Prospective study of body mass index and risk of stroke in apparently healthy women.

BACKGROUND: Obesity is an escalating pandemic in the United States, and its association with coronary heart disease is well understood. Several studies have found positive associations between body mass index (BMI) and stroke in men, but the association with stroke and its subtypes is less clear in women.

METHODS AND RESULTS: This was a prospective cohort study among 39 053 women participating in the Women's Health Study. BMI was measured as self-reported weight (in kilograms) divided by height (in meters) squared. Incident stroke was self-reported and confirmed by medical record review. We used the Cox proportional hazards model to evaluate the association between BMI and stroke. After a mean follow-up of 10 years, a total of 432 strokes (347 ischemic, 81 hemorrhagic, and 4 undefined) occurred. We found a statistically significant trend for increased risk of total and ischemic stroke across 7 BMI categories. With World Health Organization criteria, women who were obese (BMI > or =30 kg/m2) had hazard ratios of 1.50 (95% CI 1.16 to 1.94) for total stroke, 1.72 (95% CI 1.30 to 2.28) for ischemic stroke, and 0.82 (95% CI 0.43 to 1.58) for hemorrhagic stroke compared with women with BMI <25 kg/m2. Additional control for history of hypertension, diabetes, and elevated cholesterol substantially attenuated the hazard ratios for total and ischemic stroke. There was no effect modification for age, exercise, or smoking.

CONCLUSIONS: In this large prospective cohort study among women, BMI was a strong risk factor for total and ischemic stroke but not for hemorrhagic stroke. The association was highly mediated by hypertension, diabetes, and elevated cholesterol.