Effect of dietary calcium on metabolic balance of normal infants.

Metabolic balance studies were performed with six normal infants fed in a balanced cross-over design three formulas differing only in calcium concentration: 389, 659, and 1024 mg/liter. Absorption of phosphorus was significantly affected by increasing calcium intake decreasing from 68 to 63 to 52% of intake. However, since urinary excretion of phosphorus also decreased, there was no significant effect on retention of phosphorus. Absorption of fat was slightly (but significantly) depressed by calcium, decreasing from 97.5 to 95.5 to 92.1% as calcium intake increased. The level of dietary calcium had no significant effect on absorption or retention of nitrogen, magnesium, copper, and zinc. One of the subjects had consistently lower absorption of calcium and fat than the other subjects, suggesting an absorptive defect. The decrease in fractional absorption of calcium (57 to 47 to 39% of intake) with increasing intake of calcium is compatible with adaptive regulation. On the other hand, the observation that absorbed calcium (mg/kg/day) was linearly related to intake of calcium with a slope of 0.373 suggests an alternative explanation: constant passive, concentration-dependent (slope), and constant active (y-intercept) transport of calcium over the range of calcium intakes. Thus, the findings are explainable in more than one way and do not necessarily demonstrate regulatory changes.