Non-starch polysaccharides (NSP) reduce digestibility, but relative contributions of their functional properties viscosity (V) and fermentability (F) are not known. Thus, 8 ileal cannulated pigs were fed 4 diets based on cornstarch and casein and supplemented with purified NSP sources, either 5% low F, low V cellulose (CEL), low F, high V carboxymethylcellulose (CMC), high F, low V oat β-glucan (LG), or high F, high V oat β- glucan (HG) in a double 4×4 Latin square. Apparent ileal (AID) and total tract digestibility (ATTD) were calculated using TiO2 as indicator. The AID of energy, crude protein (CP) and dry matter (DM) was highest for high viscous CMC. The ATTD of energy and DM was highest for CMC and ATTD of CP did not differ among diets. The AID and ATTD of ash was only positive for CMC indicating secretion of minerals with other NSP. Post-ileal DM digestibility, an indicator of fermentation, was highest for CEL and HG. The relationship of AID of energy to post-ileal DM digestibility was stronger (R2=0.85) than the relation with digesta V (R2=0.45). In conclusion, negative effects of NSP on AID of energy and CP are explained better by increased fermentability than by increased digesta viscosity.

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