Biosynthesis and tissue deposition of docosahexaenoic acid (22∶6n−3) in rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss) weighing ca. 5 g and previously acclimated for 8 wk on a diet comprising vegetable oil (11%), fish meal (5%), and casein (48%) as the major constituents were fed a pulse of diet containing deuterated (D5) (17,17,18,18,18)‐18∶3n−3 ethyl ester. The synthesis and tissue distribution of D5‐22∶6n−3 was determined 3,7,14, 24, and 35 d after the pulse. The whole‐body accumulation of D5‐22∶6n−3 was linear over the first 7 d, corresponding to a rate of 0.54±0.12 μg D5‐22∶6n−3/g fish/mg D5‐18∶3n−3 eaten/d. Maximal accretion of D5‐22∶6n−3 was 4.3±1.2 μg/g fish/mg of D5‐18∶3n−3 eaten after 14 d. The amount of D5‐22∶6n−3 peaked in liver at day 7, in brain and eyes at day 24, and plateaued after day 14 in visceral and eye socket adipose tissue and in the whole fish. The majority of D5‐22∶6n−3 was found in the carcass (remaining tissues minus the above tissues analyzed separately) at all times. On a per milligram lipid basis, liver and eyes had the highest concentration of D5‐22∶6n−3. The experimental diet also contained 21∶4n−6 ethyl ester as a marker to estimate the amount of food eaten by individual fish. From such estimates it was calculated that the great majority of the D5‐tracer was catabolized, with the combined recovery of D5‐18∶3n−3 plus D5‐22∶6n−3 being 2.6%. The recovery of 21∶4n−6 was 57.6%. The concentration of 22∶6n−3 in the fish decreased during the 13‐wk period, and the amount of 22∶6n−3 synthesized from 18∶3n−3 was only about 5% of that obtained directly from the fish meal in the diet.