Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2006
Publication Date: 11/7/2006
Citation: Hu, Y., Cheng, Z., Heller, L., Krasnoff, S.B., Glahn, R.P., Welch, R.M. 2006. Kaempferol in red and pinto bean seed (PHASEOLUS VULGARIS L.) coats inhibits iron bioavailability using an in vitro digestion/human CACO-2 cell model. Journal of Agricultural and Food Chemistry. 54:9254-9261. Interpretive Summary: Iron deficiency afflicts over 3 billion people globally, mostly resource-poor women and children. Beans are an important dietary source of iron. However, beans contain substances that can inhibit the amount of utilizable iron (i.e., bioavailable iron) in diets. We isolated, identified and determined the effects of two major seed coat polyphenols (kaempferol and astragalin) in colored beans on iron bioavailability using an in vitro Caco-2 cell model. These polyphenols were found to be potent inhibitors of iron bioavailability. White beans did not contain significant amounts of polyphenols and were better sources of bioavailable iron. Their seed coats were found to contain a potential promoter of iron bioavailability. Studies are underway to identify this white bean seed coat promoter. Knowing the identity of these substances in beans will allow the development of beans with higher levels of bioavailable iron making beans an important dietary source of iron.
Technical 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 colored beans. These differences in bioavailable iron were not due to bean-iron and bean-phytate levels. Flavonoids in the colored bean hulls were found to be contributing to the low bioavailability of iron in the colored beans. White bean hulls contained no detectible flavonoids but did contain a factor(s) that may promote iron bioavailability. The flavonoids, astragalin (kaempferol-3-O-glucoside) and kaempferol, were identified in red and pinto bean hulls via HPLC. Some unidentified anthocyanins were also detected in the black bean hulls but not in the other colored bean hulls. Kaempferol and quercitrin were shown to inhibit iron bioavailability, in which quercitrin was used as a proxy to study the effects of astragalin on iron bioavailability. Treating in vitro bean digests with 20 to 500 M kaempferol and quercitrin significantly inhibited iron bioavailability in a concentration dependent fashion. Thus, astragalin and kaempferol were found to be important inhibitory factors affecting iron bioavailability in the red and pinto beans studied.