1. Homogenized and diluted faeces (50 g/l) from one human source were incubated with the complex plant polysaccharide, guar gum, to investigate the degradation of viscous polysaccharides by intestinal bacteria.
2. Incubation of the faecal homogenate with guar gum produced a rapid decrease in viscosity and in pH, accompanied by the release of hydrogen.
3. No changes in viscosity or pH were observed and there was no production of H2 gas when guar gum was incubated with autoclaved faecal homogenate (20 min, 1.03 × 105 Pa).
4. A bacteria-free filtrate of faeces was prepared by centrifuging the faecal homogenate (2400 g for 100 min) followed by filtration through a Seitz filter and then a millipore filter (size 0.45 μm). Incubating this with guar gum produced a slow decrease in viscosity, but no significant change in pH and no generation of H2.
5. Our results show that guar gum can be fermented by human colonic bacteria and suggest the possibility of predigestion by extracellular free enzymes.