Developing Staple Food Crops for Improved Nutritional Quality: Identification of Compounds in Bean Seed Coats That Inhibit and Enhance Fe Absorption.

Pulse crops such as beans have been targeted for Fe biofortification.  However, polyphenolic compounds in the seed coat of colored beans, generally considered to be inhibitors of Fe bioavailability, can actually be present in higher concentration in the Fe-biofortified lines and thus limit the nutritional benefit.  Research now indicates that not all polyphenols are inhibitors of Fe uptake and that some polyphenols can actually promote Fe absorption.  In this presentation, we present research on the content of specific major polyphenolic compounds in bean seed coats and their individual and combined effects on iron uptake by Caco-2 cells, an established screening tool for measuring Fe bioavailability. We demonstrate in this work that extracts from black bean seed coats strongly inhibited iron uptake.  Of the eight most abundant, non-anthocyanin polyphenols extracted from black bean seed coats, four compounds (catechin, 3,4-dihydroxybenzoic acid, kaempferol and kaempferol 3-glucoside) clearly promoted iron uptake.  The other four (myricetin, myricetin 3-glucoside, quercetin and quercetin 3-glucoside) inhibited iron uptake.  Tests of interactions between iron uptake-promoting and -inhibiting polyphenols showed that myricetin and myricetin 3-glucoside were  much more effective at inhibiting iron uptake than epicatechin was at promoting iron uptake.  In experiments replicating relative concentrations of the eight black bean polyphenols, progressive removal of inhibiting polyphenols led to a net promotion of iron uptake when the two most abundant inhibitors were withdrawn from the mixture.  Of further interest, some of the iron bioavailability-promoting compounds, specifically kaempferol and kaempferol glycosides, have also been linked to anti-obesity and anti-diabetic effects. Therefore, since obese individuals are also at increased risk to develop Fe deficiency, enhancing the levels of these Fe uptake promoting compounds in beans represents a potential breeding or processing strategy to increase the health promoting qualities of beans and possibly other staple food crops where such compounds are present.  Research funded by USDA.