Simple SummaryFor animal agriculture to be environmentally sustainable, nitrogen excretion must be reduced. This means diet must be formulated to closely meet the requirements of the animal. Accounting for basal endogenous amino acid (EAA) losses during diet formulation helps in formulating diets that closely match amino acid requirements. These studies examined the effect of three or two different ingredient types on basal losses of amino acids in broiler chickens or pigs, respectively. Corn fiber resulted in higher ileal EAA losses compared with wheat bran in broiler chickens. With the exception of His, Leu, and Tyr, there was no difference in basal ileal EAA losses between corn fiber and pectin in cannulated growing pigs. These studies show that in addition to fiber effects, dietary nitrogen sources could have a different effect on basal ileal EAA losses.The objective of these studies was to determine the effect corn fiber (CF), wheat bran (WB) and pectin (PEC) on basal ileal endogenous amino acid (EAA) losses in broiler chickens (Exp. 1) and cannulated pigs (Exp. 2) using the regression method. Semi-purified diets containing 100 g/kg of CF, WB, or PEC (broiler chickens) and CF or PEC (pigs) were fed to replicate cages consisting of eight birds per cage of 21-d-old broiler chickens and six replicates of pigs consisting of one pig per pen. Ileal endogenous His, Glu, and Pro losses were higher (p < 0.05) for CF and WB compared with birds fed diets containing PEC. Contrasts between CF and WB showed higher (p < 0.05) ileal endogenous nitrogen, total amino acid, His, Ile, Met, Glu, Pro, and Tyr losses in birds fed the CF diets (Exp. 1). Contrasts of EAA losses between birds fed the WB and PEC diets showed higher (p < 0.05) losses for His, Glu, and Pro. In the cannulated pigs, CF resulted in higher (p < 0.05) ileal endogenous His, Leu, and Tyr losses. In summary, CF induced higher ileal EAA losses in broiler chickens and cannulated pigs.