Dietary lactic acid and rosemary leaf supplementation enhances growth and immune responses in Nile tilapia (Oreochromis niloticus)

The present study investigated the individual and combined effects of lactic acid and rosemary meal on growth performance, biochemical parameters, immune responses, the expression of growth and antioxidant-related genes, intestinal morphology, and oxidative status in Nile tilapia (Oreochromis niloticus). A total of 120 apparently healthy fish, with an average weight of 3.03 ± 0.02 g, were randomly assigned to four equal groups, each consisting of three replicates. Four experimental diets were formulated: a basal control diet (CON), a basal diet supplemented with 1 g of lactic acid per kg of diet (LA), a basal diet containing 10 g of rosemary per kilogram of diet (RM), and a basal diet that included both supplements (LA + RM). Fish were fed these diets for a duration of 60 days. The results indicated that either lactic acid or rosemary alone or in combination had a greater growth-stimulating impact than the CON group (P ≤ 0.05) with superiority to the combination group (LA + RM group). Activities of aspartate aminotransferase and alanine aminotransferase, along with creatinine and urea levels, were significantly reduced (P ≤ 0.05) in the groups of lactic acid and rosemary alone or in combination relative to the CON group. Total protein and albumin concentrations were elevated in the LA + RM group (P ≤ 0.05). Intestinal histology revealed normal morphology across groups, with increased villus height, intestinal villi spacing, and goblet cell density in LA + RM (P ≤ 0.05) without pathological lesions in the liver and spleen. Antioxidant, immune, and growth-related gene expressions were upregulated in RM and LA + RM groups. In conclusion, rosemary supplementation, alone or combined with lactic acid, enhanced fish health status and upregulated target genes without pathological lesions with superiority to the combination treatment.