Members of the Tzu Chi Buddhist communities in Taiwan who ate vegetarian diets showed markedly lower stroke risk than nonvegetarian members, researchers found.
In one cohort, vegetarians had lower ischemic stroke risk (HR 0.26; 95% CI 0.08–0.88). In a second, they showed a lower risk of both ischemic (HR 0.41; 95% CI 0.19–0.88) and hemorrhagic (HR 0.34; 95% CI 0.12–1.00) stroke, according to Chin-Lon Lin, MD, of Tzu Chi University in Hualien, and co-authors.
In a subgroup analysis, vitamin B12 intake appeared to modify the association between vegetarian diet and overall stroke (P interaction = 0.046), they reported in Neurology.
Most participants refrained from alcohol and tobacco, eliminating one source of possible confounding, although 15%-20% had used one or both prior to joining the Tzu Chi sect.
“Overall, our study found that a vegetarian diet was beneficial and reduced the risk of ischemic stroke, even after adjusting for known risk factors like blood pressure, blood glucose levels and fats in the blood,” Lin said in a statement. “This could mean that perhaps there is some other protective mechanism that may be protecting those who eat a vegetarian diet from stroke.”
Research about diet and stroke risk has produced mixed results. Among women, a Mediterranean diet has been tied to reduced stroke risk, and a meta-analysis has tied meatless diets to lower blood pressure.
“Vegetarian diets might offer even more benefit but for B12 deficiency, which increases the risk of stroke by raising plasma total homocysteine,” wrote J. David Spence, MD, of Western University in London, Canada, and Christy Tangney, PhD, of Rush University Medical Center in Chicago, in an accompanying editorial. Based on diagnostic markers, B12 deficiency is common among vegetarians. “The prevalence is high among lacto-ovo vegetarians: 45% or more in adults and infants, and higher in vegans,” Spence and Tangney pointed out.
In their study, Lin and colleagues followed 5,050 people in the Tzu Chi Health Study (cohort 1) and 8,302 people in the Tzu Chi Vegetarian Study (cohort 2) to identify stroke events in Taiwan’s National Health Insurance Research Database.
Most study participants were Tzu Chi volunteers, Buddhists who committed to community service, and who completed at least 2 years of training in Tzu Chi core values and stopped smoking and drinking alcohol to become certified volunteers. Cohort 1 was recruited from 2007 to 2009; cohort 2 was recruited in 2005. Mean follow-up time was 6 years for cohort 1 and 9 years for cohort 2.
At baseline, participants received a health exam, answered questions about their medical history, lifestyle habits, and diet, and completed a food frequency questionnaire.
About 30% of participants were full-time vegetarians in both cohorts. Overall, vegetarians ate more fiber and plant protein — including more nuts, vegetables, and soy — and less animal fat and protein than non-vegetarians. Both groups consumed similar amounts of eggs and fruit.
Baseline age in both cohorts was about 50. Of the vegetarians, only 25% were men, whereas nonvegetarians showed a more even gender split. Serum B12 levels were lower in vegetarians (26%) versus non-vegetarians (1%). Homocysteine elevation was more common in vegetarians (25% vs 8%).
In cohort 1, a total of 54 events occurred over 30,797 person-years. Stroke was lower among vegetarians (1.16 vs 1.99 per 1,000 person-years). An adjusted analysis showed risk reduction for ischemic stroke was 74% (hemorrhagic strokes were not analyzed in this cohort because only eight occurred).
In cohort 2, a total of 121 events occurred over 76,797 person-years. Vegetarians experienced lower risk of overall stroke (0.95 vs 1.88 per 1,000 person-years; HR 0.52; 95% CI 0.33–0.82), including about a 60% risk reduction for ischemic stroke and about a 65% risk reduction for hemorrhagic stroke.
“The low overall risk of stroke likely reflects the mean age at baseline of only 50 years in both cohorts and the relatively short duration of follow-up (5–7 years in cohort 1 and 9 years in cohort 2),” the editorialists observed.
A higher intake of vitamin B12 (≥2.4 μg daily) negated the observed benefit of reducing stroke risk among vegetarians in cohort 1, Spence and Tangney noted. Why this occurred is unclear; it may be that a “cutoff of 2.4 μg/d (usually regarded as sufficient to prevent hematologic abnormalities) may not be appropriate with regard to risk of stroke,” they suggested.
The findings contrast with the recent EPIC-Oxford study which found a higher stroke risk for vegetarians, driven largely by a higher rate of hemorrhagic stroke, Lin and co-authors noted. “Nearly 80% of vegetarians in the EPIC-Oxford study drank varying degrees of alcohol, but most participants in our studies avoid alcohol (about 6% of non-vegetarians and fewer than 1% of vegetarians were current drinkers at baseline),” they pointed out. Asian Buddhists also avoid garlic and other allium vegetables, they added.
Diet was assessed only at baseline; this was an important limitation of the study. Results may not apply to other populations with different smoking, drinking, and dietary habits. The study was observational, and unmeasured confounders may have influenced results.
The study was supported by the Taiwan Department of Health, the Buddhist Dalin Tzu Chi General Hospital, the Buddhist Tzu Chi Medical Foundation, and the Taiwan Ministry of Science and Technology.
The researchers reported no disclosures. The editorialists reported no disclosures.