Dietary Fat Inclusion Decreases Endogenous Amino Acid Losses in Broiler Chickens

Fats and oils are widely used in poultry diets as a source of energy and to increase palatability. Increasing dietary fat level has been reported to improve the apparent ileal digestibility of amino acids (Cowieson and Ravindran, 2008). These improvements may be related, in part, to reduced endogenous amino acid (EAA) losses, but the influence of dietary fat on EAA losses has not been studied in chickens. The present study was carried out to measure the basal EAA losses in male broilers (Ross 308) fed three nitrogen-free diets namely, a control diet with no added fat and test diets with 60 g/kg of either soybean oil or tallow added. Since no protein was fed to the broilers, the assumption is that all nitrogen and amino acids in the ileal digesta will be of endogenous origin and represent the basal losses. Titanium dioxide (5 g/kg) was added to all diets as an indigestible marker. Each diet was assigned to six replicate cages (8 birds per cage) from day 18 to 21. On day 21, the digesta were collected from the lower half of the ileum. The endogenous losses of all amino acids, except cysteine, were higher (P < 0.05) in broilers fed diets with no added fat, compared to those fed diets with fat. For all amino acids, there was no significant difference between the endogenous losses in diets supplemented with tallow or soybean oil. Several mechanisms are probably involved in the observed EAA flow responses. First, the addition of fat lowers the passage rate (Mateos et al., 1982), retaining the digesta longer in the intestine, which may increase the digestion and re-absorption of EAA and reduce EAA losses. Endogenous losses represent the net balance between protein ingested plus endogenous proteins and absorbed dietary plus reabsorbed endogenous protein. Thus, it is reasonable to assume that any factor that increases amino acid absorption would favour reductions in EAA flow. Second, reduced EAA losses could be related, at least in part, to morphological changes in the microvilli with fat addition. Goda and Takase (1993) reported that a high-fat diet increased the length of intestinal microvilli, crypt depth and proliferation of microvilli in the intestinal wall. The resulting increase in the surface area could be expected to increase the efficiency of re-absorption of endogenous proteins. Changes in the composition of microbiota with fat addition may be another possible contributing factor. Although the primary area of bacterial activity in the intestine of chickens is the hindgut, the small intestine also contains a significant microbiome presence, particularly associated with mucus layer (Tomas et al., 2016), and many of these species use mucin as their energy and amino acid sources (Pan and Yi, 2014). The above speculations are consistent with the suggestion by Jenkins and Thompson (1992) that higher dietary fat content might increase amino acid absorption in the proximal intestinal segments. The present data indicate that the inclusion of fat in broiler diets decreases endogenous amino acid losses and implies that dietary fats have beneficial effects beyond energy contribution and palatability.