To investigate whether oral intake of highly branched α-glucan isomaltodextrin (IMD) could stimulate ileal GLP-1 secretion, we examined 1) the digestibility of IMD, 2) the digestion and absorption rates of IMD, in rat small intestine, and 3) portal GLP-1 concentration in rats given IMD. In Experiment 1, ileorectostomised rats were given a 3% IMD diet for 10 days. Separately, a 16-h in vitro digestion of IMD, using porcine pancreatic α-amylase and brush-border membrane vesicles from rat small intestine, was conducted. In Experiment 2, upon 24-h fasting, rats were given any of glucose, IMD and high-amylose maize starch (HAMS) (1 g/kg of body weight). In Experiment 3, caecectomised rats were given 0.2% neomycin sulphate and a 5% IMD diet for 10 days. The in vivo and in vitro digestibility of IMD was 70-80%. The fraction of IMD digested in vitro for the first 120 min was 67% of that in maize starch. The area under the curve for 0-120 min of plasma glucose concentration was significantly lower in HAMS group and tended to be lower in IMD group than in the glucose group. Finally, we also observed that, when compared with control rats, glucose of IMD significantly stimulated and improved the concentration of portal active GLP-1 in antibiotic-administered, caecectomised rats. We concluded that IMD was slowly digested and the resulting glucose stimulated GLP-1 secretion in rat small intestine. Oral delivery of slowly released IMD glucose to the small intestine likely exerts important, yet unknown, physiological effects on the recipient. (count: 248 words).