The present study was carried out to investigate whether the dietary
histidine requirement to reduce
cataract development is higher than that for growth in Atlantic salmon smolts (Salmo salar L.) after seawater transfer and whether dietary
vegetable oils contribute to cataractogenesis. Duplicate groups of salmon smolts were fed ten experimental diets with either
fish oil (FO) or a
vegetable oil (VO) mix replacing 70 % FO and
histidine at five target levels (10, 12, 14, 16 and 18 g His/kg diet) for 13 weeks after seawater transfer. The VO diet-fed fish exhibited somewhat inferior growth and feed intakes compared with the FO diet-fed fish, irrespective of the dietary
histidine concentration. Both
cataract prevalence and severity were negatively correlated with the dietary
histidine concentration, while lens N-acetyl-
histidine (NAH) concentrations were positively correlated with it. The
fatty acid profiles of muscle, heart and lens reflected that of the
dietary oils to a descending degree and did not affect the observed
cataract development. Muscle, heart and brain
histidine concentrations reflected dietary
histidine concentrations, while the corresponding tissue
imidazole (
anserine,
carnosine and NAH) concentrations appeared to saturate differently with time. The expression level of liver
histidase was not affected by the dietary
histidine concentration, while the liver
antioxidant response was affected in the VO diet-fed fish on a transcriptional level. The lowest severity of
cataracts could be achieved by feeding 13·4 g His/kg feed, independently of the dietary
lipid source. However, the present study also suggests that the dietary
histidine requirement to minimise the risk of
cataract development is 14·4 g His/kg feed.