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To determine whether the acidifying effect of malabsorbed carbohydrate on fecal pH differed between methane-excreting and nonexcreting individuals, we administered the poorly absorbed disaccharide lactulose to five CH4 excretors and six CH4 nonexcretors. Lactulose, 20 g twice daily for 1 wk, significantly lowered fecal pH among CH4 nonexcretors as compared with CH4-excreting individuals (5.38 ± 0.66 vs 6.90 ± 0.61, p < 0.01). To determine whether this observation was due to differences in acid production from bacterial carbohydrate fermentation, feces from each subject were incubated with lactulose. There were no differences in in vitro acid or hydrogen production between groups. We conclude that malabsorbed carbohydrate fails to induce in vivo colonic acidification in CH4 excretors. In contrast, in vitro fecal carbohydrate fermentation is similar among CH4 excretors and nonexcretors. These results suggest differences between these two groups in the colonic absorption of fermentation products. [ABSTRACT FROM AUTHOR]