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 < 0.05), and a tendency (P < 0.10) for arginine (Arg), Met, Phe, Thr, and Val in pigs fed the LP-FAA diet; in pigs fed the HP-PBAA diet, there was a quadratic response for Arg, His, Phe, Thr, and Val (P < 0.05) and a tendency for Ile and Leu (P < 0.10), but Lys and Met were not affected either in a linear or a quadratic way (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 < 0.05). At 1-h postprandial, SC of all AA did not differ from preprandial SC in LP-FAA pigs, but it was higher (P < 0.05) in HP-PBAA pigs. These results confirmed that the SC of AA varies depending on the form (free or protein-bound) dietary AA were ingested. In conclusion growing pigs fed a 140 g protein/kg diet formulated on the basis of nine SID indispensable AA and net energy, performed similarly to those fed a 220 g protein/kg diet, although the overall efficiency of utilization of dietary AA appears to increase when pigs are fed low protein diets supplemented with free AA.