ObjectiveBody weight-related insulin resistance probably plays a role in progression to type 1 diabetes, but has an uncertain impact following diagnosis. In this study, we investigated whether BMI measured at diagnosis was an independent predictor of C-peptide decline 1-year post-diagnosis.DesignMulticentre longitudinal study carried out at diagnosis and up to 1-year follow-up.MethodsData on C-peptide were collected from seven diabetes centres in Europe. Patients were grouped according to age at diagnosis (<5 years, n=126; >5 years <10 years, n=295; >10 years <18 years, n=421; >18 years, n=410). Linear regression was used to investigate whether BMI was an independent predictor of change in fasting C-peptide over 1 year. Models were additionally adjusted for baseline insulin dose and HbA1c.ResultsIn individuals diagnosed between 0 and 5 years, 5 and 10 years and those diagnosed >18 years, we found no association between BMI and C-peptide decline. In patients aged 10–18 years, higher BMI at baseline was associated with a greater decline in fasting C-peptide over 1 year with a decrease (β 95% CI; P value) of 0.025 (0.010, 0.041) nM/kg per m2 higher baseline BMI (P=0.001). This association remained significant after adjusting for gender and differences in HbA1c and insulin dose (β=0.026, 95% CI=0.0097, 0.042; P=0.002).ConclusionsThese observations indicate that increased body weight and increased insulin demand are associated with more rapid disease progression after diagnosis of type 1 diabetes in an age group 10–18 years. This should be considered in studies of β-cell function in type 1 diabetes.