Plasma metabolomics reveals distinct responses to acute and chronic heat stress in broilers

Heat stress (HS) poses a significant threat to the global broiler industry by impairing growth performance and welfare of the birds. The metabolic adaptations to varying durations of heat exposure remain poorly understood. This study aimed to characterize the plasma metabolic profiles of broilers subjected to thermoneutral conditions (TN; 32-22°C, age depended), very short-term (8 h), intermediate (1 week), and long-term (5 weeks) cyclic heat stress (HS; 31°C on average), as well as a subsequent recovery (REC) phase after a 1-week period of HS. A total of 450 day-old male Cobb 500 broilers were randomly allocated to five treatments. Each treatment consisted of 6 replicate pens, with 15 birds per pen. The treatments were distributed in two environmentally controlled rooms based on their assigned climatic conditions (thermoneutral or heat stress). We evaluated growth performance, cloaca temperature, and performed liquid chromatography-mass spectrometry (LC-MS) based metabolomics on plasma from six broilers per treatment. As expected, exposure to heat stress for 1 week or longer significantly increased cloaca temperature (>1.5°C), reduced feed intake, and impaired growth performance. Metabolomic analysis revealed distinct, duration-dependent metabolic signatures. Very short-term (8 h) heat stress was uniquely characterized by elevated plasma uridine, suggesting an activation of nucleotide salvage pathways. In contrast, long-term (5 weeks) heat stress induced a catabolic phenotype, evidenced by increased plasma levels of kynurenine, creatinine, and argininosuccinate, reflecting muscle proteolysis and altered amino acid metabolism. Both short- and long-term heat stress suppressed de novo nucleotide synthesis, marked by decreased orotate and N-carbamoyl-aspartate. Notably, these metabolic alterations were largely reversible, with the profiles of recovery broilers returning to thermoneutral levels within 8 hours. Our findings demonstrate that the broilers' metabolic response to heat stress is a dynamic, duration-dependent process. The identified metabolites, particularly uridine and kynurenine, represent promising candidate biomarkers for distinguishing between acute and chronic heat stress, offering a valuable tool for improving welfare monitoring in poultry production.