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Dietary fibre is an important component of dietary ingredients but is not digested by porcine digestive enzymes. Dietary fibre may be fermented, mostly in the large intestine, and may reduce the digestibility of other macronutrients. The physiological effects of dietary fibre are attributable to two functional properties, viscosity and fermentability. Viscous fibre binds water, increases digesta viscosity, modifies digesta passage rate and thereby reduces nutrient digestibility in the small intestine. Fermentable fibre contributes a major part of the non-digested nutrients that pass into the large intestine and are fermented by microbial populations, thereby producing SCFA. The effects of types of dietary fibre on nutrient digestibility have been linked to structural characteristics and solubility of fibre. However, the role and specific contributions of viscosity and fermentability are largely unknown. The objectives were to understand the independent and interactive effects of the two functional properties of dietary fibre in semi-purified diets on apparent ileal digestibility (AID), apparent total tract digestibility (ATTD), digesta passage rate, N retention and SCFA concentration in ileal-cannulated grower pigs. Thus, eight ileal-cannulated pigs randomised in a double 4 x 4 Latin square were fed four diets based on maize starch and casein supplemented with 5% of actual fibre in a 2 x 2 factorial arrangement: low-fermentable, low-viscous cellulose (CEL); low-fermentable, high-viscous carboxymethylcellulose (CMC); high-fermentable, low-viscous oat β-glucan (LBG); high-fermentable, high-viscous oat β-glucan (HBG).
Viscosity and fermentability interacted to affect (P < 0.001) digesta viscosity and AID and ATTD of nutrients. These properties tended to interact to affect (P < 0.10) digesta passage rate and butyrate. Pigs fed the CMC diet had the lowest (P < 0.05) digesta passage rate and the highest (P < 0.001) AID of energy, crude protein and DM, and ATTD of energy and DM. Post-ileal DM digestibility was highest (P < 0.001) for pigs fed the CEL and HBG diets. Post-ileal DM digestibility had a negative, curvilinear relationship with the AID of energy and crude protein (R2 0.85 and 0.72, respectively; P < 0.001). Digesta viscosity had a less strong relationship with the AID of energy and crude protein (R2 0.45 and 0.36, respectively; P < 0.001).
In conclusion, high-viscous, low-fermentable dietary fibre increases the proportion of a diet that is digested in the small intestine by reducing digesta passage rate.
S Hooda, BU Metzler-Zebeli, T Vasanthan and RT Zijlstra, 2011. British Journal of Nutrition, 106: 664-674.
