Renal handling of phosphate in the first six months of life.

Abstract

SUMMARY 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.The serum concentration of phosphate is much higher in neonates and infants than in older children or adults. The reason for this phenomenon remains obscure, although a relation between red cell 2,3 DPG, adenosine triphosphate, blood haemoglobin concentration, and serum phosphorus concentration has been previously reported. ' The mechanisms involved in maintaining a high serum phosphorus concentration in neonates and infants are numerous. The main food of this age is milk, which contains high quantities of freely absorbable phosphorus. Moreover, during the postneonatal period various hormonal and non-hormonal factors act on bones and kidneys, influencing the metabolism of phosphorus.Excretion of phosphate in the urine represents the final result of all these interactions and therefore can be used as an index of phosphate metabolism. Reports have been made on phosphorus excretion index2 in Bantu infants older than 4 months,3 urinary phosphate excretion in normal children older than one year,4 and renal threshold phosphate concentration (TmP/GFR) in children older than 6 years.5 These studies emphasised that proper interpretation of all indices of phosphorus excretion firstly required definition of age specific normal ranges, a condition that applies also for indices of phosphate reabsorption.6 The neonatal period and the first six months of life have their own specific metabolic characteristics, which for phosphate metabolism is primarily the high dietary intake of phosphorus. For this reason and also because of the unavailability of normal ranges for the renal handling of phosphate in this age group in the published reports we undertook the present study. Our results define the indices of urinary phosphate excretion and reabsorption in normal neonates and infants 3 and 6 months old. Subjects and methodsThree groups of subjects were studied. The first group consisted of 20 neonatal infants of both sexes aged 2-4 weeks. The second and third groups each consisted of 20 infants of both sexes and aged 3±0 5 and 6±0*5 months old, respective...

Indices of intact serum parathyroid hormone and renal excretion of calcium, phosphate, and magnesium.

“…The 97th centile simplify calculation of TmPO4IGFR.5 ralue in our study of 069 mmol:mmol is close There are some differences seen between our o the value of 0-74 mmol:mmol obtained by study and other published data on TmPO4/GFR ihazali and Barratt6 so it would appear that in children." 12 Kruse et al demonstrated an ralues of this magnitude can be applied to inverse correlation between TmPOJ4GFR and British children but may not be applicable to age which was not seen in our study. " This may ther population groups.…”

Phosphorus Intake in Preterm Babies and Variation of Tubular Reabsorption for Phosphate per Liter Glomerular Filtrate

“…We have found a positive cor relation of TP/GFR with serum phosphorus as it was demonstrated for TmPOVGFR [9], In adults, TmPOVGFR varies little with di etary phosphorus [19], However, in spite of the very low phosphaturia after feeding pooled human milk, our study demonstrates that TP/GFR remains too low for age [12]. It is in contradiction with the results of Bistarakis et al [9] who found that phosphate indices were in no way different in breast-fed babies or babies fed unmodified cow milk [9], In spite of immaturity of the kidney of the pre term infant, its adaptation with modifying phosphorus load is remarkable. After 3 days of feeding milk formula adapted for preterm babies, which contains more proteins, cal cium and phosphorus with adequate Ca/P ratio, TP/GFR reached normal values for age and then remained stable.…”

“…In clinical practice, this pa rameter is calculated in adults using an em piric normogram described by Bijvoet et al [10] and by Walton and Bijvoet [11]. This normogram was, however, used in pediatric subjects to define normal values for renal phosphate handling [8,9]. In infancy, the serum concentrations of inorganic phospho rus are much higher, and the applicability of this normogram is uncertain.…”

Genetic Testing Confirmed the Early Diagnosis of X-Linked Hypophosphatemic Rickets in a 7-Month-Old Infant

“…Based on this, the renal threshold for phosphate concentration was 1.18 mmol/100 mL glomerular filtration rate (GFR), which was higher than the normal range for infancy (reference range 0.15-0.34 mmol/100 mL GFR), suggesting that he was not losing phosphate in the urine. 13 , 14 …”