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Potential Renal Solute Load of Infant Formulas¹

Department of Pediatrics, University of Iowa, Iowa City, IA 52242

The potential renal solute load (PRSL) of infant feedings is the sum of dietary nitrogen (expressed as mmol of urea, i.e., mg nitrogen divided by 28), sodium, potassium, chloride and phosphorus. The PRSL determines the renal solute load, and, therefore, the osmolar concentration of the urine. When water intake is reduced and/or water losses are increased, the renal concentrating ability may be exceeded, and negative water balance (dehydration) may ensue. Under these circumstances, feedings providing high PRSL lead more rapidly to dehydration than do feedings providing lower PRSL. On the basis of simulated clinical situations and epidemiologic data, it is concluded that conventional infant formulas (PRSL 135–177 mosmol/l, or 20–26 mosmol/100 kcal) provide a satisfactory margin of safety. A feeding providing the upper limits for concentrations of protein and electrolytes specified by the Food and Drug Administration rule does not afford a satisfactory margin of safety. It is recommended that the upper limit for protein content of infant formulas be decreased from 4.5 g/100 kcal to 3.2 g/100 kcal and that an upper limit for phosphorus concentration of infant formulas be set at 93 mg/100 kcal. Maximum PRSL will then be 221 mosmol/l (33 mosmol/100 kcal).

KEY WORDS: • potential renal solute load • infant formulas • upper limits of nutrients • water balance

¹ This paper was presented at a symposium, "Upper Limits of Nutrients in Infant Formulas," November 7–8, 1988, in Iowa City, IA.