Iron and Zinc Absorption from Two Bean (Phaseolus vulgaris L.) Genotypes in Young Women

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Abstract
Extrinsic and intrinsic iron and zinc labels were used to test iron and zinc absorption from two bean (Phaseolus vulgaris) genotypes, containing normal (common beans, CB) or higher (HFeZnB) iron and zinc concentrations, fed in single meals to young women with low iron reserves. The women were divided into two groups, with one receiving a CB test meal (n = 12) and the other, an HFeZnB test meal (n = 11). The beans were intrinsically labeled hydroponically with 55Fe (CB and HFeZnB) and with 70Zn (HFeZnB). Concentrations of zinc and iron were 98 and 65% higher, respectively, in HFeZnB as compared to CB, but phytic acid contents were similar. Extrinsic labels were 59Fe (CB and HFeZnB), 67Zn (CB), and 68Zn (HFeZnB). Iron and zinc percent absorption levels were calculated from radio-iron activity in red blood cells and from urinary excretion of zinc isotopes. Intrinsic and extrinsic iron absorption measures were highly correlated (R 2 = 0.986) (average extrinsic/intrinsic ratio was 1.00). Iron absorption was low (geometric mean < 2%) in both bean types, and total iron absorbed was not different between types. Intrinsic zinc absorption from the HFeZn beans was higher than extrinsic absorption (15.2% vs 13.4%, p < 0.05) (average extrinsic/intrinsic was 0.90). The correlation between intrinsic and extrinsic zinc measures was not as high as that for iron (R 2 = 0.719). Percent zinc absorption levels were similar in both bean types, but total extrinsic zinc absorbed was 90% higher (p < 0.05) from the HFeZnB meal. Thus, the less expensive and time-consuming extrinsic labeling may be used to screen various varieties of beans for iron bioavailability in humans, but it underestimates zinc absorption by ∼10%. Selective breeding for high-zinc bean genotypes may improve zinc status. However, high-iron genotypes appear to have little effect on iron status when fed alone in single meals to women with low iron reserves. Keywords: Iron and zinc bioavialability; humans; plant breeding; isotope labeling