OBJECTIVES: The two objectives were 1) to evaluate the solubility of two iron casein complexes (ICCs) under a condition mimicking gastric pH, 2) to evaluate the impact of ascorbic acid (AA) on the in vitro iron absorption of ICCs after incorporation in reconstituted whole milk powder. METHODS: The in vitro solubility was determined over time after addition of diluted HCl (pH 1.7), ultracentrifugation and measurement of iron appearing in the supernatant by ICP-OES (n = 2). The impact of AA on iron uptake from the Fe compounds in reconstituted milk was determined using the in vitro digestion coupled with the Caco-2 cell model and the measurement of ferritin/total protein produced by the cells (n = 3). The molar ratio of AA to iron of 2 to 1 recommended by the WHO for iron absorption optimization has been tested with an iron level corresponding to 3.3 mg Fe/serving of milk. Ferrous sulfate (FeSO(4)), the reference compound for iron bioavailability and micronized ferric pyrophosphate (FePP), main salt used for milk fortification were used as references. RESULTS: The dissolution test showed a rapid solubilization of iron from the ICCs i.e., >75 ± 19.3% at 5 min and >89 ± 0.3% at 90 min. The kinetics of soluble iron from the complexes were like that from FeSO(4). The solubility of FePP was only 37.6 ± 4.7% at 90 min. Without AA, the iron uptake from FeSO(4) was lower than expected translating into a relative in vitro bioavailability (iRBA) of FePP and of the two ICCs to FeSO(4) of 66, 169 and 215%. This might be explained by a rapid conversion of soluble iron from FeSO(4) into Fe(3+) and insoluble iron hydroxide when the pH increased from 2 to >7 during in vitro digestion. However, with the addition of AA in the milk, iron uptake by the cells was found to be increased to levels of 341.8 ± 8.9, 124 ± 12.2, 403.1 ± 117.8 and 362.9 ± 36.9 ng ferritin/mg protein for FeSO(4), FePP and the two ICCs respectively. This translates into iRBAs to FeSO(4) of 36% for FePP and of 118 and 106% for the two ICCs. CONCLUSIONS: The solubility and the demonstrated impact of AA on Fe uptake suggest that ICCs are absorbed to a similar amount as FeSO(4) and thus provide an excellent source of Fe. FUNDING SOURCES: Société des Produits Nestlé, NPTC Konolfingen, Switzerland.
