Dietary low-glucosinolate rapeseed meal affects thyroid status and nutrient utilization in rainbow trout (Oncorhynchus mykiss)

Two rapeseed (Brassica napus) meals, RM1 and RM2, with two levels of glucos inolates (GLS; 5 and 41 mu mol/g DM respectively) were incorporated at the levels of 300 and 500 g/kg of the diets of juvenile rainbow trout (Oncorhyn chus mykiss) in replacement of fish meal, and compared with a fish-meal-bas ed diet. A decrease in the digestibility of the DM, protein, gross energy a nd P was observed with high-rapeseed meal (RM) incorporation. In trout fed on PM-based diets, growth performance was reduced even after only 3 weeks o f feeding. Feed efficiency was adversely affected by RM and GLS intake. Pro tein and energy retention coefficients were significantly lower in fish fed on the diet containing the higher level of GLS. P retention was significan tly lower with all the RM-based dirts than with the fish-meal diet. Irrespe ctive of the degree of growth inhibition, fish fed on RM-based diets exhibi ted similar typical features of hypothyroid condition due to GLS intake, ex pressed by lower plasma levels of triiodothyronine and especially thyroxine and a hyperactivity of the thyroid follicles. This hypothyroidal condition led to a strong adjustment of the deiodinase activities in the liver, the kidney and the brain. A significant increase of the outer ring deiodinase a ctivities (deiodinases type I and II respectively) and a decrease of the in ner ring deiodinase activity (deiodinase type III) were observed. It is con cluded that the observed growth depression could be attributed to the conco mitant presence of GLS, depressing the thyroid function, and of other antin utritional factors affecting digestibility and the metabolic utilization of dietary nutrients and energy.