Phenylalanine as a biomarker of disease in animals: Current evidence and future perspectives.

Phenylalanine (Phe), an essential aromatic amino acid, is emerging as a potential biomarker of metabolic and health status in animals. Altered Phe concentrations reflect changes in metabolic, inflammatory, and disease processes. In companion animals, shifts in serum or biofluid Phe have been linked to hepatopathies, inflammatory enteropathies, and neoplastic conditions. In livestock, variations in milk or serum Phe accompany mastitis, metabolic stress, lameness, and other production-related disorders. Evidence for cats, horses, poultry, and aquaculture species remains limited. Interpretation is complicated by analytical variability, small sample sizes, and confounding factors such as diet, stress, and microbiome composition. Non-invasive sampling of milk, urine, saliva, and breath, shows promise for Phe monitoring across animal species. Advances in multi-omics, particularly metabolomics, proteomics, and microbiome analyses, can clarify mechanisms and support development of composite biomarker panels. This review evaluates current evidence on Phe as a biomarker across species, highlights gaps in research coverage and methodology, and outlines priorities for future work. Expanding studies to underrepresented species, standardizing measurement protocols, and conducting longitudinal research. Addressing these priorities is essential to establish Phe as a robust biomarker, for veterinary diagnostics, health monitoring, and the optimization of animal welfare and production systems. SHORT SUMMARY: This review looks at phenylalanine (Phe), an amino acid, as a possible health indicator in animals. Changes in Phe levels may show problems with metabolism, inflammation, or disease. Evidence is strongest for dogs and dairy cows, but limited for cats, horses, poultry, and fish. Results vary due to measurement methods and other factors. Future studies should standardize testing, include more species, and combine methods to improve Phe's usefulness in monitoring animal health.