A low-molecular-weight factor in human milk whey promotes iron uptake by Caco-2 cells

The iron bioavailability of human milk (HM) is substantially greater than that of cow's milk (CM), but the factor responsible for this high bioavailability is unknown. This study evaluated the effects of various HM and CM fractions on iron bioavailability. Milk was separated into fat, casein and whey fractions by ultracentrifugation. Whey was further fractionated by ultrafiltration with a 10-kDa membrane to produce a 10-kDa retentate (10kR) and a 10-kDa filtrate (10kF). Samples were prepared by mixing various combinations of the fractions, bringing the samples to prefractionation weight with minimum essential medium (MEM), and adding iron (10 micro mol/L) as ferrous sulfate. Samples were divided into two aliquots: one was subjected to in vitro digestion, the other was not. Bioavailability was assessed by applying the samples to Caco-2 cell monolayers and incubating for 24 h. Ferritin formation in the cells was used as an index of iron uptake. Removing the fat from undigested HM samples doubled the ferritin formation, but removing the whey or casein had no effect. Results with digested HM samples were similar, except that removing the whey decreased ferritin formation by 48%. Removing the fat from digested CM samples had no effect, but removing the casein doubled the ferritin formation. Removing the 10kF from HM reduced ferritin formation by 60%, but removing the 10kR had no effect. These data suggest that a low-molecular-weight factor (<10 kDa) in human milk enhances iron absorption.