The present study examined the acute effects of co-ingestion of fat (37.5 g) on the post-prandial metabolic responses to 75 g of carbohydrate which was either slowly absorbed (lentils) or rapidly absorbed (potatoes). Co-ingestion of fat resulted in a significant flattening of the post-prandial glucose curves, the effect being more pronounced for the rapidly absorbed potatoes. This was probably due to delayed gastric emptying. However, the post-prandial insulin responses to either carbohydrate were not significantly reduced by fat, suggesting that the insulin response to a given glucose concentration was potentiated in the presence of fat. The gastric inhibitory polypeptide (GIP) responses to both carbohydrates were greatly increased in the presence of fat. To investigate further the possible roles of GIP in the entero-insular axis, a 5-g bolus of glucose was injected intravenously 1 h after lentils +/- fat. This was sufficient to raise the glucose levels above the threshold reported for GIP to potentiate insulin secretion. However, despite the large differences in circulating GIP levels, the insulin response to glucose was not affected by the presence of fat. These results suggest that (1) the rate of absorption of carbohydrate is a major determinant of post-prandial metabolic responses even in the presence of fat, (2) fat-stimulated GIP secretion does not potentiate glucose-induced insulin secretion, and (3) the potentiation of the insulin response to glucose when carbohydrate is co-ingested with fat is consistent with the well-documented insulin resistance associated with high fat diets.