Could the apparent increased stroke risk in vegetarians be reverse causation? And what about vegetarians versus vegans? In the “Risks of Ischaemic Heart Disease and Stroke in Meat Eaters, Fish Eaters, and Vegetarians Over 18 Years of Follow-Up” EPIC-Oxford study, vegetarian diets were associated with less heart disease—10 fewer cases per 1,000 people per decade compared to meat eaters—but vegetarian diets were associated with three more cases of stroke.
So, eating vegetarian appears to lower the risk of cardiovascular disease by 7 overall, but why the extra stroke risk? Could it just be reverse causation? When studies have shown higher mortality among those who quit smoking compared to people who continue to smoke, for example, we suspect “reverse causality.” When we see a link between quitting smoking and dying, instead of quitting smoking leading to people dying, it’s more likely that being “affected by some life-threatening condition” led people to quit smoking.
What’s going on? Let’s dive deeper into the data to look for clues. What happens when you break down the results by type of stroke and type of vegetarian (vegetarian versus vegan)? There are two main types of strokes—ischemic and hemorrhagic. Most common are ischemic, clotting strokes where an artery in the brain gets clogged off, as opposed to hemorrhagic, or bleeding strokes, where a blood vessel in the brain ruptures.
Even though this was the first study of vegetarian stroke incidence, there have been about half a dozen studies on stroke mortality. The various meta-analyses have consistently found significantly lower heart disease risk for vegetarians, but the lower stroke mortality was not statistically significant.
It may be worth reiterating that vegetarians do not have a higher risk of dying from a stroke, but they do appear to be at higher risk of having a stroke. There must be something about eating plant-based that so increases stroke risk that it counterbalances the meat-free benefit. Might it be because plant-based eaters don’t eat fish? We turn to omega-3s next.
