'Healthy' Plant-Based Diet Linked to Reduced Stroke Risk

Erik Greb

March 12, 2021

Maintaining a healthy plant-based diet is associated with a lower risk for both total stroke and ischemic stroke, results from a study of three large cohorts suggest.

A plant-based diet was not associated with risk for hemorrhagic stroke, however, and the data also showed no association between a fully vegetarian diet and total stroke risk, the researchers report.

Most of the previous studies of this question defined a plant-based diet as a vegetarian diet, without differentiating the quality of plant foods, they note.

Dr Megu Y. Baden

"In this study, we developed three versions of plant-based diet indices to assess the quality of plant-based dietary intake without excluding all animal foods and examined the quality of plant-based diet in relation to stroke risk," Megu Y. Baden, MD, PhD, postdoctoral research fellow at Harvard T. H. Chan School of Public Health, Boston, Massachusetts, told Medscape Medical News. "This approach is more similar to the strategies that many individuals are doing to increase plant-based foods and decrease animal-based foods in their diets."

The study was published online March 10 in Neurology.

Research suggests that plant-based diets reduce the risk for cardiovascular disease, but few studies have examined the effect of these diets on the risk for stroke. Moreover, published research on this question has yielded conflicting results, the researchers note. Two previous investigations showed no association between a vegetarian diet and risk for stroke mortality, but one study found that vegetarian patients were at increased risk for total and hemorrhagic stroke.

For their study, the researchers analyzed data from three large American cohorts. Two, the Nurses' Health Study (NHS) and the NHS II, included only women. The third, the Health Professionals Follow-Up Study (HPFS), included only men. Among exclusion criteria were history of cardiovascular disease or cancer at baseline, missing dietary data, and implausible intake of dietary energy.

Few Hemorrhagic Strokes

Participants in the three studies responded to questionnaires every 2 years, which provided information on lifestyle factors and medical history. Every 2 to 4 years, participants filled out a food frequency questionnaire.

To examine the populations' intake of plant-based foods, the investigators created a plant-based diet index (PDI), a healthful plant-based diet index (hPDI), and an unhealthful plant-based diet index (uPDI). They cited foods containing refined grains and added sugars as examples of unhealthful plant-based foods.

The researchers classified patients as vegetarian if they ate plant foods or other animal foods such as butter, dairy, or eggs but ate less than one serving per month of meat or fish. Other participants were considered nonvegetarians.

Medical records and the National Death Index were used to confirm incident and fatal stroke, respectively. The investigators examined total, ischemic, and hemorrhagic stroke separately.

The analysis included 73,890 women in the NHS, 92,352 women in the NHS II, and 43,266 men in the HPFS. In each cohort, participants with the highest PDI had lower BMI, engaged in more physical activity, were less likely to smoke, had higher healthy and lower unhealthy plant food consumption, and had lower animal food consumption compared with participants with the lowest PDI scores.

For participants in the highest PDI quintile, the hazard ratio (HR) for total stroke was lower compared to that of participants in the lowest quintile (HR, 0.94). Participants with the highest hPDI had a significantly lower risk for total stroke (HR, 0.90) than those with the lowest hPDI. Those with the highest uPDI had a slightly higher risk for total stroke than those with the lowest uPDI (HR, 1.05).

With regard to ischemic stroke, for participants with the highest PDI, the HR was 1.04, compared to those with the lowest PDI. For participants with the highest hPDI, the HR was 0.92, and for those with the highest uPDI, the HR was 0.96.

There was no association between PDI and risk for hemorrhagic stroke. "Fewer strokes were of hemorrhagic type, so our power may have been limited," said Baden.

They did not find an association between vegetarianism and total stroke risk, but this result could have followed from the small number of cases. The investigators did not examine the risk for stroke subtypes among vegetarians.

Diet Quality Matters

"The inconsistent results from prior studies looking at a vegetarian diet and stroke risk may in part be due to differences in the quality of plant-based diets — the proportion of heathy and less healthy plant-based foods — among those that follow vegetarian diets," said Baden. "In other words, no prior studies examined the dietary quality of vegetarian diets."

Research has indicated that plant-based foods are rich in nutrients that may reduce the risk for cardiovascular disease. These foods include dietary fiber, flavonoids, and polyphenols, which are more abundant in whole grains, fruits, vegetables, tea, and coffee. Consumption of these nutrients may be the mechanism through which a plant-based diet reduces the risk for stroke, they speculate.

The investigators would like to examine the mechanisms underlying the association between improved plant-based diet quality and lower stroke risk by assessing the metabolomic signatures or microbiomes of healthful and unhealthful plant-based diets, said Baden.

Dr Kevin N. Sheth

One of the study's strengths is its use of two large, well-established cohort studies of men and women, Kevin N. Sheth, MD, professor of neurology and neurosurgery at Yale School of Medicine and chief of neurocritical care and emergency neurology at Yale New Haven Hospital, New Haven, Connecticut, told Medscape Medical News. The findings, however, may not be generalizable to broad sections of society that are characterized by large socioeconomic and other disparities, he added.

"The results suggest that all plant-based diets are not the same and that further research is needed to identify the plant-based diets that are associated with reduction in stroke risk," said Sheth.

These findings underscore the importance of further clarifying which dietary strategies are most effective in preventing stroke, including diets that focus to various degrees on fruit, fiber, vegetables, and whole grains. "We need to understand what roles these kinds of diets may play in stroke risk in broader populations," said Sheth.

Dr Erica Camargo Faye

Also commenting for Medscape Medical News, Erica Camargo Faye, MD, PhD, a stroke neurologist at the Massachusetts General Hospital and an instructor in neurology at Harvard Medical School, Boston, Massachusetts, pointed out that the significant number of stroke endpoints in this study allowed for better assessment of associations with the outcomes of interest.

However, a large proportion of stroke endpoints (38%) were not classified as ischemic or hemorrhagic, which limits the assessment of associations between plant-based diets and stroke subtypes, she added.

"There is a large body of literature supporting the concept that the Mediterranean diet, DASH diet, and diets rich in fruits and vegetables reduce the risk of stroke," said Faye. "This study adds to these findings, demonstrating that the quality of plant-based diets also has a significant impact on the risk of stroke."

Future research could explore whether these findings remain significant in different racial and ethnic populations, Faye added. Another relevant question is whether healthful plant-based diets have a stronger effect on the risk for ischemic stroke or the risk for hemorrhagic stroke.

"Considering prior conflicting data regarding the risk of stroke in relation to vegetarian diets, studying how the quality of vegetarian diets (ie, healthful vs unhealthful plant-based diets) impacts the risk of stroke is an important question to address," she concluded.

The study was funded by grants from the National Institutes of Health. Baden and Sheth have disclosed no relevant financial relationships.

Neurology. Published online March 10, 2021. Abstract

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