To describe the body mass index (BMI) distribution of children developing autoimmune type 1 diabetes (T1D) compared with the general population and to assess factors associated with BMI at T1D onset.

Children age 2-<19 years enrolled in the Pediatric Diabetes Consortium at 7 US pediatric diabetes centers at T1D onset were included. Eligibility for analysis required a diagnosis of T1D, ≥1 positive diabetes autoantibody, and availability of BMI within 14 days of diagnosis. BMI at diagnosis was compared with the general population as described by the 2000 Centers for Disease Control. Regression analysis was used to assess the association between BMI and various participant characteristics.

BMI scores for the 490 participants were slightly lower than the 2000 Centers for Disease Control population (P = .04). The median BMI percentile for age and sex was 48^th, 11% of the children were overweight (BMI ≥85^th and <95^th percentile), 8% obese (BMI ≥95^th and <99^th percentile), and 2% severely obese (≥99^th percentile), percentages that were comparable across age and sex groups. Higher BMI Z-scores were associated with African American and Hispanic race/ethnicity (P = .001) and lower hemoglobin A1c (P < .001), and diabetic ketoacidosis, age, and Tanner stage were not associated.

Although the BMI distribution in children developing autoimmune T1D was lower than that of the general population, 21% of children were obese or overweight. Youth who are overweight, obese, racial/ethnic minority, and/or present without diabetic ketoacidosis should not be presumed to have type 2 diabetes because many patients with autoantibody-positive T1D present with the same clinical characteristics.