Effects of dietary linolenic acid to linoleic acid ratio on growth, tissue fatty acid profile and immune response of the juvenile grouper Epinephelus malabaricus

A 12-wk feeding trial was conducted to determine the dietary effects of α-linolenic (LN; 18:3n − 3) and linoleic acids (LA; 18:2n − 6) on growth, tissue fatty acids profile and non-specific cellular immune response in kidney of juvenile grouper Epinephelus malabaricus (initial weight: 11.3 ± 0.6 g). Eight experimental diets and a reference diet each containing 11.6% lipids were compared. Four experimental diets contained either 1 or 2% diet of pure LN or LA (N1, N2, L1, and L2). The other 4 diets contained oil mixtures of linseed oil, safflower oil, LA, and oleic acid and had LN:LA ratios of 3.3, 1.4, 0.7 and 0.4 (NL3, NL1.4, NL0.7, and NL0.4). The reference diet contained a mixture of cod liver oil, linseed oil and safflower oil at a ratio of 2:1:1. Weight gain of the grouper was significantly increased with increasing dietary LN:LA ratios. The group that fed on the NL3 diet showed the highest growth. Weight gains of this group equaled those of the reference group. Dietary inclusion of LN increased the presence of n − 3 polyunsaturated fatty acids (PUFAs) in the grouper, except 22:6n − 3 and 22:5n − 3 in neutral lipid fraction of the liver. 18:2n − 6 and 18:3n − 6, but not 20:4n − 6, were the predominant n − 6 PUFAs in the body of the grouper, and their quantities were closely related to their quantitative dietary inclusion. Non-specific cellular immune response was significantly increased with increasing dietary LN:LA ratios. The grouper that fed on the NL3 diet showed the significantly highest head-kidney leucocyte phagocytic and respiratory burst activities among all treatments. Our results suggest that LN and LA at a dietary level of 2% and a ratio of 3:1 are beneficial to weight gain and non-specific cellular immune responses of the juvenile grouper.

► Grouper growth is improved with increasing dietary linolenic acid/linoleic acid ratios.
► Grouper fed the diet with the highest linolenic acid/linoleic acid ratio (3:1) grow best.
► Grouper's immunity is enhanced with high dietary linolenic acid/linoleic acid ratio.