Influence of ingested load on postprandial insulin secretion. Rôle of gastric inhibitory polypeptide (GIP)

Two standard mixed meals including the same ingredients, and with different caloric values were ingested by 10 normal subjects. The plasma gastric inhibitory polypeptide (GIP) and immunoreactive insulin (IRI) as well as blood glucose (BG) were assayed during these meals at 0 (beginning of the meal) and after 30, 60, 120 and 180 min. BG was not significantly different between the two meals. At 30 min, the GIP peak was significantly higher in the case of the meal with the highest caloric (HC) value (499.5 +/- 112.0 vs. 273.4 +/- 57.5 pg/ml). Insulin was higher as well, although not in a significant way. At 120 min, the IRI was significantly higher (63.0 +/- 9.8 vs. 34.4 +/- 6.2 microU/ml) in the case of the HC meal. The HC meal induced a significantly higher insulinogenic index (0.29 +/- 0.05 vs. 0.14 +/- 0.07 mU litre g-1 ml-1). Integrated IRI and GIP responses of the HC meal were significantly higher than those of the meal with the lowest caloric value (IRI : 7.9 +/- 1.1 vs. 4.9 +/- 0.6 mU ml-1 180 min-1; GIP : 53.3 +/- 20.5 vs. 28.2 +/- 9.9 ng ml-1 180 min-1). The early (30 min) augmentation of IRI secretion after the ingestion of a larger meal is related to the insulinotropic action of the enhanced GIP secretion. The reasons for the late IRI increase are not obvious from this experiment. They might be of neural, nutrient, and/or intestinal origin.