The association of body mass index with the risk of type 2 diabetes: a case-control study nested in an electronic health records system in the United States.

Objectives: Obesity is a known risk factor for type 2 diabetes (T2D). We conducted a case-control study to assess the association between body mass index (BMI) and the risk of being diagnosed with T2D in the United States.

Methods: We selected adults (≥ 18 years old) who were diagnosed with T2D (defined by ICD-9-CM diagnosis codes or use of anti-diabetic medications) between January 2004 and October 2011 ("cases") from an electronic health records database provided by an integrated health system in the Middle Atlantic region. Twice as many individuals enrolled in the health system without a T2D diagnosis during the study period ("controls") were selected based on age, sex, history of cardiac comorbidities or hyperinflammatory state (defined by C-reactive protein and erythrocyte sedimentation rate), and use of psychiatric or beta blocker medications. BMI was measured during one year prior to the first observed T2D diagnosis (for cases) or a randomly assigned date (for controls); individuals with no BMI measure or BMI < 18.5 kg/m2 were excluded. We assessed the impact of increased BMI (overweight: 25-29.9 kg/m2; Obesity Class I: 30-34.9 kg/m2; Obesity Class II: 35-39.9 kg/m2; Obesity Class III: ≥40 kg/m2), relative to normal BMI (18.5-24.9 kg/m2), on a T2D diagnosis using odds ratios (OR) and relative risks (RR) estimated from multiple logistic regression results.

Results: We included 12,179 cases (mean age: 55, 43% male) and 25,177 controls (mean age: 56, 45% male). We found a positive association between BMI and the risk of a T2D diagnosis. The strength of this association increased with BMI category (RR [95% confidence interval]: overweight, 1.5 [1.4-1.6]; Obesity Class I, 2.5 [2.3-2.6]; Obesity Class II, 3.6 [3.4-3.8]; Obesity Class III, 5.1 [4.7-5.5]).

Conclusions: BMI is strongly and independently associated with the risk of being diagnosed with T2D. The incremental association of BMI category on the risk of T2D is stronger for people with a higher BMI relative to people with a lower BMI.