Effects of methionine, cysteine and medium chain triglycerides on nutrient digestibility, absorption of amino acids along the intestinal tract and nutrient retention in Atlantic salmon (Salmo salar L.) under pair-feeding regime

Atlantic salmon, kept in seawater (8-11 degrees C), initial weight 180 g, were pair-fed four high energy, low protein diets for 65 days in triplicate. The diets were supplemented isonitrogenously with amino acids: methionine (6.2 g kg(-1)), cysteine (5.0 g kg(-1)) or alanine (3.7 g kg(-1)) (control). In the fourth diet, also supplemented with alanine, medium chain triglycerides (MCT) (100 g kg(-1)) replaced fish oil. Feed intake was estimated and feeding level adjusted based on the difference between delivered and wasted feed. Apparent digestibilities and pancreatic enzyme activities along the intestinal tract were determined from analyses of chyme from different regions and of faeces, using yttrium oxide as the marker. Plasma amino acid concentrations were determined in blood from the caudal vein. Whole fish taken at start and termination of the experiment were used to estimate nitrogen and energy retention. Feed intake in fish fed the MCT-diet determined the feeding level of fish on the other diets. MCT increased digestibilities of protein and starch and increased nitrogen retention from 52% to 66%, while energy retention was significantly reduced. MCT caused accumulation of cysteine in the proximal regions of the post-gastric intestinal tract and increased pancreatic proteolytic activity of the chyme. Methionine supplementation increased fat digestibility. Cysteine increased digestibilities of protein, fat, starch, as well as nitrogen retention. Hence, sulphur amino acids seemed to be the first limiting amino acids and methionine less effective than cysteine in relieving the limitation. Plasma levels of taurine did not, however, indicate any limitation in the capability to convert methionine into cysteine. (C) 2000 Elsevier Science B.V. All rights reserved.