Sources of protein-induced endogenous acid production and excretion by human adults

The origin of the increase in endogenous acid production and excretion associated with the calciuretic action of high protein intakes was investigated in human adults. Eight subjects, 4 males and 4 females, aged 25-36 years, were fed a low protein diet (50 g/day) and a high protein diet (120 g/day in males and 106 g/day in females) for 7 days each. The high protein diet was formulated by supplementing the low protein diet with a mixture of four purified proteins. Increased protein intake was associated with increases in urinary Ca, sulfate, titratable acidity (acid phosphates) and ammonium, and decreases in urinary pH and bicarbonate. There was no increase in organic anion excretion. The increases in endogenous acid production (EAP) and net acid excretion (NAE) were entirely attributable to the oxidation of excess sulfur amino acids (SAA), which yields 2 moles of hydrogen ions per mole of amino acid catabolized. The results differ in this respect from those reported for studies on the effect of SAA loading, which indicate that non-SAA make a major contribution to the increase in EAP seen under these conditions.