Kaempferol in Red and Pinto Bean Seed ( Phaseolus vulgaris L.) Coats Inhibits Iron Bioavailability Using an In Vitro Digestion/Human Caco‐2 Cell Model

Abstract

Four different colored beans (white, red, pinto and black beans) were investigated for factors affecting iron bioavailability using an in vitro digestion/human Caco-2 cell model. Iron bioavailability from whole beans, de-hulled beans and their hulls was determined. The results show that white beans contained higher levels of bioavailable iron compared to red, pinto and black beans. These differences in bioavailable iron were not due to bean-iron and bean-phytate concentrations. Flavonoids in the colored bean hulls were found to be contributing to the low bioavailability of iron in the non-white colored beans. The flavonoids, kaempferol and astragalin (kaempferol-3-O-glucoside), were identified in red and pinto bean hulls via HPLC and mass spectrometry. Kaempferol, but not astragalin, was shown to inhibit iron bioavailability. Treating in vitro white bean digests with 40, 100, 200, 300, 400, 500, and 1000 μM kaempferol significantly inhibited iron bioavailability (e.g., 15.5%, 40 μM and 62.8%, 1000 μM) in a concentration dependent fashion. Thus, seed coat kaempferol was identified as a potent inhibitory factor affecting iron bioavailability in the red and pinto beans studied. Results comparing the inhibitory effects of kaempferol, quercitrin and astragalin on iron bioavailability suggest that the 3′,4′-dihydroxy group on the B-ring in flavonoids is contributing to the lower iron bioavailability.