Threonine supplementation stimulates immune, antioxidant and liver functions of largemouth bass, Micropterus salmoides

Threonine is an essential amino acid required in the diets for regular growth of fishes, but information is inadequate regarding the effects of threonine on immunity and health status of many aquaculture species. In the present study, a total of 360 juvenile largemouth bass, Micropterus salmoides (19.0 ± 0.1 g) were divided into six experimental groups, and fed diets supplemented with varying levels of crystalline L-threonine at 0 g kg⁻¹ (control), 2.5 g kg⁻¹ (Thr-2.5), 5.0 g kg⁻¹ (Thr-5.0), 7.5 g kg⁻¹ (Thr-7.5), 10.0 g kg⁻¹ (Thr-10.0) and 12.5 g kg⁻¹ (Thr-12.5) for eight weeks. The results revealed that white blood cell count (P = 0.024) and albumin content (P = 0.006) in fish were significantly modulated by the dietary threonine, with the highest levels recorded in fish fed diets Thr-5.0 and Thr-12.5, respectively. White blood cell increased with dietary threonine levels up to Thr-5.0, then decreased thereafter. The other hematological variables such as red blood cell (P = 0.571), hemoglobin (P = 0.259), hematocrit (P = 0.985), mean corpuscular volume (P = 0.631), mean corpuscular hemoglobin (P = 0.280) and mean corpuscular hemoglobin concentration (P = 0.691) remained unchanged as compared to the control group. Serum lysozyme increased (P = 0.000) with increasing levels of threonine up to 5.0 g kg⁻¹. Fish receiving diet Thr-2.5 had relatively higher (P = 0.000) immunoglobulin M and the level decreased when fed with increasing levels of threonine. The activities of superoxide dismutase (P = 0.008) and malondialdehyde (P = 0.005) were elevated at 10.0 g kg⁻¹ and 12.5 g kg⁻¹ supplementation levels, respectively. Moreover, diets Thr-5.0 and Thr-10.0 induced lower serum alanine aminotransferase (P = 0.011) and aspartate aminotransferase activities (P = 0.000) in fish. These results suggest that threonine supplementation led to a positive effect on the hematology, humoral immunity, antioxidant capacity and liver function enzyme activities of economically important largemouth bass. Therefore, threonine has a clear role in fish nutrition and is important in formulating nutritionally balanced feeds.