Previously, the physiological effects of dietary fiber in the small intestine have been discussed mainly on the basis of the interaction of dietary fiber with simultaneously ingested nutrients in the gastrointestinal tract, and only a limited number of studies have focused on events in the gastrointestinal tract itself. This review covers the results of analyses of small-intestinal goblet cell responses and mucin secretion during dietary fiber intake, based mainly on the bulk and viscosity of the dietary fiber. Mucin from the small intestine acts as an endogenous fiber that underpins the symbiotic relationship between the host and intestinal bacteria via short-chain fatty acids, a metabolic product of fermentation. Finally, the validity of the current Prosky digestion-based dietary fiber quantification method to estimate the gastrointestinal kinetics of resistant starch and digestion-resistant dextrin derivatives is discussed. These have recently attracted attention as new dietary fiber materials in addition to classic dietary fibers derived from plant cell walls.

Abdominal aortic aneurysm (AAA) is a disease characterized by progressive dilation of the abdominal aorta. Because details of the pathogenesis of AAA remain unknown, no effective therapeutic agent has been developed. Using human arterial and mouse abdominal aortic models, we have demonstrated for the first time that osteoclasts are present in these vessels, and that they are involved in the development of aneurysms. Hyperglycemia is also known to be a negative risk factor for the development of aneurysms, but no detailed analysis has been reported. When aneurysm formation was induced in a diabetic mouse model, macrophage activity was decreased by hyperglycemia. The formation of aneurysms was then suppressed, and it was clarified that liver x receptors (LXRs) were involved in this suppression. Furthermore, administration of puerarin, an isoflavone kudzu, significantly suppressed osteoclast differentiation and aneurysm formation in this murine aneurysm model. Based on these findings, it is expected that clinical research into the treatment of AAA targeting osteoclasts and preventive methods using food and nutritional components will be established in the future.

There is growing interest in the physiological functions of polyphenols. In this study, the author focused on poorly absorbable polyphenols such as procyanidins and theaflavin and their physiologically beneficial functions. These compounds are condensed tannins and have no functionality due to their poor bioavailability. It was found that procyanidins exhibited a novel mechanism whereby hyperglycemia and obesity were prevented by promoting the secretion of glucagon-like peptide-1 (GLP-1) from intestinal L cells. Such secretion of GLP-1 also had an ameliorating effect on vascular dysfunction by activating endothelial nitric oxide synthase. GLP-1 secretion triggered by these polyphenols was also involved in circadian rhythm. These results suggest that poorly absorbable polyphenols can exert various bioregulatory functions via a signaling network among multiple organs, and that these functions exhibit a specific timing.