HIGH DIETARY LEVELS OF BRANCHED-CHAIN AMINO ACIDS AFFECT GROWTH PERFORMANCE AND AMINO ACID UTILIZATION IN RED DRUM Sciaenops ocellatus

Sergio Castillo* and Delbert M. Gatlin III
 
Department of Wildlife and Fisheries Sciences
Texas A&M University, College Station, TX 77843
srgcastilloalv@gmail.com

The minimum dietary requirements for the branched-chain amino acids (BCAAs) by red drum have been quantified at 1.6, 1.1, and 1.2% of dry diet (35% crude protein [CP]) for leucine (Leu), isoleucine (lle) and valine (Val), respectively. However, it should be noted that all three BCAAs share the same common transporter for intestinal absorption and most organisms metabolize these three amino acids (AAs) using the same enzymatic system. Because of this, interactions between BCAAs are known to produce antagonistic effects in pigs, rats and humans, where an excess of Leu may reduce the concentration of Ile and Val in plasma, ultimately altering protein synthesis and affecting growth. In fish, an antagonistic interaction involving BCAAs has not been fully assessed; therefore, this study aimed to elucidate the effects of high dietary levels of BCAAs on growth performance and AA utilization of red drum. This is of vital importance because a current priority in the aquaculture industry is to replace the use of fishmeal with alternative protein ingredients, including plant-based feedstuffs; however, unbalanced AA profiles in plant-based ingredients (e.g., high Leu levels in corn protein feedstuffs) could potentially impair fish growth.

A semi-purified control diet (35% CP) was prepared by combining lyophilized red drum muscle (RDM) (10.5% of dietary CP) and supplementing with crystalline L-AAs to simulate the AA pattern of RDM, while keeping the levels of Leu, Ile and Val at the minimum dietary requirement levels for red drum. In addition, six experimental diets were prepared by supplementing the control diet with (1) an excess of Leu (6.2% of dry diet), (2) an excess of Ile (4.4% of dry diet), (3) an excess of Val (5.0% of dry diet), (4) an excess of Leu and Ile (6.2 and 4.4%), (5) an excess of Ile and Val (4.4 and 5.0%), and (6) an excess of Leu and Val (6.2 and 5.0%). These levels correspond to 200% of the amount found in 35% CP provided by RDM. A 50:50 mixture of aspartate and glutamate was used to maintain all diets isonitrogenous. Red drum juveniles were stocked in 38-L glass aquaria (12 fish/aquarium), and diets were fed to fish in triplicate aquaria at a rate approaching apparent satiation, twice daily, for 49 days. At the end of the feeding trial, growth performance parameters were calculated. In addition, the postprandial (4 h after feeding) concentration of plasma and muscle free AAs was analyzed.

A summary of the obtained results can be found in Table 1. Weight gain was significantly (P<0.05; Dunnett's test) affected with an excess of Leu and an excess of any two BCAAs, when compared to the control diet. Based on weight and BCAAs levels in plasma, it seems that, as in mammals, Leu plays a critical role in regulating the concentrations of Ile and Val for red drum's optimal growth. A thorough discussion of all the results obtained and their implications will be presented at the meeting.