The dependence of the in vitro fermentation of dietary fibre to short-chain fatty acids on the contents of soluble non-starch polysaccharides.

The fermentability of cellulose and dietary fibre in common clinical use (Inolaxol, Fiberform, Vi-Siblin, Lunelax, pectin) was measured as the in vitro production of short-chain fatty acids, lactate, and ammonia in 16.6% faecal homogenates from 18 healthy volunteers. The results were compared with the contents of soluble and insoluble non-starch polysaccharides as determined by the method of Englyst. The amounts of soluble non-starch polysaccharides in the fibre were closely associated with the mean productions of short-chain fatty acids after 6 h (R = 0.94, p < 0.002) and 24 h (R = 0.98, p < 0.0002) of incubation. The mean production of ammonia was inversely related to the soluble fraction of the fibre (after 6 h, R = -0.93, p < 0.003; after 24 h, R = -0.90, p < 0.006). These variables were not dependent on the insoluble fractions of the fibre. The in vitro fermentability differed considerably among the fibres: cellulose and Inolaxol (sterculia gum) were almost non-fermentable, Fiberform (wheat bran-based) was low-grade fermentable, Vi-Siblin and Lunelax (both ispaghula husk) were intermediately fermentable, and pectin was highly fermentable. These findings support that the water solubility determines the degree of fermentability of dietary fibre and thereby the corresponding bacterial assimilation of ammonia. In vitro measurements of short-chain fatty acid production in faecal homogenates may hence supplement commonly used methods to classify dietary fibre.