All plant-based diets are not equal when it comes to estimating their impact on lowering the risk of specific types of strokes, a new investigation reports.

Each year, more than 795,000 people in the United States have a stroke, estimates the CDC, and about 87% of these strokes are ischemic strokes, where blood flow to the brain is blocked. A healthy lifestyle including a healthy plant-based diet can help prevent a stroke.

According to a new study led by researchers from Harvard T.H. Chan School of Public Health, high quality plant-based diets—defined as diets rich in foods such as leafy greens, whole grains, and beans—as opposed to lower quality plant-based diets that include foods such as refined grains, potatoes, and added sugars, may lower overall stroke risk by up to 10%.

The findings are reported in the article, “Quality of Plant-based Diet and Risk of Total, Ischemic, and Hemorrhagic Stroke,” published in the journal Neurology, the medical journal of the American Academy of Neurology.

“Our findings have important public health implications, suggesting that future nutrition policies to lower stroke risk should take the quality of food into consideration,” says Megu Baden, PhD, postdoctoral fellow in the Department of Nutrition at the Harvard T.H. Chan School of Public Health, and first author on the study.

Earlier studies suggest plant-based diets may lower the risk of diabetes, cardiovascular disease, and other diseases. However, few studies have explored whether plant-based diets are related to the risk of stroke, particularly different types of stroke.

The few studies that have investigated this question show inconsistent results, the authors note. For instance, this research team has earlier reported an inverse association between a healthful plant-based diet and the risk of cardiovascular disease including total stroke, whereas another an EPIC-Oxford study showed a higher risk of total and hemorrhagic stroke among vegetarians.

The authors explore whether these confusing results arise because the earlier studies do not differentiate between the quality of plant-based foods. Toward this goal, the researchers developed three plant-based diet indices—the overall plant-based diet index (PDI), the healthful plant-based diet index (hPDI), and the unhealthful plant-based diet index (uPDI)—to evaluate the quality of plant food intake. The authors do not exclude all animal foods but define vegetarians as those who reported their meat and/or fish intakes were zero or less than one serving per month.

The researchers analyzed health data from 209,508 women and men in the Nurses’ Health Study, Nurses’ Health Study II, and Health Professionals Follow-Up Study, who did not have cardiovascular disease or cancer at the start of their participation. The participants in the study were followed for more than 25 years and completed diet questionnaires every two to four years. The research team then scored participants on diet quality based on the healthfulness of the plant-based foods that they ate.

“Many individuals have been increasing the amount of plant-based components in their diet,” says Kathryn Rexrode, PhD, associate professor of medicine at Brigham and Women’s Hospital and co-author on the paper. “These results show that higher intake of healthy plant-based foods may help reduce long-term stroke risk, and that it is still important to pay attention to diet quality of plant-based diets.”

During the 25 year follow up, the study documents a total of 6,241 cases of stroke, including 3,015 ischemic strokes and 853 hemorrhagic strokes.

The researchers report that a healthy plant-based diet—in addition to being linked with 10% lower overall stroke risk—is associated with a modest reduction in risk of ischemic stroke.

The authors find no association between a healthy plant-based diet and risk of hemorrhagic stroke, which occurs when an artery in the brain leaks blood or ruptures.