Insulin and Glucose Responses to Glucose Injection in Fed and Fasted Obese and Lean Sheep

Effects of short-term fasting on the insulin and glucose responses to injected glucose were determined in obese (n = 6) and lean (n = 6) Dorset ewes that were fed a maintenance level of energy intake. Sheep were assigned by Latin-square design to be fasted for 0 (fed), 12 or 24 h before glucose (350 mg/kg) was injected via jugular cannula at 2000 h with at least 7 d between successive tests. Insulin and glucose were quantified in jugular plasma samples. Pretreatment concentrations of insulin were affected (P < 0.005) only by body condition with higher mean values in obese (23.5 ± 33. µU/ml) than in lean (9.4 ± 1.0 µU/ml) sheep. Pretreatment concentrations of glucose (53.6 ± 1.8 mg/dl) were unaffected by body condition and fasting. The insulin responses to glucose, whether determined as absolute levels or response areas above base-line levels, were greater (P < 0.005) in obese than in lean sheep regardless of fasting period. Insulin and glucose concentrations after glucose injection in lean sheep were unaffected by fasting. In contrast, the insulin response to glucose was greater (P < 0.005) in fed obese than 12- or 24-h fasted obese sheep while glucose levels in the fed sheep were similar to those in the fasted obese sheep. Thus, factors associated with feeding enhanced the insulin response to glucose in obese sheep. In addition, obesity in sheep was associated with insulin resistance because basal hyperinsulinemia coexisted with euglycemia and because fractional removal rates of injected glucose were similar in obese and lean sheep despite much greater concentrations of insulin in obese sheep.