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 betwe en type of fat and lipid digestibility, for diets which induce an increased digesta viscosity. Pairs of semisynthetic diets were formulated which cont ained 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 soyab ean oil diet C18:1 and C18:2 fatty acids predominated and for the coconut o il 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 th e viscosity of the liquid phase of the small intestinal digesta and increas ed the ratio of liquid to solids. CMC depressed apparent faecal lipid diges tibility for chickens fed the tallow diet (76.1 versus 66.0%) but not for t he birds given the soyabean oil or coconut oil diets. The apparent digestib ility of C16:0 was significantly depressed by CMC in the birds fed the tall ow diet (62.7 versus 52.3%) and the digestibility of C18:0 tended to be low er. There was no significant effect of CMC on the digestibility of C18:1 an d 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 li pid digestibility in chickens is dependent on the lipid source and its fatt y acid composition.