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.
Journal of Agricultural and Food Chemistry (2008) 56 (16) 6881–6885 [DOI: 10.1021/jf800658s]