Caecal and faecal short-chain fatty acids and stool output in rats fed on diets containing non-starch polysaccharides

The exact mechanisms by which non-starch polysaccharides increase stool output are unknown. In the present study the hypothesis that the site of fermentation and short-chain fatty acid (SCFA) accumulation is related to the action of non-starch polysaccharides (NSP) on stool output was tested. The basal diet (45 g NSP/kg) of forty-three male Wistar rats was supplemented with 50 g/kg of either guar, karaya, tragacanth, gellan, xanthan or ispaghula for 28 d. A further twenty-three rats were maintained on the basal diet for the same time period. Faeces were then collected over 2 d and caecal contents obtained post-mortem. Caecal and faecal wet and dry weights and SCFA were measured. Each supplement had a different effect on the caecal and faecal contents but they appeared to fall into three groups when compared with the basal diet. In group 1, guar gum affected only caecal SCFA. It had no effect on stool output or faecal SCFA. In group 2, karaya increased caecal SCFA and tragacanth, karaya and xanthan increased faecal SCFA and faecal water. In group 3, ispaghula and gellan had no consistent effect on caecal or faecal SCFA concentrations but increased total faecal SCFA output and increased faecal wet and dry weight. Although the knowledge that SCFA are rapidly absorbed in the large intestine has led us to believe that they play no role in determining faecal output, these results suggest that in some cases where NSP are slowly fermented, and increase faecal SCFA, the role of the SCFA may need to be reassessed.