Dietary α-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) (linoleic acid) and 18:3(n-3) (α-linolenic acid) on the growth, feeding, 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 or 14 wk, respectively. Low levels of dietary 18:2(n-6) and 18:3(n-3) supplemented with pure methyl esters or linseed (flax) oil had an additive effect on fish 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 compared with the treatment diets with lower amounts of 18:2(n-6). Whereas 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) conversion 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 inhibition of dietary 18:2(n-6) on 18:3(n-3) conversion was noted only when the ratio of dietary 18:2(n-6) to 18:3(n-3) changed from 1.0 to 1.5. Feeding 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) and 22:6(n-3), regardless of the level of 18:3(n-3) in the diets.