Amino Acid Absorption and Endogenous Amino Acids in the Lower Ileum and Excreta of Chicks

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Abstract
Five diets containing protein levels supplying 5–35 g N/kg, and an N-free diet, were fed ad libitum to six groups of 15 chicks each, from 10 to 16 d of age. Soybean meal was the only source of protein. Diets contained magnesium ferrite as marker. Individual amino acid absorption was determined by analysis in the lower ileum or excreta as apparent absorption; true absorption was calculated from the slope of the regression curves, obtained by plotting dietary amino acid levels versus amino acids in the lower ileum or excreta. In the excreta, the true values were higher than the apparent ones determined with the three levels of dietary protein (diets 1–3, respectively, were 34.8, 25.7 and 18.1 g N/kg). This difference is the result of elimination of the endogenous fraction as represented by the intercept of the regression line obtained by the calculation method. In the lower ileum both absorption values, the apparent (diets 1–3) and the true (calculated), are almost similar to the true (calculated) ones in the excreta. The method based on regression analysis allows calculation of true absorption from excreta data, without the need to kill the chicks. This has been validated with soybean meal as the source of protein. The determined endogenous amino acids levels in chicks fed an N-free diet were higher than those calculated from the intercept (the endogenous amino acids related to the feed protein tested), both in the excreta and in the lower ileum. In the lower ileum even the pattern of calculated endogenous amino acids seems to be different from the one determined with the N-free diet. These results support the assumption that the so-called true absorption values calculated by subtracting endogenous amino acids determined with chicks fed an N-free diet are too high. Moreover, values determined in the lower ileum might even be misleading for some amino acids.