DIETARY ALPHA-LINOLENIC AND LINOLEIC ACIDS COMPETITIVELY AFFECT METABOLISM OF POLYUNSATURATED FATTY-ACIDS IN ARCTIC CHARR (SALVELINUS-ALPINUS)

We investigated the effects of various levels of dietary 18:2(n-6) (li noleic acid) and 18: 3(n-3) (alpha-linolenic acid) on the growth, feed ing, body composition and metabolism of polyunsaturated fatty acids in liver and muscle of Arctic charr. Fish were fed 8 or 12 casein-based semipurified diets in two separate experiments for 12 of 14 wk, respec tively. Low levels of dietary 18:2(n-6) and 18:3(n-3) supplemented wit h pure methyl esters or linseed (flax) oil had an additive effect on f ish growth. For each 100 g of diet, the addition of 1.6-1.7 g of 18:3( n-3) and 2.6 g of 18:2(n-6) led to significantly lower fish growth com pared with the treatment diets with lower amount's of 18:2(n-6). Where as 18:2(n-6) was converted to 20:4(n-6) and 22: 5(n-6) in the absence or presence of dietary 18: 3(n-3), the dominant product of 18:3(n-3) c onversion was consistently 22:6(n-3). High levels of dietary 18: 3(n-3 ) markedly inhibited the conversion of 18:2(n-6), whereas the inhibiti on of dietary 18:2(n-6) on 18: 3(n-3) conversion was noted only when t he ratio of dietary 18:2(n-6) to 18:3(n-3) changed from 1.0 to 1.5. Fe eding diets rich in 18:3(n-3) led to a marked accumulation of 18:3(n-3 ) and 18:4(n-3) in fish muscle but a negligible change in 20:5(n-3) an d 22:6(n-3), regardless of the level of 18:3(n-3) in the diets.