OBJECTIVE: To test the hypothesis that metformin therapy, given as an adjunct to insulin therapy, improves metabolic control in insulin-treated NIDDM patients with suboptimal glycemic control.
RESEARCH DESIGN AND METHODS: A total of 33 subjects with insulin-treated NIDDM were investigated; all had commenced insulin after secondary failure of antihyperglycemic agents. Two randomized double-blind placebo-controlled crossover studies were run. In study 1 (n = 19), insulin-treated subjects with suboptimal glycemic control received 12 weeks of metformin 1 g b.i.d. and 12 weeks of placebo. In study 2 (n = 14), subjects already established on adjunctive metformin/insulin therapy stopped the metformin component and received 12 weeks of metformin at their baseline dosage (range 1-2.5 g) and 12 weeks of equivalent placebo. Fasting plasma glucose, HbA1c, and serum lipids were measured at baseline and midway through and at the end of each treatment phase. The effect of 12 weeks of metformin treatment was compared with the effect of 12 weeks of placebo in each study and in both studies combined.
RESULTS: In study 1, metformin treatment was associated with significant improvements in fasting plasma glucose (mean 12-week difference from placebo [95% CI]: 5.8 mmol/l [3.5-8.1], P < 0.001) and HbA1c (1.6% [0.9-2.4], P < 0.001). In study 2, metformin treatment was associated with significantly lower fasting plasma glucose (5.3 mmol/l [0.6-9.9], P = 0.029) and lower HbA1c (2.4% [1.0-3.8], P = 0.003) compared with those for placebo. Study 2 also showed metformin treatment to be associated with significantly lower total cholesterol than that for placebo (1.0 mmol/l [0.1-1.9], P = 0.032) and lower LDL cholesterol (1.0 mmol/l [0.1-1.9], P = 0.028). This significant difference in serum lipids seen in study 2 was not seen in study 1, but was present when both sets of data were combined (n = 33, mean total cholesterol difference at 12 weeks [95% CI]: 0.6 mmol/l [0.1-1.1], P = 0.015). Metformin had no significant effect on triglyceride, HDL cholesterol, weight, or blood pressure. Two subjects on metformin withdrew because of side effects.
CONCLUSIONS: Metformin, when given as adjunctive therapy, was well tolerated and improved glycemic control and lipid concentrations in patients with insulin-treated NIDDM whose diabetes was poorly controlled. These improvements could be maintained over the long term.
