The effect of dietary vitamin A and astaxanthin on the immunocompetence of rainbow trout

Rainbow trout (Oncorhynchus mykiss) were maintained for 4 months on diets formulated with supplementary vitamin A (18 mg/kg dry diet) and astaxanthin (100 mg/kg dry diet) (diet A⁺Ax⁺), astaxanthin alone (diet A^- Ax⁺), vitamin A alone (diet A⁺Ax^-) and neither vitamin A nor astaxanthin (diet A^- Ax^-) to investigate the immunomodulatory effects of these compounds in fish. Serum concentrations of both vitamin A and astaxanthin reflected dietary levels, with vitamin A supplementation resulting in levels of 32-40 vs. 4 μg/dl in fish fed unsupplemented diets and astaxanthin supplementation resulting in levels of 9-11 μg/ml with levels being undetectable in fish fed unsupplemented diets. However, there was no marked effect of diet on group food conversion efficiencies or growth rates. Of the immune parameters measured, total serum antiprotease activity was found to be significantly lower in the group fed diet A^-Ax^- and classical serum complement activity was also lowest in this group. In addition, a trend for decreased leucocyte migration was observed in the fish fed diets A^-Ax⁺ and A^-Ax^- compared with those fed diets A⁺Ax⁺ and A⁺A^-. On the other hand, total serum immunoglobulin level, specific immunoglobulin level following immunization with Aeromonas salmonicida, serum lysozyme activity and phagocyte respiratory burst activity were unnaffected by dietary vitamin A and/or astaxanthin intake. Overall, the small immunomodulatory effect of vitamin A and/or astaxanthin supplementation observed in this study suggests that these compounds have only limited potential as immunostimulatory agents in practical rainbow trout diets.