Metformin does not increase energy expenditure of brown fat

The antihyperglycaemic agent metformin (dimethylbiguanide) increases metabolic energy expenditure. The possibility that this might involve increased fuel utilization by brown adipose tissue (BAT) was studied. Basal and noradrenaline-stimulated (10(-6) mol/L) oxygen consumption by isolated brown adipocytes from normal and streptozotocin diabetic mice was unaffected by incubation with metformin (10(-4) mol/L) for 30 min. Basal and insulin-stimulated (10(-8) and 10(-6) mol/L) aerobic and anaerobic glucose metabolism was also unaffected during 2 hr incubations of BAT with metformin (10(-4) mol/L). Treatment of normal mice with metformin (250 mg/kg/day) for 7 days did not significantly alter the amount or cytochrome oxidase activity of BAT, and a preliminary semi-quantitative estimation of mitochondrial uncoupling protein showed no apparent change. The results suggest that metformin does not increase energy expenditure of brown adipose tissue.