Renal excretion of calcium and phosphate in preterm and term infants.

We studied urinary phosphate and calcium excretion in preterm and term infants during the first 3 months of life. The infants were mainly breast-fed, and the average phosphate intake ranged between 0.5 and 1 mmol/kg/day. During the first week of life urinary phosphate excretion was significantly higher in preterm than in term infants, whereas parathyroid hormone values were the same. After the first week of life urinary phosphate and calcium excretion were the same in preterm and term infants. Fractional excretion of phosphate was low (range 1% to 6%). In both groups calcium excretion was low during the first weeks of life, and increased thereafter to 5 and 3 mmol/1.73 m2/day, respectively. The urinary calcium/creatine ratio generally exceeded 2.0 (mmol/mmol) in preterm infants after the second week of life. These results are compatible with a state of relative phosphate deficiency, resulting in an adaptively low urinary phosphate excretion and an inability to form bone minerals, and therefore relatively high urinary calcium excretion.

Salt and water homeostasis during oral rehydration therapy.

Changes in sodium balance and urinary and stool output during orally administered rehydration therapy were studied in 22 well-nourished Turkish infants, aged 2 to 13 months, with acute diarrhea mainly of viral origin. The infants randomly received a rehydration solution containing either 90 mmol Na/L (ORS90) or 40 mmol Na/L (ORS40). Slight transient hypernatremia was noted in a few infants receiving ORS90, and slight transient hyponatremia in a few infants receiving ORS40. In both groups, sodium balance increased most rapidly during the first 12 hours of rehydration, and then more slowly because of increased urinary as well as stool sodium output. Sodium balance was always more positive after ORS90 than after ORS40, but the difference did not change much from 12 to 36 hours after therapy was started. Changes in fractional sodium excretion, urinary K/Na quotient, and urinary aldosterone-creatinine quotient were used as indexes of changes in sodium balance. All values were interpreted to indicate that the sodium deficit on admission was corrected within 12 to 18 hours after ORS90 and, in most cases, after 24 to 36 hours after ORS40. Both groups of infants responded well to orally administered rehydration therapy from the clinical viewpoint.