 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 a B12 supplement or eat B12 fortified foods regularly, can lead to an increased level of homocysteine in the blood, which is accepted as an important risk factor for stroke, accepted maybe 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 crotted arteries that lead up to the brain compared to those with single-digit homocysteine levels and do seem to be at higher risk of clotting strokes in observational states, 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 at about two 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 five times as likely to reduce stroke compared to placebo. 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 tested, never been studied until now. And vegetarians do appear to be at higher risk, and no wonder, as about a quarter of vegetarians in nearly three quarters of the vegan 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 US, B12 fortification of organic foods isn't allowed. So while US soy milk and other products may be fortified with B12, UK products may not. We don't see the same problem among US vegans in the Adventist study, presumably because of the B12 fortification of commonly eaten foods, and 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 as a risk factor for cardiovascular disease in vegetarians is to put it to the test. And when researchers measured the amount of atherosclerosis in the crotted arteries, the main arteries supplying the brain between vegetarians and non-vegetarians, no significant difference was found. They both looked just as bad, even though vegetarians tend to have better risk factors, such as cholesterol and blood pressure. They should just be 12 deficiencies 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. 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 would need is an interventional study, where you give them B12 and see if that fixes it. And here we go, double-blind placebo-controlled randomized crossover study, and the title gives it away. Vitamin B12 supplementation improves our arterial function in vegetarians with subnormal vitamin B12 status. So that may explain it, right? Compromise vitamin B12 status among those eating more plant-based 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 proper intake of B12 despite excluding meat. And now that we may have nailed the cause, a maybe future study should focus on identifying ways of convincing vegetarians to routinely take vitamin B12 supplements in order to prevent a deficiency.