Background and purpose: Despite previous investigations, substantial uncertainty remains about the relation between body mass index (BMI) and stroke, especially in populations with a relatively low BMI but a high stroke rate.
Methods: A nationally representative prospective study of mortality included 212,000 Chinese men 40 to 79 years old without known cardiovascular disease in 1990 to 1991 who were followed up for 10 years. Standardized hazard ratios were calculated for stroke mortality by baseline systolic blood pressure (SBP) and BMI.
Results: Mean SBP and BMI were 124 mm Hg and 21.7 kg/m(2), respectively. During 10 years of follow-up, 5766 stroke deaths were recorded. There were strong, positive relations between BMI and SBP and between SBP and stroke mortality, with a 3-mm Hg higher baseline SBP associated with a 5.6% (95% CI, 5.3% to 6.0%; P<0.00001) higher stroke mortality. The association between BMI and stroke mortality was, however, not linear, with the hazard increasing substantially only for BMI >25 kg/m(2) (P<0.001 for nonlinearity). Approximately 90% of men had a baseline BMI <25 kg/m(2), and among them, BMI was not associated with stroke mortality despite its strong association with BP (which continued to a BMI <18 kg/m(2)). The relation with BMI was similar for ischemic and hemorrhagic stroke but appeared to be steeper among lifelong nonsmokers than among current smokers (P=0.01 for difference between slopes) despite similarly positive relations between BMI and SBP and between SBP and stroke risk in both smoking categories.
Conclusions: High BMI was strongly associated with increased stroke mortality only among men who were overweight or obese.