Effect of a high protein intake on acid-base balance in adult rats

Adult rats are able to maintain Ca balance under protein loads that produce Ca loss in adult humans. This species difference was investigated by determining the relationship between protein intake, endogenous acid production (EAP), net acid excretion (NAE), and urinary Ca in adult rats for comparison with a similar study on adult humans. Diets containing 10, 30, and 50% casein were fed in conjunction with proportionate increments in the sulfur amino acid (SAA) methionine (0.6, 1.8, and 3.0%). Urine volume, Ca, sulfate, organic anions, TA (titratable acidity as acid phosphates), and ammonium increased progressively with increases in protein intake, and pH decreased. When protein intake was increased at a constant level of SAA, no increase in urinary Ca, sulfate, and TA or decrease in pH was observed. Both SAA and non-SAA enhanced ammonium excretion but only non-SAA enhanced organic anion excretion, an indicator of incomplete oxidation of organic acids. SAA were responsible for 89 and 91% of the increase in EAP and Ca excretion, respectively, caused by increasing protein intake from 10-30% of the diet. In a comparison experiment, human adults on a high protein intake exhibited a much smaller increase in acid excretion as ammonium, a greater increase as TA, no change in organic anion excretion, and no increase in EAP from non-SAA. The importance of these species differences in acid-base response to a high protein intake in the greater ability of rats to maintain Ca balance on high protein intakes is unclear; however, the smaller fraction of endogenous Ca excreted in the urine of rats is probably an important factor.