A new study reveals that blood type, and in particular the ABO blood group, is related to a higher risk of stroke.

Researchers at the Harvard School of Public Health (HSPH; Boston, MA, USA) analyzed data from the US Nurses' Health Study (NHS), which included 61,973 women, and the Health Professionals Follow-up Study (HPFS), which included 27,808 men, two prospective longitudinal cohorts with 26 and 20 years of follow-up respectively.

Baseline characteristics were similar in each cohort, with an average age of 46 for women and 53 for men, and average body mass indexes (BMIs) of 24 kg/m2 and 25 kg/m2 for women and men, respectively. In the NHS group, there was a total of 1,995 participants who developed stroke, while in the HPFS group, 906 participants developed stroke.

Information on ABO blood group was self-reported, and were adjusted for age, smoking, BMI, alcohol intake, physical activity, aspirin use, ethnicity, family history of coronary heart disease (CHD), a history of hypertension, high cholesterol, and type 2 diabetes, menopausal status, and postmenopausal hormone use. At least 93% of participants in each blood type group (O, A, B, AB) were white. In both cohorts, the number of participants was greatest in O, followed by A, B, and then AB, which is consistent with the overall population.

The results showed that both men and women in the AB blood group were very significantly associated with a 26% increase for the risk of developing stroke, compared with those with type O blood. Women with type B had a 15% increase in stroke risk, but no such risk appeared in the male cohort. The study was limited by the population being mostly white, and it was underpowered to detect the associations with the subtypes of stroke (such as hemorrhagic). The study was presented at the American Heart Association (AHA) Scientific Sessions 2011, held during November 2011 in Orlando (FL, USA).

“The increased risk for women could be due to the larger sample size compared with men, or to chance,” said lead author and study presenter Lu Qi, MD, PhD. “The decreased risk in men was also surprising because, in other studies, men with type B blood were associated with increased coronary heart disease and myocardial infarction.”

The relative contribution of founder effects and natural selection to the observed distribution of human blood groups has been debated since blood group frequencies were shown to differ between populations almost a century ago. With the subsequent discovery of the complexity of the Rh blood groups, and the finding of many new independent blood-group systems, the outstanding example of association between blood groups and a disease is in hemolytic disease of the newborn. Other examples are stomach cancer, which is more common in people of group A than in those of groups O and B; duodenal ulceration, on the other hand, is more common in nonsecretors of ABH substances than in secretors.

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