Fortified mothers' milk for very low birth weight infants: results of macromineral balance studies.

The mineral adequacy of fortified mothers' milk for very low birth weight (VLBW) infants was tested during their first two postnatal months. Metabolic balance and serum Ca, P, Mg, Na, and K values were evaluated at 2.5 and 6 weeks of life in 32 VLBW infants (less than 1.3 kg). Infants were fed either their mothers' milk fortified with skim and cream components derived from heat-treated, lyophilized mature human milk (FMM) or commercial cow milk-derived formulas. Despite Ca and P concentrations 50% to 100% higher in the fortified human milk than is usual in unfortified human milk, group FMM's Ca and P intakes remained significantly below those fed formula (P less than 0.001). Serum calcium levels and alkaline phosphatase activity were higher and serum phosphorus lower (P less than 0.002) in group FMM, whereas serum levels of magnesium, sodium, and potassium were similar in both groups. Ninety-six-hour urinary excretion of Ca was greater and 96-hour urinary excretion of P was less in group FMM (P less than 0.02). Retention of Ca and P in both groups was significantly below estimates of intrauterine accretion. Mg retention was significantly higher in group FMM (P less than 0.002) despite intakes of Mg significantly below those in infants fed formula. Although intakes of Na were below recommended levels in both groups, by the second balance period all infants demonstrated Na retention that was greater than expected from the sum of estimates of intrauterine accretion and dermal losses. Similar findings were noted for K. Biochemical and balance data indicate that relative to the needs of the VLBW infant, fortified mothers' milk was deficient in Ca and P, but adequate in Mg, Na, and K.