Potential Renal Solute Load: Considerations Relating to Complementary Feedings of Breastfed Infants

Abstract

Renal solute load (RSL) refers to all solutes of endogenous or dietary origin that require excretion by the kidneys. Potential renal solute load (PRSL) refers to solutes of dietary origin that would need to be excreted in the urine if none were diverted into synthesis of new tissue and none were lost through nonrenal routes.PRSL=N/28+Na+Cl+K+Pawhere N is nitrogen, Na is sodium, Cl is chloride, K is potassium and Pa is available phosphorus, and the units are in millimoles (or milliosmoles), except for N, which is total N in mg. The term N/28 represents nitrogenous solutes (mmol) based on the assumption that the modal number of N atoms per molecule is 2. Pa is equal to total P except in soy-based diets, in which Pa is about two-thirds of total P.When an infant is well and consuming a predominantly liquid diet, the renal concentrating ability of nearly all infants is sufficient to maintain water balance even if the feeding provides a PRSL as high as that of cows' milk. When an infant is ill, does not grow, and does not have diarrhea, PRSL is nearly the same as RSL. PRSL is an important consideration in maintaining water balance in the following 3 circumstances: 1) during acute illness, when fluid intake is decreased, especially if the illness is accompanied by fever; 2) when an energy-dense diet is fed; and 3) when renal concentrating ability is decreased, as in chronic renal disease and diabetes insipidus.Calculations of the PRSL of various infant foods are presented in Table 1. Beikost in the United States provides 2.4 g protein per 100 kcal, which may be greater than in many lesser industrialized countries.A number of studies3–7 have demonstrated that nearly all breastfed infants who are otherwise well maintain satisfactory water balance in hot environments.To illustrate the effect of feeding beikost on water balance during illness, Table 2 presents calculations regarding a hypothetic 6-month-old, 7-kg infant fed varying quantities of human milk and beikost. It is evident from the calculations that replacement of some of the human milk intake with beikost will have a slightly negative effect on water balance whereas increasing the total fluid intake, either by achieving greater consumption of human milk or a combination of human milk and beikost, despite a concurrent increase in PRSL, will have a major beneficial effect on water balance. Thus, during illness if the feeding of beikost is accompanied by increase in total volume of intake, water balance will be improved.