Iron absorption by rats fed tannins extracted from bean hulls

Abstract

Nutritional balance and whole-body radioassay procedures were used to assess the effects of tannins extracted from hulls of beans ( Phaseolus vulgaris) on iron (Fe) absorption by male rats fed test meals labeled extrinsically with 59Fe. Tannin in single meals did not affect 59Fe absorption by moderately anemic, growing rats when 59Fe was added directly to the whole meal or when it was added to casein before it was incorporated into the meal. However, addition of 59Fe to either bean tannin or to tannic acid prior to including it in the meal depressed 59Fe absorption compared to that from tannin-free meals. In growing, anemic rats fed diets with O, .06, .125, .25 or .5% tannin before and after test meals, 59Fe absorption averaged about 82, 81, 72, 72 and 71% of the dose, respectively; absorption of 59Fe by rats fed diets with either O or .06% tannin was greater than that of rats fed the other diets. These data suggest a threshold effect of tannins on inhibition of Fe absorption. Similarly, compared to rats fed a tannin-free diet, 59Fe absorption was depressed in growing, Fe-adequate rats fed a diet with .5% tannin. In contrast, tannins did not affect 59Fe absorption by mature rats. Although some impairment in Fe absorption in growing rats resulted from consumption of tannin-containing diets for several days, bean tannins consumed as part of typical diets probably have little adverse effect on Fe absorption.