ENDOGENOUS PRODUCTION OF FIXED ACID AND THE MEASUREMENT OF THE NET BALANCE OF ACID IN NORMAL SUBJECTS*

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Abstract
In a series of 10 balance experiments, normal subjects in an apparently steady state were fed a liquid-formula diet which had as the N source a purified soy phosphoprotein essentially free of mineral cations. It was demonstrated that the net renal excretion of acid was closely matched by the sum of (1) inorganic sulfate in the urine, (2) total "organic acids" in the urine, and (3) the acid calculated to be released by the metabolism of the combustible cationic groups covering the phosphate residues in the dietary phosphoprotein. In 3 experiments net production or excretion of acid was temporarily altered and, at the end, when a steady state had been restored, the equivalence of cumulated acid excretion with the cumulated production of acid could again be demonstrated. These observations are taken to mean that, under the present conditions, the total net production of fixed acid arose from the oxidation of organic sulfur, plus the production of nonmetabolized organic acids from neutral precursors, and the release of protons during the metabolism of the organic cations neutralizing phosphate esters and/or during the hydrolysis of phosphate diesters in the dietary phosphoprotein. However, when non-combustible cations neutralize the phosphate residues, no acid production can be attributed to the presence of phosphate mono-esters in the protein. The relationship between acid production measured in this way and the renal excretion of acid appears to be that of a true physiological equilibrium. Provided that body composition remains relatively constant during the period of study, it should be possible to apply this technique to the direct measurement of the external balance of acid under a variety of experimental and clinical conditions.