Introduction: Bad nutritional habits and administration of insulin in supraphysiological doses lead to the development of insulin resistance and poor metabolic control in patients with type 1 diabetes. Accumulation of visceral fat is the main cause of the decrease in insulin sensitivity.

Objectives: We aimed to evaluate changes in anthropometric parameters, indirect measures of insulin resistance, and safety of treatment with metformin added to intensive insulin therapy in patients with type 1 diabetes and excess body fat.

Patients and methods: We analyzed 114 patients (60 women and 54 men; median age, 31 years [range, 18–60 years]), with a median diabetes duration of 14 years (range, 10–20 years). Metformin was administered for at least 6 months in 74 patients, while 40 patients did not receive metformin. The study group was randomized in a 2:1 ratio. Total body fat assessment and laboratory tests were performed before the study and at 6‑month follow‑up.

Results: At 6 months, in the metformin group, compared with the non–metformin group, an improvement was noted for adiposity parameters (reduction in body mass index, –0.4 kg/m2 vs 0.6 kg/m2, P = 0.006; waist circumference, –5 cm vs 3.5 cm, P = 0.02; and total body fat, –1.7 kg vs 1.4 kg; P <0.001; glycated hemoglobin A1c: –0.6% vs 0.2%, P <0.001), as well as for lipid parameters and blood pressure. An increase in the estimated glomerular filtration rate was greater in the metformin compared with the non–metformin group: 0.9 mg/kg/min vs –0.2 mg/kg/min, P <0.001).

Conclusions: In patients with type 1 diabetes and excess body fat, treated with intensive functional insulin therapy, the addition of metformin improves metabolic control of diabetes at 6 months. Metformin added to insulin therapy in patients with type 1 diabetes and excess body fat appears to be safe.