Distribution of iron and its bioavailability from iron-fortified milk and formula

Abstract

Infant formulas are commonly fortified with iron, but the optimal form of fortification has not been determined. We have determined the distribution of added iron from various salts (Fe(II)SO 4, Fe(III) Citrate, or Fe(III) EDTA) added to milks and formulas at levels of 12, 30 or 60 mg/L. Milks and formulas used were iron-fortified and unfortified cow's milk formula, human milk, cow's milk, and soy formula. The bioavailability of iron from different iron salts added to cow's milk and cow's milk formula at a level of 12 mg/L was also determined. Forty-eight hours was shown to be the optimal time for allowing both isotope exchange of the extrinsic label in the milks and formulas and formula stability. The percentage of iron bound to the fat fraction did not change after the addition of the iron salts, but there was a redistribution of iron between the casein and the whey fractions. The distribution of iron did not change when Fe(II) sulfate or Na 2Sulfate was added, probably due to the low stability constant of Fe(II) Sulfate. As the stability constant of the iron salts and chelates formed increased (EDTA>Citrate>Sulfate) a higher percentage of the iron was found in the whey fraction. EDTA was able to remove iron from casein and soy protein. Suckling rat pups were dosed with 59Fe-labeled milk or formula; after 6 h, the animals were killed and individual tissues were counted for radioactivity. Regardless of the iron salt added, iron was more available from a low casein diet (Enfamil) than from cow's milk. Comparing the different iron salts used, iron from ferric citrate was significantly more available and iron from ferric ammonium citrate was significantly less available than the other salts used.