Dietary glutamine improves growth and intestinal morphology of juvenile GIFT tilapia (Oreochromis niloticus) but has limited effects on innate immunity and antioxidant capacity

This study evaluated the effects of dietary glutamine (Gln) supplementation on growth, intestinal morphology, and health status of Nile tilapia juveniles. Four hundred forty GIFT tilapia juveniles (4.4 +/- 0.1 g) were randomly distributed in 40 tanks (250-L) and fed five practical diets supplemented with graded levels of L-glutamine (0, 0.5, 1.0, 1.5, and 2.0% Gln) with eight replicate tanks/diet. The experimental diets were formulated to contain 29% digestible protein and 14.5 MJ kg-1 digestible energy. After 90 days of feeding, growth performance, in-testinal morphology, and body proximate composition were evaluated. Additionally, eight fish per treatment were sampled for the assessment of hematological profile, antioxidant capacity and innate immune responses. Glutamine supplementation improved relative weight and length of the intestine, as well as villus length and villus:crypt ratio, which could be related to better feed efficiency observed in fish fed Gln-supplemented diets. Final body weight and weight gain also were positively affected by dietary Gln supplementation. Fish fed Gln-supplemented diets showed increased glutathione (GSH) concentrations in the intestine. However, Gln supple-mentation had no significant effects on hematological and biochemical parameters and presented limited effects on innate immune responses. In summary, dietary Gln supplementation improved growth, feed efficiency, and promoted Nile tilapia intestinal development and function. Based on polynomial regression analysis of growth performance parameters, the recommended level of Gln supplementation in diets for juvenile Nile tilapia was estimated to be 1.33% of dry weight. These results also imply that higher levels of Gln supplementation may be required to support enhanced immune responses and antioxidant capacity, but further studies are necessary to evaluate these latter notions.