Aims: To test the effect of continuing metformin on weight gain and glycaemic control in patients with poorly controlled Type 2 diabetes who need to start insulin.
Methods: Patients with Type 2 diabetes on maximum tolerated oral agents referred for insulin conversion were recruited from hospital diabetes clinics into a double-blind randomized placebo-controlled trial. The 183 participants received metformin or placebo, titrated up to 2 g a day or maximum tolerated dose, with insulin started according to local practice. The main outcome measures were weight change over 12 months, HbA1c, insulin dose, frequency of hypoglycaemia, treatment satisfaction, and well-being.
Results: Over 12 months, metformin was associated with less weight gain than placebo [mean 6.1 kg vs. 7.6 kg; adjusted difference 1.5 kg (95% confidence interval 0.2-2.9); P=0.02], a greater reduction in HbA1c[1.5% vs. 1.3%; adjusted difference 0.5% (0.1-0.9); P=0.02] and a lower insulin requirement [62 units vs. 86; adjusted difference 25 units (15-34); P<0.001], but also more hypoglycaemia [relative risk of any episode 1.24 (1.02-1.1); P=0.03]. Treatment satisfaction improved more in patients on metformin than on placebo (P<0.001), as did the positive well-being score (P=0.02).
Conclusions: Metformin decreases weight gain, lowers insulin requirement, and improves glycaemic control, and should be continued in patients with Type 2 diabetes who transfer to insulin.