Isotopic exchange in 59Fe labeled rice-based meals

The process of isotopic exchange between extrinsic 59Fe tracer and native nonheme Fe in a rice-based meal was studied by performing Sephadex G-50 column chromatography of soluble supernatant of (1) 59Fe-labeled meal after in vitro pepsin digestion and (2) duodenal aspirate obtained from a human subject fed a similarly labeled meal. The elution patterns of radioactivity and chemically determined nonheme Fe differed markedly when the supernatant of in vitro digested meal at pH 1.8 was analyzed, whereas the patterns were similar when pH was adjusted to 6.2 and also for duodenal aspirate at pH 7. Our results imply that complete isotopic exchange between 59Fe tracer and native nonheme Fe in rice-based meal does not take place in vitro or even during gastric digestion but that it is probably completed in the duodenum.