Renal Solute Load

• RSL = all solutes that must be excreted by the kidney. These solutes are derived either from the diet or endogenous sources. The RSL relates to the ability of the infant to maintain water balance, even if presented with a variety of solute loads. For example, if the infant's intake is derived solely from cow milk, the RSL differs from that resulting from human milk intake. Thus, it is imperative that the composition of infant formulas be based upon the ability of the kidney to reabsorb and excrete proper amounts of solute (and water or solvent) to maintain growth.
• Potential RSL (PRSL) = the reduced solute load presented to the kidney when some solutes (X) are normally diverted away for protein synthesis or extrarenal losses. So, normally, RSL > PRSL, or  RSL - X = PRSL.
• However, in illness in which the infant does not have any unusual losses but intake is reduced and all intake is directed toward maintenance instead of growth, X=0 and PRSL is equal to the RSL.
  
  The PRSL is calculated from the equation:
  
                                                                        PRSL = N/28 + Na + Cl + K + P
  
                                      where N=total nitrogen; Na=sodium; Cl=chloride; K=potassium; P=phosphorous
  
  Based upon this equation, sodium, potassium, chloride, and phosphorus contribute to the RSL and PRSL. In contrast, calcium is a relatively minor contributor to RSL. Similarly, although protein contributes to the RSL, it is not the major factor. Finally, urea has a significant role in the RSL. There are sources to ascertain the PRSL in various formulas (see references). In general, the PRSL is similar among most commercially available formulas. The following equation can be used to predict the RSL from the PRSL:
  
                                                                     RSLet = PRSL � (0.9 X weight gain)
  
  Typically, infants can tolerate formulas that have a range of PRSLs. However, data show that infants fed formulas that have a high (≥39 mOsm/100 kcal) PRSL are more prone to hypertonic dehydration during illness compared with infants fed formulas in which the PRSL is below that value. Indeed, providing a high-solute formula to an infant who has developed hypertonic dehydration (eg, hypernatremic dehydration) may exacerbate the hyperosmolality. In turn, the infant will be at risk for increased water losses from the brain. These infants already are in need of higher amounts of free water, and formulas that have high RSLs or PRSLs may exacerbate the hyperosmolality.
  
  
  References:
  Fomon SJ, Ziegler EE. Renal solute load and potential renal solute load in infancy. J Pediatr. 1999;134:11-14
  
  Fomon SJ. Water and renal solute load. In: Nutrition of Normal Infants. St. Louis, Mo: Mosby; 1993:91-102
  
  Ziegler EE, Ryu JE. Renal solute load and diet in growing premature infants. J Pediatr. 1976;89:609-611