Indices of renal excretion and reabsorption of
phosphate were studied in 20 neonatal infants, 20 infants aged 3 months, and 20 infants aged 6 months. All subjects were normal and were fed a modified formula enriched with
vitamin D. In neonatal infants all indices of
phosphate excretion were found to be significantly lower and those of
phosphate reabsorption significantly higher than in older infants.
Phosphate excretion gradually increased with age, while its reabsorption decreased. The positive correlation between serum
phosphorus and renal threshold
phosphate concentration (
TmP/GFR) and the negative correlation between
phosphorus excretion index and
TmP/GFR found in this study shows that in young infants as in adults
TmP/GFR is the principal determinant of renal
phosphate homeostasis. Among the many indices of renal
phosphate handling in use
TmP/GFR is the best for studies of
phosphorus or
calcium metabolism disorders, or both, especially in the first three months of life.