Xanthan gum (15 g/d) was given for 10 d to eighteen normal volunteers. In vivo measurements of stool output, transit time, frequency of defaecation and flatulence were compared with a preceding control period of 10 d. At the end of the control and test periods fresh faecal homogenate from each subject was anaerobically incubated with xanthan gum and control solutions to assess the ability of the bacteria to break down the gum. Xanthan gum was found to be a highly efficient laxative agent causing significant increases in stool output (P < 0.01), frequency of defaecation (P < 0.05) and flatulence (P < 0.01) whilst having variable effects on transit time. Before feeding xanthan gum, faecal samples from twelve of the eighteen subjects could reduce the viscosity of the gum in vitro. This rose to sixteen of the eighteen with significantly greater amounts (P < 0.05) of hydrogen and short-chain fatty acids also being produced, indicating bacterial adaptation in the presence of the substrate. Correlations between the in vivo and in vitro findings did not substantiate claims that the in vivo effect of a given polysaccharide can be predicted from its fermentation characteristics in vitro.
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