Background Obese patients with type two diabetes mellitus (T2DM) may have a better prognosis than patients of normal weight, but reports are limited by study size, duration and confounders.

Methods We investigated the relationship between body mass index (BMI) and prognosis in 10,568 patients with T2DM without known CV disease followed for a median of 10.6 (IQR: 7.8–13.4) years. Hospital admissions for acute coronary syndrome (ACS), cerebrovascular accidents (CVA) and heart failure (HF) and all-cause mortality were recorded. Information on comorbidity (cancer, lung diseases and chronic renal failure) was also collected as confounder.

Results Median age was 63 ± 19 years, 54% were men and median BMI was 28.8 (IQR: 25.2–32.4 kg.m²). Adjusting for differences in age and other variables, patients in the highest quartile of BMI had the greatest risk of hospitalisation for ACS or HF (HRs with 95% CI for highest versus lowest quartile were 1.39 1.14–1.70; p = 0.001 and 1.36 1.08–1.71; p = 0.01 respectively) but the risk of CVA was similar amongst BMI quartiles. However, patients in higher BMI quartiles had lower mortality rates, with the nadir of risk at BMI 25–30 kg/m² (adjusted for confounders). Greater BMI was associated with lower mortality especially amongst older patients.

Conclusions Patients with T2DM and moderately increased BMI, in the overweight range of 25–30 kg/m², have a lower mortality than slimmer patients. Amongst older patients, even higher BMI may be associated with lower risk. However, obesity is associated with an increased risk of hospitalisation for cardiovascular events.