Lesser amounts of organic acid derive from the following: Incomplete metabolism of glucose and fatty acids into lactic acid and ketoacids Metabolism of sulfur-containing amino acids (cysteine, methionine) into sulfuric acid Metabolism of cationic amino acids (arginine, lysine) Hydrolysis of dietary phosphate This "fixed" or "metabolic" acid load cannot be exhaled and therefore must be neutralized or excreted by the kidneys.