Effects of Dietary Vitamin A Concentration and Stress on Astaxanthin Utilization in Atlantic Salmon (Salmo salar)

Diets with 30 and 60 ppm astaxanthin in combination with vitamin A (VA) concentrations of 6500, 35,000, and 100,000 IU/kg were fed to Atlantic salmon post smolts in seawater for 17 weeks. Flesh color (SalmoFan 27.1-27.5), astaxanthin concentration (3 mg/kg), retention (4.8% and 8.9% at 30 and 60 mg astaxanthin/kg respectively), and digestibility (29% and 12% at 30 and 60 mg astaxanthin/kg, respectively) were lowest at the highest dietary VA concentration. Increasing VA concentration in the diet also reduced the astaxanthin concentrations in liver and plasma, but not in intestine. Hepatocytes and enterocytes were isolated from fish fed the six diets and incubated for 48 h with radiolabeled astaxanthin to study metabolic transformation of astaxanthin in vitro. Astaxanthin was metabolized to idoxanthin, β-carotene, retinol, and retinal in both cell types, but astaxanthin made up a larger proportion of total radioactivity in hepatocytes than in enterocytes. Hepatocytes from salmon fed high VA in the diet had the highest astaxanthin concentration (39% of the radioactivity), whereas the highest VA concentration (6% of the radioactivity) was found in hepatocytes from fish that received the lowest dietary VA concentration. Enterocytes from salmon fed the medium VA diet, had highest concentrations of astaxanthin (17%-26%), retinol (6.4%-7%) and the lowest concentration of idoxanthin (14%-31%). After the 17-week feeding trial, salmon fed four of the diets (30 and 60 ppm astaxanthin in combination with 6500 and 100,000 IU/kg VA) were exposed to repeated stress (crowding and low oxygen) for a period of 5 weeks. Stress reduced the astaxanthin concentrations in muscle with around 0.5 mg/kg, except in salmon fed the diet with high VA and 60 ppm astaxanthin, which maintained an astaxanthin concentration of 3.3 mg/kg. Changes in the part of the carotenoid NMR spectrum in salmon exposed to stress indicated that stress affected the conversion of astaxanthin in interaction with dietary VA and astaxanthin. The effects of diet VA and astaxanthin concentration in combination with stress on gene expression in intestine, liver, and muscle were measured, and minor effects on several immune, stress, and metabolic genes were found.