The absorption and retention of magnesium, zinc, and copper by low birth weight infants fed pasteurized human breast milk

Abstract

Using serial metabolic balance techniques, the absorption and retention of magnesium, copper, and zinc have been measured in six preterm infants (mean gestation 29 weeks), and two term small for gestational age infants (<1.5 kg birth weight). The amounts of the minerals retained were compared with the amounts retained by a fetus of equivalent gestational age in utero. The preterm infants absorbed 5.3 ± 3.5 mg/kg day magnesium (43% of intake, range 1-74%) and retained 2.8 ± 1.3 mg/kg day (25% of intake, range -15 to +48%). The mean postnatal retention over the whole study period, as a percentage of intrauterine accumulation by a fetus of equivalent maturity, was 41% (range 17-66%). The light for dates infants absorbed 5.7 mg/kg day, range 4.0-7.2 (81% of intake, range 51-95%) and retained 5.4 mg/kg day, range 3.3-6.6 (76% of intake, range 41-88%). In the preterm infants balance for zinc was negative on average until the 60th day of life. The most negative balance averaged -0.611 ± 0.432 mg/kg day (n=6) on days 20-22. All the preterm infants experienced a net loss of body zinc over the period of study (mean -18.1 mg, range -4.2 to -37 mg.) In contrast the light for dates infants were in positive balance and they absorbed 0.44 mg/kg day, range -0.06 to +0.75 (56% of intake, range -35 to +86%) and retained 0.42 mg/kg day, range -0.08 to +0.73 (52% of intake, range -46 to +82%). In the preterm infants balance for copper was negative on average until the 35th day of life, and the most negative balance was -0.35 ± 0.061 mg/kg day on days 10-12. Three of the six preterm infants, during the period of study, experienced a net loss of body copper which averaged 0.73 mg (range -1.3 to -0.3 mg); the other three retained on average 1.74 mg (range 0.12 to 3.2 mg). The light for dates infants absorbed and retained 0.045 mg/kg day, range 0.004 to 0.079 (57% of intake, range 12-88%). It is concluded that breast milk supplies enough magnesium but insufficient copper and zinc for preterm infants, with the result that their body stores of copper and zinc cannot develop normally.