Role of the source of phosphate salt in improving the mineral balance of parenterally fed low birth weight infants Philippe Chessex, MD, Marjolain Pineault, BPharm, Gilies Brisson, PhD, Edgar E, Delvin, PBD, and Francis H, GIorieux, MD, PhD From the Service de n~onatologie, Centre de recherche, H6pital Sainte-Justine, D6part- ements de p6diatrie, de pharmacie, de biochimie, Universit6 de Montr6al, Shriners Hospital~ McGill University, Montreal, Quebec, Canada Because the monobasic potassium phosphate salt (monobasic) improves the solubility of calcium and phosphorus in amino acid plus dextrose solutions, compared with the current mixtures of monobasic plus dibasic salts (dibasic), we tested the bioavailability and clinical effects of monobasic in 16 parenter- ally fed low birth weight infants at standard (n = 8) and high levels (n = 8) of mineral intakes. A constant infusion of macronutrients and vitamin D was provided in a crossover design of two four-day periods. With standard intakes of calcium (35 mg/kg/day, 0.9 mmol/kg/day) and phosphorus (30 mg/kg/day, I mmol/kg/day), there was no difference between monobasic and dibasic reg- imens on balance data or plasma biochemical monitoring (calcium, phospho- rus, pH, carbon dioxide pressure, base excess, 1,25-dihydroxyvitamin D, 25-hy- droxyvitamin D). With the use of the monobasic regimen, the mineral intakes were doubled without precipitation in the infusate: calcium, 70 mg/kg/day (1.8 mmol/kg/day), and phosphorus, 55 mg/kg/day (1.7 mmol/kg/day). This led to increased apparent retention of both calcium (63 _• 5 mg/kg/day, 1.58 • 0.12 mmol/kg/day) and phosphorus (52 +_ 4 mg/kg/day, 1.67 +_ 0.14 mmol/kg/day) compared with that for standard levels of mineral intake. The improvement of calcium-phosphorus balance was accompanied by more severe calciuria (9 ___ 2 mg/kg/day, 0.2 _+ 0,05 mmol/kg/day) and by metabolic compensation for an increased acid load. In addition to the possibility of exceeding the buff- ering capacity of the infant, this relative acidosis could also be evidence of im- proved bone mineralization. (J PEDIATR 1990;116:765-72) Low birth weight infants have hypomineralizationl; the pathogenesis is multifactorial, but it is generally accepted Supported by the Medical Research Council of Canada (grant No. UI-0035), as well as by grants from Baxter Corporation and Clintec Nutrition, Canada, Inc., of Mississauga,Ontario, and Foundation Justine Lacoste Beaubien, Montreal, Quebec, and the Shriners of North America. Submitted for publication June 26, 1989; accepted N'ov. 21, 1989. Reprint requests:PhilippeChessex, MD, Research Center, H6pital Sainte-Justine, 3175 Cfte Sainte-Catherine, Montreal, Quebec H3T 1C5, Canada. 9/23/18339 that the main cause is insufficient intake of calcium and phosphorus), 2 There is no consensus on optimal parenteral requirements for calcium and phosphorus, but the fetal ac- KH2PO4 + K2HPO4 KH2PO4 25-OHD 1,25-(OH)2D Mixture of monobasic and dibasic potassium phosphate salts Monobasic potassium phosphate salt (monobasic regimen) 25-HydroxyvitaminD 1,25- DihydroxyvitaminD cretion rates of 140 mg/kg/day for calcium and 75 mg/kg/day for phosphorus are often used as guidelines. 2 765