THEORETICAL MODEL TO ESTIMATE RENAL NET ACID EXCRETION

Abstract

Natural food is originated from cell material with constant pH. Patients fed natural food show renal net acid excretion (NAE) far below maximum. During parenteral nutrition or under synthetic diets high NAE was observed. Therefore, a model was developed to calculate NAE under a synthetic diet from the analytical data of this diet. According to Relman (J.Clin. Invest. 40, 1621, 1961) NAE corresponds to the difference of urinary anions [sulfate (SO4) + organic acids (OA) + chloride (Cl) + phosphate (P)] and urinary cations [sodium (Na) + potassium (K) + calcium (Ca) + magnesium (Mg)]. If urinary excretion of each parameter can be estimated from the data of intake, NAE can be predicted. In steady state conditions the urinary excretion of electrolytes correlates with their intestinal absorption, which can be estimated from intake and absorption rate. Absorption from the gut was estimated in the following way: Na + Cl: 100%; K: 90%; Ca: 0.17 × Ca intake (mg) + 60; Mg: 0.39 × Mg intake (mg) −23; P: 0.642 × P intake (mg) − 11. 75% of the ingested organic sulfur (methionine and cystine) is oxidized to SO4 and excreted in the urine. OA excretion is almost independent from the diet (38 meq/d/1.73m2). The usefulness of this theoretical model to estimate NAE was demonstrated in acid-base studies of children with phenylketonuria receiving two synthetic amino acid mixtures (PAM with a high NAE of 4-6 meq/kg/d, PAM74 with a low NAE of 1 meq/kg/d).