Objective Several studies have found a U-shaped association between body mass index (BMI) and mortality in the general population. In similar studies among people with diabetes, the shape of the association is inconsistent. We investigated the relationship of BMI and waist circumference with mortality among people with diabetes.

Setting The Third National Health and Nutrition Examination Survey (NHANES III) and the 1999–2004 NHANES Mortality Studies were designed to be representative of the US general population. Baseline data were collected in 1988–2004.

Participants 2607 adults ≥20 years of age with diabetes.

Primary Outcome Measure Participants were followed through 31 December 2006 for mortality (n=668 deaths).

Results Compared with people with a BMI 18.5–24.9 kg/m², the HRs (95% CI) of mortality were 0.85 (0.60 to 1.21) for 25–29.9 kg/m², 0.87 (0.57 to 1.33) for 30–34.9 kg/m² and 1.05 (0.72 to 1.53) for ≥35 kg/m² after adjustment for age, sex, race-ethnicity, smoking status, education, income and diabetes duration. Compared with people in the lowest sex-specific quartile of waist circumference, the adjusted HRs (95% CI) of mortality were 1.03 (0.77 to 1.37) for the second quartile, 1.02 (0.73 to 1.42) for the third quartile and 1.12 (0.77 to 1.61) for the highest quartile of waist circumference. When modelled as a restricted quadratic spline with knots at the 10th, 50th and 90th centiles, BMI and waist circumference were not associated with mortality. Several sensitivity analyses were conducted and most found no significant association between measures of adiposity and mortality, but there were significant results suggesting a U-shaped association among people in the highest tertile of glycated haemoglobin (≥7.1%), and there was an inverse association between BMI and mortality among people 20–44 years of age.

Conclusions In a nationally representative sample of the non-institutionalised US population with diabetes, BMI and waist circumference were not associated with risk of mortality.