Glucagon-like peptide-1 responses to carbohydrate, fat and protein in rats during the development of diet-induced obesity and the role of the distal small intestine.

The secretion of glucagon-like peptide-1 (GLP-1) is stimulated by luminal nutrients after meal ingestion. Diet-induced obesity (DIO) may affect nutrient-induced GLP-1 secretion in humans and rodent models. We previously demonstrated that mixed meal-induced GLP-1 secretion is enhanced in rats with DIO compared with normal rats. However, it is unclear to which nutrient the GLP-1 secretion is adaptively enhanced or reduced during the development of DIO. The present study investigated the effect of obesity on the GLP-1 secretion to individual nutrients and further on GLP-1 secretory functions of the proximal and distal small intestine in rats. Male Sprague-Dawley rats were fed a control diet or a high-fat diet with sucrose solution (HFS) for 4–5 weeks. GLP-1 responses to a single oral administration of a liquid diet, dextrin, soyabean oil or whey protein were examined after 4 weeks of dietary intervention. In addition, a liquid diet was administered to the proximal or distal small intestine of anaesthetised rats (control or HFS), and GLP-1 levels in the portal vein plasma were measured. In HFS-fed rats, GLP-1 secretion to dextrin, soyabean oil and whey protein slightly increased compared with those in normal rats. Furthermore, the GLP-1 response to liquid diet administration into the lumen was greater in the distal, but not proximal, small intestine of HFS-fed rats than that in control rats. In rats with DIO, GLP-1 secretion increased, regardless of the type of nutrient. Furthermore, the distal small intestine is responsible for adaptive enhancement of the GLP-1 secretion.