Dietary glycerol partially replaces excess lipids in the diet of GIFT tilapia (Oreochromis niloticus) under hyperthermal stress conditions.

The study evaluated dietary glycerol as a lipid substitute for genetically improved farmed tilapia (GIFT, Oreochromis niloticus) juveniles reared under hyperthermal stress. A 60-day feeding trial was conducted with fish (initial weight 6.25 ± 0.34 g) exposed to ambient temperature (28.3 ± 0.02 °C) or high temperature (33.3 ± 0.07 °C) in a freshwater recirculatory aquaculture system. Three isonitrogenous (340 g kg⁻1 crude protein) and isocaloric (16.69 ± 0.04 MJ digestible energy kg⁻1) diets were formulated with decreasing dietary lipid levels (12%, 10%, 8%) replaced by graded glycerol inclusion (0%, 4%, 8%). Moderate glycerol substitution (4%) significantly enhanced growth performance and feed efficiency under high temperature by stimulating glycerol metabolism, as evidenced by elevated hepatic glycerol kinase and glycerol-3-phosphate dehydrogenase activities. This dietary treatment reduced protein catabolism through suppressed glutamate dehydrogenase activity while promoting a metabolic shift toward lipid catabolism, characterized by increased lipolytic markers (muscle lipoprotein lipase, hepatic carnitine palmitoyltransferase I, and peroxisome proliferator-activated receptor alpha gene expression) while decreased lipogenic indicators (hepatic glucose-6-phosphate dehydrogenase, malic enzyme, and fatty acid synthase gene expression). Histological analysis revealed reduced hepatic lipid vacuolization and increased hepatocyte density in fish fed 4% glycerol at high temperature, accompanied by decreased plasma cortisol and hepatic heat shock protein 70 expression, indicating enhanced thermal stress responses. These findings demonstrate that 4% purified glycerol substitution for 2% dietary lipid optimizes growth, feed utilization, and metabolic efficiency in GIFT under elevated temperatures.