People who eat diets with higher amounts of healthy plant-based foods and lower amounts of less-healthy plant-based foods may reduce their risk of stroke compared to people with lower-quality diets, according to a study led by researchers from Harvard TH Chan School of Public Health. They found that healthy plant-based diets – defined as rich in foods such as leafy greens, whole grains, and beans, and including lower levels of foods like refined grains, potatoes, and added sugars – may lower overall stroke risk by up to 10%.
“Our findings have important public health implications, suggesting that future nutrition policies to lower stroke risk should take the quality of food into consideration,” said first author Megu Baden, a postdoctoral fellow in the department of nutrition. Evidence suggests that plant-based diets may lower the risk of diabetes, cardiovascular disease, and other diseases; however, few studies have looked at whether these diets lower the risk of stroke, and their results have been inconsistent.
In this study, researchers analysed 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. They were followed for more than 25 years and completed diet questionnaires every two to four years.
Participants were scored on diet quality based on the healthfulness of the plant-based foods that they ate. People who ate a serving or less of meat or fish per month were classified as vegetarians.
The researchers found that a healthy plant-based diet – in addition to being linked with 10% lower overall stroke risk – was associated with a modest reduction in risk of ischaemic stroke, the most common type of stroke, which occurs when blood flow to the brain is blocked. There was no association found between a healthy plant-based diet and reduced risk of haemorrhaegic stroke, which occurs when an artery in the brain leaks blood or ruptures.
In a separate analysis, the researchers did not find any association between a vegetarian diet and lowered risk of stroke, although they noted that the number of study participants classified as vegetarians was small. The researchers suggested that this result – as well as inconsistent results in prior studies looking at plant-based diets and stroke risk – may in part be explained by a high proportion of low-quality plant-based foods in participants’ diets.
“Many individuals have been increasing the amount of plant-based components in their diet,” said Kathryn Rexrode, associate professor of medicine at Brigham and Women’s Hospital and co-author of 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.”
Other Harvard Chan School co-authors of the study included Zhilei Shan, Fenglei Wang, Yanping Li, JoAnn Manson, Eric Rimm, Walter Willett, and Frank Hu.
One of the study's strengths is its use of two large, well-established cohort studies of men and women, Dr Kevin N Sheth, 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, is quoted in Medscape Medical News as saying. The findings, however, may not be generalisable 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.
Also commenting, Dr Erica Camargo Faye, a stroke neurologist at the Massachusetts General Hospital and an instructor in neurology at Harvard Medical School, 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 haemorrhagic, 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 haemorrhagic stroke.
"Considering prior conflicting data regarding the risk of stroke in relation to vegetarian diets, studying how the quality of vegetarian diets (healthful vs unhealthful plant-based diets) impacts the risk of stroke is an important question to address," she concluded.
Quality of Plant-based Diet and Risk of Total, Ischemic, and Hemorrhagic Stroke
Megu Y Baden, Zhilei Shan, Fenglei Wang, View ORCID ProfileYanping Li, JoAnn E Manson, Eric B Rimm, Walter C Willett, Frank B Hu, Kathryn M Rexrode
Published in Neurology on 10 March 2021
To determine whether a healthful plant-based diet is related to lower stroke risk, we examined the associations of plant-based diet quality with risk of total, ischemic, and hemorrhagic stroke.
The participants were 73,890 women in Nurses’ Health Study (NHS; 1984 to 2016), 92,352 women in NHSII (1991 to 2017), and 43,266 men in Health Professionals Follow-Up Study (1986 to 2012) without cardiovascular disease and cancer at baseline. Plant-based diet quality was evaluated by the overall plant-based diet index (PDI), the healthful PDI (hPDI), and the unhealthful PDI (uPDI). Participants who reported their meat and/or fish intakes were zero or less than one serving per month were categorized as vegetarians, and others were classified as non-vegetarians. Strokes with available medical records were subtyped as ischemic or hemorrhagic.
During the follow-up, 6,241 total stroke cases (including 3,015 ischemic and 853 hemorrhagic strokes) were documented. Compared to participants with the lowest PDIs, among participants with the highest PDIs the hazard ratios (HRs) for total stroke were 0.94 (95% confidence interval 0.86 to 1.03) for PDI, 0.90 (0.83 to 0.98) for hPDI, and 1.05 (0.96 to 1.15) for uPDI. Participants in the highest hPDI showed marginally lower HR for ischemic stroke (0.92 [0.82 to 1.04]) and no consistent associations for hemorrhagic stroke. We observed no association between a vegetarian diet and total stroke (1.00 [0.76 to 1.32]), although the number of cases was small.
Lower risk of total stroke was observed by those who adhered to a healthful plant-based diet.
Harvard TH Chan School of Public Health material
Full Medscape report (Open access)
Neurology study (Restricted access)