Serum concentration of free amino acids in pigs of similar performance fed diets containing protein-bound or protein-bound combined with free amino acids

Abstract The serum concentration (SC) of amino acids (AA), as indicative of their availability, varies depending on the dietary AA source (free or protein-bound). However, the AA availability may not differ when pigs fed low CP-free AA-supplemented diets or high protein-bound AA diets perform similarly. A study was conducted to analyze the SC of free AA at three different postprandial times in pigs with similar performance fed a low protein-AA supplemented or a high protein diet. Twenty-four pigs were randomly assigned to one of two wheat-soybean meal diets: low protein (140 g/kg) plus free L-lysine (Lys), L-threonine (Thr), DL-methionine (Met), L-leucine (Leu), L-isoleucine (Ile), L-valine (Val), L-histidine (His), L-tryptophan (Trp) and L-phenylalanine (Phe) (LP-FAA); high protein (220 g/kg) without added free AA (HP-PBAA). There were two phases; a 21-day growth one followed by a 4-day blood collection phase from 6 pigs fed twice a day (0700 and 1900 h). Blood was collected at 2-h before the morning meal (preprandial) and at 1, 2, and 3-h after the morning meal (postprandial). There was no effect (P ≥ 0.208) of dietary treatment on body weight gain, feed intake, and gain:feed during the 21-day growth phase. Regarding postprandial time, there was a quadratic effect for serum Leu and Lys (P   0.10). In all pigs, the highest values were observed at 2-h postprandial. Serum Lys and Thr, at 2-h postprandial, was higher in pigs fed the LP-FAA diet than in HP-PBAA pigs (P