Body mass index and mortality in China: a 15-year prospective study of 220 000 men

Background In China, there have been few large prospective studies of the associations of body mass index (BMI) with overall and cause-specific mortality that have simultaneously controlled for biases that can be caused by pre-existing disease and smoking. Methods Prospective cohort study of 224 064 men, of whom 40 700 died during follow-up between 1990-91 and 2006. Analyses restricted to 142 214 men aged 40-79 years at baseline with no disease history and, to further reduce bias from pre-existing disease, at least 5 years of subsequent follow-up, leaving 17 800 deaths [including 4165 stroke, 1297 coronary heart disease (CHD), 3121 chronic obstructive pulmonary disease (COPD)]. Adjusted hazard ratios (HRs) and 95% confidence intervals (95% CIs) per 5 kg/m(2) calculated within either a lower (15 to < 23.5 kg/m(2)) or higher (23.5 to < 35 kg/m(2)) range. Results The association between BMI and all-cause mortality was U-shaped with the lowest mortality at similar to 22.5-25 kg/m(2). In the lower range, 5 kg/m(2) higher BMI was associated with 14% lower mortality (HR 0.86, 95% CI 0.82-0.91); in the upper range, it was associated with 27% higher mortality (HR 1.27, 95% CI 1.15-1.40). The absolute excess mortality in the lower range was largely accounted for by excess mortality from specific smoking-related diseases: 54% by that for COPD, 12% other respiratory disease, 13% lung cancer, 11% stomach cancer. The excess mortality in the upper BMI range was largely accounted for by excess mortality from specific vascular diseases: 55% by that for stroke, 16% CHD. In this range, 5 kg/m(2) higher BMI was associated with similar to 50% higher mortality from stroke (HR 1.61, 95% CI 1.36-1.92) and CHD (HR 1.48, 95% CI 1.12-1.95). Conclusions For China, previous evidence may have overestimated the excess mortality at low BMI but underestimated that at high BMI. The main way obesity kills in China appears to be stroke.