The inhibitory effect of a highly viscous carboxymethylcellulose on dietary fat digestibility in the growing chicken is dependent on the type of fat

The aim of the study was to determine whether there is an interaction between type of fat and lipid digestibility, for diets which induce an increased digesta viscosity. Pairs of semisynthetic diets were formulated which contained either tallow, soyabean oil or coconut oil as the main source of fat. To one diet of each pair, carboxymethylcelullose (CMC) was added. CMC is a soluble, viscous nonfermentable fibre. The fat fraction of the tallow diet comprised mainly C16:0, C18:0 and C18:1 fatty acids, whereas for the soyabean oil diet C18:1 and C18:2 fatty acids predominated and for the coconut oil diet C12:0 and C14:0 fatty acids were predominant. The diets were fed to broiler chickens (7-21 days of age) in a conventional digestibility study using chromium oxide as an indigestible marker. CMC significantly raised the viscosity of the liquid phase of the small intestinal digesta and increased the ratio of liquid to solids. CMC depressed apparent faecal lipid digestibility for chickens fed the tallow diet (76.1 versus 66.0%) but not for the birds given the soyabean oil or coconut oil diets. The apparent digestibility of C16:0 was significantly depressed by CMC in the birds fed the tallow diet (62.7 versus 52.3%) and the digestibility of C18:0 tended to be lower. There was no significant effect of CMC on the digestibility of C18:1 and C18:2 in the soyabean oil diet nor on the digestibility of C12:0 or C14:0 in the coconut oil diet. The effect of CMC-induced digesta viscosity on lipid digestibility in chickens is dependent on the lipid source and its fatty acid composition.