A recently released Harvard Study regarding plant-based food intake and stroke risk is in the news. It shows a modest association between healthy plant-based food intake with lower risk of stroke. Researchers concluded that there was a 10% lower total stroke risk among people eating the most healthy plant foods compared to people eating the fewest healthy plant foods.
It adds a little bit to the evidence suggesting a diet/stroke connection, but not much, to be honest. I find it to be a bit of a conundrum that the evidence for diet and stroke is not stronger, because the totality of the evidence for plant-based nutrition leading to lower heart disease risks is very strong and heart disease and stroke share some of the same risk factors and disease processes.
I wrote a commentary about plant-based diets and stroke for the Journal of Geriatric Cardiology and if you want a deeper dive, I encourage you to read that article. In short, while there are several major stroke risk factors (high blood pressure, diabetes, excess weight, etc…) that might be successfully prevented or even treated with healthy plant-based nutrition (Table 1), I think it is reasonable to say that large observational studies have not shown major, consistent nutritional effects on stroke risk.
This Harvard study is no different. In many of the researchers analyses, nutrition was not associated with stroke risk, or only very marginally so - so marginal that you couldn’t say the reduction in risk was anything more than just random chance.
Perhaps the reason there is less of a clear picture between nutrition and stroke is that stroke can have a variety of causes. Hemorrhagic stroke, or strokes caused by bleeding out of a compromised blood vessel, are quite different from ischemic strokes, or strokes caused by blockages in blood flow. Hemorrhagic strokes have been particularly unrelated to plant-based nutrition, but there aren’t a lot of large studies looking at this association, so the relationship is still largely unexamined. Even among ischemic strokes, there are several different causes. A large percentage of ischemic stroke in older adults are related to clots thrown up from the heart, related to a heart rhythm or anatomy issue (atrial fibrillation or valve problem, for example), which is yet again a different biological process than the process that causes the typical blockage in a heart artery, for example.
It is further complicated because while people focus on plant foods versus animal foods, all too often they forget to consider processed foods versus whole foods. Results from this Harvard study suggest, albeit meekly, that choosing unprocessed whole foods instead of processed foods is perhaps even more important than the argument about plant from animal foods.
There are many reasons why studies like this only find very weak associations, though, having nothing to do with the effects of nutrition itself. Perhaps all the subjects are eating a relatively uniformly poor diet, with very few fruits and vegetables and too much added fat. Perhaps the way they studied nutritional intake introduced enough measurement error that the associations were weakened. Both are likely in very large observational studies of Americans.
To support that point, one of the most interesting aspects of this study was that less than 1% of the population was vegetarian, and the vegetarians obviously didn’t eat that much differently - apart from avoiding meat. There were almost no differences in fruit and vegetable intake between the two groups. All of them, bot vegetarians and non-vegetarians, were eating few fruits and vegetables and vegetarians had only about 1 serving a day more of fruits and vegetables. Vegetarians had slightly more whole grain and legume intake, but vegetable oil intake was equal between groups. So even among the vegetarian group, which you might imagine would have the most different dietary pattern of the whole cohort, you could probably safely assume that there weren’t enough differences in nutrient intake to make a major difference in outcomes.
The take home point: there are a number of reasons to think that a healthy (whole-food), plant-based diet may lower the risk of stroke, and this Harvard study adds a very small bit of support for that thought, but much more research has to be done. In the meantime, if you’re interested in reducing your risk, I’d suggest you work hard to control the risk factors related to stroke risk (Table 1), many of which are directly related to diet and lifestyle.