Influence of melatonin administration on glucose tolerance and insulin sensitivity of postmenopausal women

OBJECTIVE The effect of melatonin on human carbohydrate metabolism is not yet clear. We investigated whether melatonin influences glucose tolerance and insulin sensitivity in aged women. PATIENTS Twenty-two postmenopausal women of whom 14 were on hormone replacement therapy. DESIGN After an overnight fast, at 0800 hours on two nonconsecutive days, placebo or melatonin (1 mg) were administered randomly and in a double blind fashion. Forty-five minutes later, an oral glucose tolerance test (75 g; OGTT) was performed in 13 women, In another nine women insulin-dependent (Si) and -independent (Sg) glucose utilization was tested by a frequently sampled intravenous glucose tolerance test (FSIGT). RESULTS Areas under the response curve to OGTT (AUC) for glucose (1420 +/- 59 vs. 1250 +/- 55 mmol x minn; P < 0.01), and C-peptide (420980 +/- 45320 vs. 33528 +/- 15779 pmol x min/l; P < 0.02) were higher following melatonin than placebo, while Si values were lower (2.6 +/- 0.28 units vs, 3.49 +/- 0.4 units; P < 0.03). Si modifications induced by melatonin were inversely related to Si values of the placebo day (r(2) = 0.538; P < 0.025). CONCLUSIONS The present results indicate that in aged women administration of 1 mg of melatonin reduces glucose tolerance and insulin sensitivity. The present data may have both physiological and clinical implications.