Effects of dietary fibers varying in physicochemical properties on total endogenous protein losses and protein digestibility in broilers

This study aimed to quantify endogenous protein losses (EPL) and protein digestibility in broilers fed diets containing dietary fibers (DF) differing in particle size (PS) and solubility. A total of 360 female 1-d-old Ross 308 broilers were randomly allocated to 40 pens (9 birds/pen). From d 10, broilers received one of five dietary treatments. Four diets were tested in a 2 à 2 factorial design containing 120 g/kg coarse or fine soybean hulls (SBH) as an insoluble DF source, with or without 50 g/kg purified wheat arabinoxylans (AX) as a viscous soluble DF source. A fifth diet replaced fine SBH with 150 g/kg fine sugar beet pulp (SBP), a DF source with notable hydration properties. Between d 26 and 31, three birds per pen received a daily oral dose of a ¹⁵N-isotope solution containing 1.2 g ¹⁵N-enriched milk protein concentrate (2.27 atom ) and 0.12 g ¹⁵NH₄Cl (¥98 atom ), with an initial dose of milk protein concentrate of 2.4 g at d 26. Six days after ¹⁵N-solution withdrawal (d 37), excreta were collected, birds were euthanized, and ileal digesta were sampled. Arabinoxylans increased EPL in feces (+0.35 g/kg DM intake, P = 0.020) and reduced true ileal protein digestibility (2.45-units, P < 0.001), regardless of SBH PS. Apparent ileal protein digestibility decreased only when AX was added to the fine SBH diet (PS Ã� AX interaction, P = 0.024). Replacing fine SBH with fine SBP increased EPL in ileal digesta (+0.31 g/kg DM intake, P = 0.002) and feces (+0.92 g/kg DM intake, P = 0.004), without affecting true protein digestibility. Overall, both AX and SBP increased EPL in broilers, but only AX reduced true protein digestibility. The PS AX interaction suggests that the viscous soluble DF impact depends on PS of the dietary matrix. These results highlight the importance of considering the counteracting DF effects on EPL and nutrient absorption, as apparent digestibility alone is insufficient to capture these responses. © 2026 The Authors