Food induces a more outspoken stimulation of insulin secretion than an equal amount of energy given IV. This incretin effect is due to gut-derived signals enhancing insulin secretion, with glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1) as foreground effectors.

Objective: 

We investigated effects of these peptides on gastric emptying, plasma glucose, insulin, and appetite.

Methods: 

Gastric emptying rate was measured with scintigraphy. Human subjects received GLP-1 0.75 pmol kg^-1min^-1, or GIP 2 or 5 pmol kg-1min-1 with saline controls. Blood was analyzed for glucose, insulin, glucagon, C-peptide, PYY, GIP and GLP-1. Hunger and satiety were measured with visual analog scales.

Results: 

GIP 2 and 5 pmol kg-1min-1 decreased half-emptying time, 93.3±6.3 and 85.2±11.0 vs. 128.5±34.0 min (p<0.05). GIP 5 pmol kg-1min-1 blunted postprandial plasma glucose (p<0.001) and insulin (p<0.05). GIP 5 pmol kg-1min-1 increased hunger, but not desire to eat, satiety or prospective consumption. GLP-1 increased half-emptying time to 329.4±71.6 vs 76.6±7.6 min (p<0.01) with accumulation of the meal in the antrum. GLP-1 lowered plasma glucose (p=0.007) and decreased the insulin response and expressed increased satiety, less hunger and less desire to eat.

Conclusion: 

An incretin effect seems primarily to be substantiated for GLP-1 with retention of food in the gastric antrum and lowered prandial plasma glucose. For comparison, GIP increased gastric emptying and reduced insulin and plasma glucose making it less likely to be a true incretin.