Production rate of acetate during colonic fermentation of lactulose: a stable-isotope study in humans

Background: Breath tests are currently used to qualitatively assess colonic fermentation: no quantitative estimations are available for healthy subjec ts. Objective: This study describes a stable-isotope-dilution method to measure acetate production quantitatively from colonic bacterial fermentation. Design: Six volunteers received a primed, constant, intravenous infusion of [1-C-13]acetate at a rate of 1.01 +/- 0.04 mu mol.kg(-1).min(-1) for 7 h. They ingested 20 g pure lactulose after 1 h of the tracer infusion. Expired air and arterialized venous blood were sampled every 15 min. Results: Before lactulose intake, the breath-hydrogen concentration was 7 /- 2 ppm and the plasma acetate concentration and isotopic enrichment were 141 =/- 14 mu mol/L and 14.8 +/- 1.4 moles percent excess, respectively, Wh ole-body acetate turnover was 6.0 +/- 0.7 mu mol.kg(-1).min(-1). After lact ulose ingestion, maximum breath hydrogen and acetate concentrations reached 63 +/- 15 ppm (P = 0.004) and 313 +/- 25 mu mol/L (P = 0.002), respectivel y, whereas [C-13]acetate enrichment decreased to 9.9 +/- 1.3 moles percent excess (P = 0.03). Whole-body acetate turnover increased to 9.8 +/- 1.5 mu mol.kg(-1).min(-1) and later decreased almost to baseline values. Colonic l actulose fermentation yielded 140 +/- 12 mmol acetate over 6 h, representin g 86% of the production based on stoichiometric equations. Conclusion: This new method provides a quantitative estimate of colonic car bohydrate fermentation via evaluation of acetate production.