Plant-based diets are best… or are they?

People choose a vegetarian or vegan diet for a number of reasons. Sometimes it's out of concern for the way animals are treated or for the environment. But it's also common to choose a plant-based diet because it's considered healthier.

And that's for good reason. Research over many years has linked plant-based diets to lower rates of heart disease, type 2 diabetes, and some cancers (as compared with diets high in meat and other animal products). Dietary guidelines and recommendations from nutrition experts reflect this, encouraging the adoption of diets (such as the Mediterranean diet and the DASH diet) that are heavy on fruits and vegetables and restrict consumption of red meat.

Popular plant-based diets include

• a vegetarian diet, which includes no meat
• a vegan diet, a type of vegetarian diet that excludes not just meat but also animal products, such as milk or eggs
• a pescatarian diet, which is largely vegetarian but also includes seafood.

Plant-based diets carry some risk of inadequate protein, vitamin, and mineral intake. But these risks are readily overcome by choosing the right vegetarian foods and, when necessary, supplements. For example, soy, quinoa, and nuts are good sources of protein, and tofu, lentils, and spinach are good sources of iron.

But a new study, published in the medical journal The BMJ, raises the possibility that despite the health benefits demonstrated by past research, plant-based diets could come with a previously unrecognized health risk.

Vegetarians and vegans may have an increased risk of stroke

Researchers in the United Kingdom analyzed the risk of stroke and other health problems over two decades among nearly 50,000 people based on the diets they followed. The types of stroke were also analyzed, including bleeding into the brain (hemorrhagic stroke) and nonbleeding stroke (ischemic stroke). Compared with meat eaters:

• rates of heart disease (such as angina or heart attack) were 13% lower in pescatarians
• rates of heart disease were 22% lower in vegetarians
• rates of stroke were 20% higher among vegetarians. However, the overall risk was small, equal to three extra cases per 1,000 people over 10 years.
• the higher stroke risk among vegetarians was mostly due to hemorrhagic stroke
• the higher stroke risk was not observed among pescatarians.

If confirmed, these findings will complicate the way we look at plant-based diets. Are there serious and underappreciated downsides to these diets that should make us think twice about choosing them? Or is the increased risk of stroke heavily outweighed by cardiac and other health benefits?

This study is also a reminder that the health impact of a particular intervention (such as diet) may not be easy to predict or explain. In most cases, the risk of stroke and heart disease tend to rise or fall together, but that wasn't the case in this research.

Beware the study's limitations

This study linking a vegetarian diet with a higher risk of hemorrhagic stroke has a number of important limitations that should temper the concerns of vegetarians.

• The study was observational. That means it simply observed what happened among different people who followed different diets over time, without being able to account for every other relevant factor. For example, if vegetarians chose plant-based diets because of a family history of stroke, it could be their genes driving the higher rates of stroke, not the diet.
• The findings might have been different if the study had included a different study population, such as one with different genetic backgrounds or higher rates of obesity.
• The data regarding diet was self-reported. While the use of dietary surveys is common and necessary in research that requires a large number of study subjects, it isn't always reliable.
• The study was not large enough to reliably sort out differences in the rates of disease between vegans and vegetarians. As a result, it's not clear whether the increased stroke risk applies to all vegetarians, or whether vegans might have a different risk.
• We don't know whether the health outcomes reported in this study might be due to what is included in the diet or what is eliminated. For example, is the lower risk of heart disease among vegetarians due to the plant-based diet, or is it just due to the restriction of animal-based products?

Even so, the results are worthy of our attention — and future study.

The bottom line

If the findings of this new research stand the test of time (and future study), a key question will be: how does a vegetarian diet increase the risk of stroke? Understanding how one's diet impacts the risk of stroke and other disease will be crucial in future dietary recommendations and other preventive measures and treatments.