In food science, natural ingredients that can inhibit dipeptidyl peptidase IV ( DPP IV ) may be useful for preventing diabetes mellitus. This study aimed to evaluate the effectiveness of bonito dashi having a high DPP IV inhibitory activity (IC50 ; 3049 µg/ ml) on the postprandial increase in blood glucose levels in 14 subjects. Bonito dashi (5 g) was subsequently subjected to oral glucose tolerance tests. Blood glucose levels of all subjects were measured at fasting and at 30 min after ingesting of bonito dashi or of warmed tap water as a control, and were also measured at 30, 45, 60, 75, 90, 120, and 150 min after ingestion of cooked rice. The maximum blood glucose level between 0 and 150 min after ingesting of bonito dashi was for 30 min, of warmed tap water for 45 min ). The blood glucose levels after ingestion of bonito dashi and warmed tap water were 135.6 ± 8.7 mg/ dl and 140.3 mg/ dl, respectively at 30 min ( p = 0.602 ); 135.6 ± 8.7 mg/ dl and 144.1 ± 10.7 mg/ dl, respectively at 45 min ( p = 0.057 ); 120.0 ± 4.9 mg/ dl and 136.8 ± 7.8 mg/ dl, respectively at 60 min ( p = 0.063 ); 110.0 ± 5.9 mg/ dl and 134.9 ± 6.9 mg/ dl, respectively at 75 min ( p = 0.006 ); 110.3 ± 6.8 mg/ dl and 129.3 ± 6.6 mg/ dl, respectively, at 90 min ( p = 0.036 ); 103.4 ± 4.1 mg/ dl and 118.7 ± 8.0 mg/ dl, respectively, at 120 min ( p = 0.091 ); 91.5 ± 3.8 mg/ dl and 102.3 ± 5.9 mg/ dl, respectively, at 150 min ( p = 0.232 ). The area under the curve for blood glucose levels after ingestion of bonito dashi and warmed tap water was 4753.1 ± 439.7 mg/ dl ×min and 6879.4 ± 728.1 mg/ dl ×min, respectively ( p = 0.005 ). Postprandial increase in blood glucose levels was lower in subjects ingestion of bonito dashi than in those ingestion of the warmed tap water. No serious adverse events related to ingestion of bonito dashi were observed. Our findings suggested that the ingestion of bonito dashi (5 g) suppressed postprandial increase in blood glucose levels in our subjects.

Dry solid matter (rutin content: 51.6 mg/g; quercetin content: 72.2 mg/g) extracted from Tartary buckwheat boiled noodles using 70% methanol as the solvent was found to have α-glucosidase inhibitory activity. As for fractions fractionated by silica gel column chromatography, the fractions rich in quercetin and rutin showed remarkable α-glucosidase inhibitory activity. Tartary buckwheat boiled noodles used as samples in this study contained quercetin produced from rutin by the action of rutinase, suggesting that both rutin and quercetin contained were involved in the α-glucosidase inhibitory activity of the dry solid extract. Changes in postprandial blood glucose levels were compared for boiled noodles made from two types of buckwheat (i.e., Tartary buckwheat and common buckwheat), revealing that blood glucose elevation after eating Tartary buckwheat boiled noodles was suppressed. The blood glucose level 40 minutes after eating Tartary buckwheat boiled noodles was significantly low (p<0.05). It can be concluded that this might be caused by the α-glucosidase inhibitory activity of rutin (270.0 mg) and quercetin (330.5 mg), which correspond to a total amount of 935 mg of rutin equivalents, in the gastrointestinal tract. As a result, the digestion of carbohydrates contained in the samples consumed and their absorption by the intestine might be inhibited, resulting in the suppression of increases in blood glucose levels. The presence of a certain amount of quercetin was considered to be key to the suppression of blood glucose elevation. It is important to control rapid postprandial blood glucose increases to prevent diabetes from developing or becoming serious. This study suggests the potential for Tartary buckwheat boiled noodles to contribute to diabetes prevention.