1. In the management of the non-insulin dependent diabetic patient, metformin often facilitates weight loss whereas sulphonylurea may predispose to weight gain. To investigate whether this is due to alterations in energy expenditure we have studied energy expenditure in seven non-insulin dependent diabetic subjects while on metformin or sulphonylurea therapy.

2. Three components of energy expenditure were measured by indirect calorimetry, namely resting metabolic rate and the thermic responses to infused noradrenaline and to a mixed constituent meal.

3. There was no significant difference in the resting metabolic rate on metformin (5.29 ± 0.41 kJ/min; mean ± se) compared with sulphonylurea (5.34 ± 0.34 kJ/min). The resting metabolic rate was also similar to predicted values for non-diabetic subjects (r = 0.96).

4. The thermic response to infused noradrenaline was similar on metformin (23.14 ± 1.87 kJ) and sulphonylurea (21.40 ± 2.98 kJ).

5. There was no significant difference in the thermic response to the meal on sulphonylurea (75.8 ± 7.5 kJ) or on metformin (86.8 ± 10.8 kJ; 95% confidence limits − 17 to + 39 kJ).

6. We conclude that in non-insulin dependent diabetic subjects metformin does not enhance energy expenditure overall, compared with sulphonylurea.

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