Here we go, a double-blind, placebo-controlled, randomized crossover study and the title gives it away: Vitamin B-12 supplementation improves arterial function in vegetarians with subnormal vitamin B-12 status. So, that may explain it. Compromised vitamin B12 status among those eating more plant-based diets due to not taking B12 supplements, or regularly eating vitamin B12 fortified foods may explain the higher stroke risk found among vegetarians.

Unfortunately, many vegetarians resist taking vitamin B12 supplements due to misconceptions, like holding on to the old myth that deficiency of this vitamin is rare among plant-based eaters. A common mistake is to think that the presence of dairy products and eggs in the diet, as in a lacto-ovo vegetarian diet can still ensure a proper intake of B12, despite excluding meat.

Now that we may have nailed the cause, maybe future studies should focus on identifying ways of convincing vegetarians to routinely take vitamin B12 supplements in order to prevent a deficiency.

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