Measurement of iron absorption and iron gains from birth to 6 months in breastfed and formula-fed infants using iron isotope dilution.

Little is known about iron kinetics in early infancy. We administered stable iron isotopes to pregnant women and used maternal-fetal iron transfer to enrich newborn body iron. Dilution of enriched body iron by dietary iron with natural isotopic composition was used to assess iron kinetics from birth to 6 months. In breastfed (BF, n = 8), formula-fed (FF, n = 7), or mixed feeding (MF, n = 8) infants, median (interquartile range) iron intake was 0.27, 11.19 (10.46-15.55), and 4.13 (2.33-6.95) mg/day; iron absorbed was 0.128 (0.095-0.180), 0.457 (0.374-0.617), and 0.391 (0.283-0.473) mg/day (BF versus FF, P < 0.01); and total iron gains were 0.027 (-0.002-0.055), 0.349 (0.260-0.498), and 0.276 (0.175-0.368) mg/day (BF versus FF, P < 0.001; BF versus MF, P < 0.05). Isotope dilution can quantify long-term iron absorption and describe the trajectory of iron depletion during early infancy.