Impact of methionine on net ketoacid production in humans.

An exogenous acid load (NH4Cl) inhibits net ketoacid production in the first week of starvation and the fourth to eighth weeks of ketogenic dieting. To determine whether an acid load produced by amino acid metabolism can similarly modify ketosis, five overweight volunteers ingested methionine (H2SO4), NH4Cl, and NaCl (control), in varying order, each day for seven days during weeks 5 to 8 of hypocaloric ketogenic dieting. During days 5 to 7 of each phase, blood pH, bicarbonate, and pCO2 were stable but lower in the NH4Cl phase (7.32 +/- 0.02, 18.1 +/- 1.2 mmol/L, 35.8 +/- 1.4 mmHg) and the methionine phase (7.33 +/- 0.01, 17.1 +/- 0.9 mmol/L, 34.0 +/- 2.0 mmHg) than in the NaCl phase (7.38 +/- 0.01, 22.3 +/- 0.2 mmol/L, 37.6 +/- 1.6 mmHg), P less than .05. Over this period, blood acetoacetate concentration was lower during the methionine and NH4Cl phases than during NaCl, P less than .05. In addition blood beta-hydroxybutyrate and total ketone-body concentrations were lower in the methionine than NaCl phases, P less than .05. Urinary acetoacetate and beta-hydroxybutyrate excretion fell with both acid loads, P less than .05. Compared with control values, urinary total ketone excretion was suppressed by 67 +/- 10% in the NH4Cl and 89 +/- 3% in the methionine periods. When NaCl was ingested after either of the acid loads, urinary ketone excretion increased by 300% to 700%. Thus, methionine ingestion, which results in an acid challenge equivalent to that of a large protein load, has an impact on net ketoacid production similar to that of NH4Cl.(ABSTRACT TRUNCATED AT 250 WORDS)