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 ∼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 ∼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.