The digestive chemistry of pinto bean iron : in vitro and cannulated swine models

Abstract

The solubility, oxidation, reduction, adsorption, and reactivity of iron are known to influence its bioavailability. The purpose of this study is to investigate the above factors in digestive mixtures using spectrophotometry and electron paramagnetic resonance (EPR) spectroscopy methods. Pinto beans were chosen as a food model since they have high iron content and the bioavailability of bean iron is sensitive to the addition of beverages and chemical additives. Pinto beans were digested in vitro and in swine fitted with gastric and duodenal cannulae. In addition to the soluble steady state Fe²⁺, which immediately reacts with ferrozine, the estimation of Fe³⁺ reduction in 10 minutes was found to be useful in examining the effect of beverages and additives on potential iron bioavailability. The low bioavailability of pinto bean iron is probably due to the presence of components which oxidize not only the iron naturally present but also exogenously added iron. When the chemistry of iron was compared in in vitro and in vivo digestive mixtures, the soluble iron estimations differed somewhat, yet, the redox distribution was quite similar with the additives used. Since results for in vitro and in vivo digestions were comparable, the Fe³⁺ and Fe²⁺ concentrations estimated at the end of in vitro digestion were used to predict the bioavailability of iron. Based on human absorption literature, absorption of 40% for Fe²⁺ and 13% for Fe³⁺ was assumed in estimating iron absorption from in vitro studies. The predicted order of pinto bean iron absorption with the additives is as follows: ascorbic + citric acids > orange juice > ascorbic acid > citric acid > control > tea. The reactivity of iron was further investigated using apotransferrin as a probe. EPR was used to quantitate the iron exchange and was found to be useful, especially when heterogeneous mixtures were used. Based on the reactivity of iron with apotransferrin, the enhancing effects of ascorbic acid and the inhibitory effects of phosvitin and tea were demonstrated. The parallel studies with swine and correlation with human absorption studies strongly support the continued use of in vitro methods in predicting the bioavailability of iron from food mixtures.