Effects of dietary limonene and thymol on the growth and nutritional physiology of Nile tilapia (Oreochromis niloticus)

Phytogenic compounds such as limonene and thymol have been shown to have growth-promoting properties in farmed animals but studies in fish are scarce. Two experiments (Experiments I and II) were carried out to investigate the individual effects of dietary limonene and thymol on the growth and nutritional physiology of juvenile Nile tilapia (Oreochromis niloticus). In Experiment I, the fish were fed on a commercial diet coated with limonene at 0 (Control), 200, 400, and 600 mg kg(-1) (ppm), while in Experiment II thymol was supplemented in the diet at 0 (Control), 250 and 500 ppm. Our results showed a significant increase in fish weight and weight gain with diets supplemented with 400 and 600 ppm limonene compared to the control. Moreover, the expression of insulin growth factor I (igf-I), mucin-like protein (muc), oligo-peptide transporter I (pept1), lipoprotein lipase (lpl), alkaline phosphatase (alp) and catalase (cat) was up-regulated by dietary limonene. Our results confirm that dietary limonene can enhance the growth of Nile tilapia juveniles through the activation of key genes involved in somatotropic axis-mediated growth, nutrient digestion and antioxidant enzyme defence. Dietary thymol did not seem to influence growth or regulate the same pathways activated by limonene in Nile tilapia juveniles at inclusion levels up to 500 ppm. Overall, the present results suggest that potential growth-promoting effects are dependent upon the phytogenetic itself and its inclusion level.