Renal regulation of phosphate homeostasis during growth

I NTRINSIC to the process of growth is the maintenance of a positive balance for a variety of substances, including minerals. Phosphate is not only an important constituent of bone but also of soft tissue and, in particular, of muscle, liver, and brain. Balance studies indicate that a l-3-moold infant fed a standard formula retains 32 k 25 (SD) mg/kg BW per day of phosphate (Fig. I).’ This estimate is well above the 3.4 mg/kg BW per day required for bone formation.* It is of interest to note that the plasma phosphate concentration bears a direct relationship to the rate of growth, suggesting-but not proving-that an environment high in phosphate is essential to the accretion of new tissue. The achievement of a positive external balance is dependent upon intake, intestinal absorption, and the ability of the kidney to reclaim the moiety passing through the glomerular capillary filter. The phosphate intake of an infant varies around 110 mg/kg BW per day’ as compared to about 20 mg/kg BW per day in the adult.3 There is no precise information available regarding the efficiency of the intestinal absorptive mechanism of phosphate during early life. If the percentage of intestinal absorption is comparable to that of the adult (65%-70%),3 then approximately 70 mg/kg BW per day will be absorbed by an infant fed exclusively formula. The urinary excretion of phosphate in the infant is about 40 mg/kg BW per day, while the corresponding value for the adult is about 13 mg/kg BW per day. It is apparent that both a high dietary intake of phosphate and a low renal excretion contribute to the positive external balance for phosphate of the developing animal.