Summary Several studies have suggested that resistant maltodextrin (RMD) suppresses intestinal lipid absorption in experimental animals and humans. However, possible mechanisms underlying this effect are not known. In this study, effects of RMD on processes of the absorption of various lipids were investigated in vitro. RMD dose-dependently suppressed the solubility of various lipid components, including 1-mono-oleoylglycerol, oleic acid, and phosphatidylcholine in bile salt micelles in vitro. When the diffusion rate of bile salt micelles through a filter membrane was investigated in vitro, bile salt micelles containing RMD diffused more slowly than those without RMD. Incorporation of [1][2][3][4][5][6][7][8][9][10][11][12][13][14] C] oleic acid into Caco-2 cells from the RMD-containing bile salt micelles was significantly smaller than that from the control micelles (without RMD). These results show that RMD suppresses intestinal absorption of lipids by decreasing their micellar solubility and the diffusion rate of bile salt micelles. Key Words resistant maltodextrin, lipid absorption, bile salt micelles, Caco-2 cells Because postprandial hypertriacylglycerolemia is one of the risk factors of coronary heart disease (1), its prevention and suppression may be important for human health. Given that the postprandial phase is characterized by the presence of plasma triacylglycerol-rich lipoproteins of intestinal origin (1), attenuation of dietary fat absorption could be an effective way to prevent postprandial hypertriacylglycerolemia.Resistant maltodextrin (RMD) is a low-viscosity viscous soluble dietary fiber and is produced by a combination of heat and enzymatic treatment of cornstarch (2). RMD is a mixture of short-chain polymers of glucose with a-1,2 and a-1,3 glucosidic bonds in addition to a-1,4 and a-1,6 bonds ( Fig. 1). Research has shown that dietary RMD can decrease serum cholesterol (3) and triacylglycerol (TAG) concentrations (4). It has been suggested that these functions can be ascribed to its suppressive effects on dietary lipid absorption (3, 4). Recently, Kishimoto et al. (5) reported that postprandial blood TAG elevation is significantly suppressed in rats treated orally with a fat emulsion containing RMD and in humans given RMD after a meal. They also showed that dietary RMD increases fecal excretion of fat in rats and humans without inhibiting pancreatic lipase activity (6). The results suggest that RMD suppresses lipid absorption in rats and humans. This can be a strategy to suppress postprandial serum TAG concentration. However, the precise mechanisms of the suppressive effects of RMD on lipid absorption are not clear.It is known that hydrolyzed products of lipids are incorporated into bile acid micelles in the small intestine, and the micelles diffuse to the intestinal epithelial cell surface through the unstirred water layer (7). Thus, the solubilization of these components in bile salt micelles and the rate of diffusion of bile salt micelles are crucial factors for lipid absorption and diges...