Plasma levels of calcium, magnesium, phosphate, and protein were measured on the first day of life (cord blood) and at the age of 1 week, in infants fed on breast milk or on one of three cow's milk preparations.The plasma calcium fell in all patients over the first 24 hours. Breast-fed infants, in all but three instances, showed a rise of calcium in the following 5 to 7 days. There was no mean change in calcium concentration for the other feeds over this period, and about a third of these infants showed a fall: in 9% to below 7·5 mg/100 ml.There was a positive correlation between concentrations of calcium and magnesium in the plasma, but an inverse relation between those of calcium and phosphate, at 1 week of age. The concentration of calcium at 1 week of age was related to that of the first day, which in turn was related to that in cord blood. The changes in calcium observed between the age of 1 day and 1 week were inversely related to changes in the plasma phosphate. A direct correlation was also found between changes in calcium and changes in magnesium concentration. All groups of infants showed an increase in plasma phosphate over the first day of life and a further increase by 1 week of age. This rise, however, was significantly smaller in breast-fed infants than in those receiving cow's milk, probably due to the much higher phosphate content of the latter. The hypocalcaemia which occurs at 1 week of life in some infants receiving cow's milk may be due to an inability of the neonatal kidney to dispose of the excessive phosphate loads in these feeds.
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