Absorption of amino acids from intact dietary proteins and purified amino acid supplements follows different time-courses in channel catfish (Ictalurus punctatus)

Abstract Two experiments were conducted to determine if the absorption of amino acids in practical feedstuffs and in purified amino acid (PAA) supplements follows different time-courses in the gastrointestinal tract of channel catfish (Ictalurus punctatus). In the first experiment, catfish (347 ± 47 g) were force-fed one of six practical ingredients of plant or animal origin (i.e., ground corn grain, soybean meal, wheat middlings, blood meal, menhaden fish meal, or meat and bone meal), or a mixture of crystalline amino acids designed to duplicate the amino acid composition of each practical ingredient, by stomach intubation. Apparent digestibility coefficients (ADCs) were calculated for the dietary essential amino acids in each practical ingredient and in each mixture of PAA, as well as the mean ADC of all amino acids in each ingredient. Very few significant differences (P ≤ 0.05) in the ADCs of individual amino acids were observed. However, the mean ADC of amino acids in each mixture of PAA equivalent to the amino acid composition of soybean meal, blood meal, menhaden fish meal, and meat and bone meal was significantly higher than the mean ADC of amino acids in the corresponding practical ingredient, while the mean ADC of amino acids in ground corn grain was higher (P ≤ 0.05) than the mean ADC of a mixture of PAA that duplicated the amino acid composition of corn. There was no difference (P > 0.05) between the mean ADC of wheat middlings and a mixture of PAA equivalent to the amino acid composition of wheat middlings. In a second experiment designed to measure the time-course of amino acid uptake, channel catfish (396 ± 49 g) were force-fed the same ingredients and PAA mixtures used in the digestibility experiment and blood was collected from the hepatic portal vein, between the intestine and liver, at 1-h intervals for 12 h after feeding. Plasma concentrations of 11 amino acids (alanine, arginine, aspartic acid, glutamic acid, glycine, histidine, leucine, lysine, serine, threonine, and valine) were significantly higher, and reached peak levels sooner (1–3 h after feeding), in catfish fed one or more of the PAA mixtures than in fish fed the corresponding practical ingredients. All of the intact proteins tested were well digested by channel catfish; however, protein in soybean meal appeared to be digested more slowly than protein in the other ingredients tested. The efficiency with which PAA supplements are utilized for protein synthesis could be affected by the rate at which intact proteins in the diet are digested. If uptake of dietary essential amino acids from PAA and intact protein follows different time-courses, amino acid utilization and protein synthesis could be reduced by asynchronous absorption of amino acids from different sources. Thus, the utilization efficiency of dietary PAA could be reduced when slowly digested proteins compose a major portion of the diet. Ingredient composition of the diet might ultimately determine how effectively purified, supplemental amino acids are utilized by channel catfish.