Blood-based biomarkers of protein digestibility, utilization, and nitrogen excretion in animals: Concepts, evidence,...

Improving dietary protein utilization is a central goal in animal nutrition because protein is costly, closely linked to growth and production, and a major driver of nitrogen losses to the environment. Protein digestibility is traditionally quantified using fecal or ileal measurements; however, these approaches are labor-intensive and difficult to apply at scale. Consequently, there is growing interest in blood-based biomarkers that can provide indirect yet practical insight into (i) the digestion and absorption of dietary protein and amino acids (AA) and (ii) the extent to which surplus absorbed nitrogen is converted into excretory products, primarily urea in mammals and uric acid in birds. Blood biomarkers are attractive because they are minimally invasive, allow repeated sampling over time, and can be readily integrated into precision feeding, herd monitoring, and genetic selection programs. This review synthesizes current knowledge on the physiological links between protein digestion, AA utilization, and nitrogen excretion, and critically evaluates blood biomarkers across ruminants and monogastric species. Biomarkers discussed include plasma urea nitrogen (PUN) or serum urea nitrogen (SUN), milk urea nitrogen (MUN), plasma uric acid, blood AA profiles and postprandial kinetics, emerging peptide-derived AA indicators, and metabolomics-based biomarker panels. Key confounding factors affecting interpretation such as renal function, energy balance, rumen nitrogen recycling, physiological stage, and sampling time are examined, and practical standards for biomarker application and validation are proposed. Overall, the review highlights the value of a panel-based biomarker approach for improving assessment of protein utilization efficiency and nitrogen excretion risk in animal production systems.