Effect of routine doses of antacid on renal acidification

Ten healthy volunteers took a magnesium and aluminium hydroxide antacid for 4 days, and their urinary acid excretion was measured. During antacid ingestion, blood bicarbonate levels did not change significantly, but there were highly significant rises in urine pH and bicarbonate excretion and falls in the 24 h excretion of titratable acid, ammonium, and net acid; the average change in net acid excretion was 41 +/- 4 mmol (72 +/- 9%) per 24 h. This large reduction in net acid excretion appears to result from neutralisation of more hydrochloric acid than sodium bicarbonate in the gastrointestinal tract rather than from absorption of exogenous alkali. Although metabolic alkalosis does not occur with their use in normal individuals, these antacids should not be termed "non-systemic". They might cause important changes in renal drug handling, solubility of excreted substances, or acid-base status in patients at risk.