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In order to investigate the constituents responsible for the enhancing effect of meat on intestinal iron absorption in humans, two different types of peptic digestion extracts were prepared from 100 g of beef, in which the thiol groups of the resulting peptides were either oxidized (CYS-), or left untreated (CYS+). The absorption of radioiron mixed with 250 g of maize was more than twofold greater when consumed along with the CYS+ extract than with the CYS- (p less than 0.05). It is suggested that the enhancing effect of meat on nonheme iron absorption is due to cysteine, and that cysteine-containing peptides, rather than the free amino acid, are responsible for this effect.

One hundred thirteen subjects participated in iron absorption studies in which histidine, cysteine, reduced glutathione or beef were administered with test meals. Increasing doses of histidine from 416 to 2080 mg did not affect the absorption of 2 mg of corn iron. Reduced glutathione significantly increased the absorption from nonheme and heme iron present in black beans, corn and hemoglobin. Increasing doses of either cysteine or glutathione produce the same trend in the increase of absorption from corn iron as that observed from beef. The results suggest that the effect of cysteine on iron absorption is similar in the peptide form and as a single molecule.

The iron absorption from heme, ferritin and vegetable foods was tested in 63 humans (10 males and 53 females). The iron absorption from heme in beef exposed to prolonged heating was markedly reduced, following the degree of denaturation of the heme. The iron absorption from ferritin was about 31% of the absorption of the reference dose when administered with meat alone, falling to 11% of the reference dose when administered in a meal containing meat and vegetable foods. The absorption of ferritin iron was less than half that of vegetable iron. The data confirm a previous proposal that ferritin forms part of a subgroup of the non-heme iron pool.