[A study of serum phosphorus and renal phosphate excretion in children of different age groups].

To further elucidate the data of high serum phosphorus concentration reported for foreign children, this study compared the values of serum phosphorus concentration in ethnic Chinese children with foreigners' results, and investigated the potential causes of high serum phosphorus concentration. This study included 33 healthy term male neonates from 2-3 days to 5-7 days old, 1,094 children of 6-18 years old (elementary school-senior high school) and 62 adults. A total of 1,189 persons were studied. Value of serum phosphorus concentration showed no significant difference between boys and girls, and no age dependency; there was notable change in serum phosphorus concentration in growing children of different ages. The mean values of serum phosphorus concentration of neonates aged 2-3 days and 5-7 days (7.20 mg/dl and 7.65 mg/dl, respectively) were the highest; boys aged 6-11 years 5.15-5.50 mg/dl and girls aged 6-11 years 5.14-5.28 mg/dl were the second; boys aged 12-18 years 4.31-4.94 mg/dl and girls aged 12-18 years 4.44-4.96 mg/dl were the lowest. These findings are similar to the conclusions of foreign reports that the older the subjects of the study, the less their serum phosphorus concentration. The maximal transport rates of tubular phosphate reabsorption were determined from a Walton-Bijvoet nomogram using the side-rule method they recommend. The renal phosphate threshold concentration of neonates aged 2-3 days 8.41 mg/100 ml GFR and 5-7 days 7.76 mg/dl GFR was also the highest; boys aged 6-11 years 5.84-6.32 mg/dl GFR and girls aged 6-11 years 5.82-6.10 mg/dl GFR and 5.44-6.10 mg/dl GFR was the second; adult males 3.51 mg/dl GFR and adult females 3.62 mg/dl GFR was the lowest. Increased intestinal absorption of phosphorus, low GFR and low clearance of phosphorus might be the contributing factors to cause high serum phosphorus concentration. Although high renal phosphate threshold capacity is important, increased absorption of phosphorus by G-I tract seems to be another factor in accordance with the need of phosphorus of the growing body cells.