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. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Methionine; Cysteine; Medium chain fatty acid; Digestion; Nitrogen retention; Pair-feeding
1. Introduction
The aquaculture feed industry is constantly striving to improve cost efficiency through increased utilisation of protein, the most expensive macronutrient of fish diets.
Higher protein utilisation may be obtained by increasing protein digestibility, improving amino acid balance, and reducing the amount of protein used for energetic purposes.
Cysteine and taurine, which is derived from cysteine, are very important digestive
Ž components. Digestive enzymes contain high levels of cysteine Blow, 1974; Ikenaka et
. Ž
al., 1974 , digestive secreta are rich in cysteine-containing glutathione Ganapathy et al., .
Ž 1991; Dahm and Jones, 1994 and bile salts are conjugated with taurine Walton et al.,
. 1982; Stryer, 1988; Yokoyama and Nakazoe, 1992b . The demand for sulphur amino
acids in digestive processes represents a major part of the total turnover of these Ž
. sulphur-containing amino acids. Cowey et al. 1992 found enhanced transulphuration
activity with increased methionine content in diets of rainbow trout and observed higher level of hepatic glutathione with dietary supplementation of cysteine, but not with
methionine. There are, however, indications that conversion of methionine to cysteine may not always provide sufficient supply and that taurine may be a limiting amino acid
Ž in fish nutrition under certain conditions Yokoyama and Nakazoe, 1992a; Yokoyama et
. Ž
. Ž
. al., 1994 . Sakaguchi et al. 1988 found increased growth in chum salmon 7–12 g
when a diet based on casein was supplemented with taurine. Moreover, in a study with Ž
Atlantic salmon fed diets with increasing levels of standard soybean meal 0 to 40 .
soybean protein of total protein supplemented with methionine, plasma methionine Ž
concentration increased while plasma taurine concentrations decreased Olli et al.,
. 1995 . Soybean inclusion may also increase excretion of proteolytic enzymes in faeces
Ž .
Ž .
Dabrowski et al., 1989 . Yokoyama and Nakazoe 1992a observed hepatic and muscle Ž
. methionine and cystathionine concentrations to increase in rainbow trout 20 g when
fed casein-based diets supplemented with either 0.5, 1.0 or 1.5
L
-methionine, while glutathione and taurine levels were unaffected.
Ž .
Medium chain triglycerides MCT have received much attention in recent years in animal and human nutrition because of their rapid absorption and oxidation without
Ž .
deposition in the body Mustafa et al., 1991; Papamandjaris et al., 1998 , and for their Ž
. potential nitrogen-sparing effect Tantibhedhyangkul and Hashim, 1975 . MCT also
Ž appear to be potential stimulators of nutrient transport into enterocytes Tajima et al.,
. 1993; Wang et al., 1996 . It appears that MCT may have the potential to enhance
nutrient absorption, decrease amino acid oxidation, reduce body fat deposition and thereby allowing increased protein utilisation. No information is available indicating
whether this potential can be utilised in salmonids.
The aim of the present study was to investigate the effects of dietary supplementation with either methionine, cysteine or MCT on growth, digestibility, nitrogen and energy
retention, levels of plasma amino acids and activity of pancreatic enzymes in Atlantic salmon under pair-feeding and conditions of limited protein supply.
2. Materials and methods
