Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/1/2008
Publication Date: 8/1/2008
Citation: Jin, F., Glahn, R.P., Rutzke, M., Zhiqiang, C., Welch, R.M. 2008. Extrinsic Labeling Method May Not Accurately Measure Fe Absorption from Cooked Pinto Beans (Phaseolus vulgaris):Comparison of Extrinsic and Intrinsic labeling of Beans. Journal of Agricultural and Food Chemistry. 56(16):6881-6885. Interpretive Summary: A model for digestion has been developed in our lab that uses a simulated digestion and Caco-2 epithelial tissue cell culture monolayer to imitate the intestinal lining and absorption of minerals into our bodies. The model is used to measure iron uptake in cells. Human studies often add an isotopic labeled form of iron mixed with food (extrinsically labeled) and then Fe absorption studies are done by measuring how much extrinsic label was absorbed. Sometimes an intrinsically labeled food is used (a plant is grown in an isotonic solution and an isotopic form of iron is internally incorporated into the plant). This study uses the Caco-2 model to assess whether intrinsically or extrinsically labeled food reflects more accurately how much iron is absorbed. The results suggest that intrinsic labeling appears necessary to accurately measure Fe uptake from pinto beans.
Technical Abstract: Isotopic labeling of food has been widely used for the measurement of Fe absorption in determining requirements and evaluating the factors involved in Fe bioavailability. An extrinsic labeling technique will not accurately predict the total Fe absorption from foods unless complete isotopic exchange takes place between an extrinsically added isotope label and the intrinsic Fe of the food. We examined isotopic exchange in the case of both white beans and colored beans (Phaseolus vulgaris) with an in vitro digestion model. There are significant differences in 58Fe/56Fe ratios between the sample digest supernatant and the pellet of extrinsically labeled pinto bean. The white bean digest shows significantly better equilibration of the extrinsic 58Fe with the intrinsic 56Fe. In contrast to the extrinsically labeled samples, both white and red beans labeled intrinsically with 58Fe demonstrated consistent ratios of 58Fe/56Fe in the bean meal, digest, supernatant, and pellet. It is possible that the polyphenolics in the bean seed coat may bind Fe and thus interfere with extrinsic labeling of the bean meals. These observations raise questions on the accuracy of studies that used extrinsic tags to measure Fe absorption from beans. Intrinsic labeling appears necessary to accurately measure Fe bioavailability from beans.