When is fasting really fasting? The influence of time of day, interval after a meal, and maternal body mass on maternal glycemia in gestational diabetes.

Objective: The object of the study was to determine whether time of day, interval after a standard meal, and maternal body mass influence plasma glucose concentrations in women with gestational diabetes mellitus.

Study Design: Identical mixed meals were administered on 2 separate occasions 1 week apart to 30 women with dietarily treated gestational diabetes and pregnancies between 28 and 38 weeks' gestation. One meal was administered at 7 AM (morning meal) and the other was administered at 9 PM (evening meal), each after a fast of >/=5 hours. The order of the meals (morning first versus evening first) was assigned randomly. Sixteen of the women had a body mass index >/=27 kg/m(2) (overweight) and 14 women had a body mass index <27 kg/m(2) (lean). Venous plasma concentrations of glucose, insulin, free fatty acids, beta-hydroxybutyrate, and bound and free cortisol were measured hourly for 9 hours after each of the test meals.

Results: When all women were considered together glucose concentrations after the morning meal were significantly greater at 1 hour, were not different at 2 hours, and were significantly lower from 3 through 9 hours postprandially than those at corresponding times after the evening meal. Plasma beta-hydroxybutyrate and free fatty acid concentrations were higher between 5 and 9 hours after the morning meal than at the same times after the evening meal. Total and free cortisol levels were higher for the first 7 hours after the morning feeding, reflecting known diurnal variation in cortisol concentrations. Overweight patients' glucose values were significantly greater than those of lean subjects during the last 4 hours of the overnight fast.

Conclusions: Among women with dietarily treated gestational diabetes the glucose concentrations were significantly higher from 3 to 9 hours after an evening meal, whereas suppression of free fatty acids and beta-hydroxybutyrate was less sustained after a morning feeding. The mechanisms underlying these differences remain to be determined but may involve diurnal influences of counterregulatory hormones. The relationships between measurements of maternal glycemia and maternal and perinatal outcomes in pregnancies complicated by gestational diabetes may be clarified by establishing a uniform duration of a fast and by developing meal-specific preprandial and postprandial maternal glucose targets for these patients.