An optimum level of vitamin A supplements for Atlantic halibut (Hippoglossus hippoglossus L.) juveniles

Abstract Six diets with different levels of retinyl acetate were fed in triplicate to Atlantic halibut larvae for 13 weeks. The dietary levels used were 0, 0.25, 0.75 (National Research Council (NRC) recommendation), 2.5, 25 and 250 mg retinol equivalents (RE) kg −1 diet (in dry weight). Any sign of deficiency or toxicity was recorded and analysed. No differences in mortality or weight gain were observed among the groups, although the groups fed 0 or 0.25 mg RE kg −1 diet had external haemorrhages and a reduced growth in length. Levels of retinol (ROH), didehydro retinol (dd-ROH), retinyl esters (ROHes) and didehydro retinyl esters (dd-ROHes) were analysed using HPLC equipped with a diode array detector. Between the groups fed 0–0.75 and 2.5–250 mg RE kg −1 diet, there was an overall change in retinoid levels from being indifferent to increasing exponentially. Groups fed 2.5 mg RE kg −1 diet had the highest specific activity in two brush border membrane (BBM) enzymes, indicating that the fish fed this diet had a highly developed or differentiated intestinal mucosa. Furthermore, DNA levels were lower in the groups fed 0.75–25 mg RE kg −1 diet (dry weight), while the other groups of fish showed higher proliferation of cells. This proliferation was confirmed by immunohistochemistry using proliferating cell nuclear antigen (PCNA). The results indicated that the optimum level of retinoids in the diets for Atlantic halibut juveniles was closer to 2.5 mg than the recommended 0.75 mg RE kg −1 diet. Our recommendation is to use 2.5 mg RE kg −1 diet but not more because retinoids are shown to be toxic in excess.