Hypothesis  The hypothesis of the project is that feeding diets unbalanced in one of the three BCAAs, valine, leucine and isoleucine (relative to lysine) will result in the appearance of metabolic breakdown products of catabolic pathways which can be detected in the blood and urine. These metabolites may serve as either qualitative or quantitative markers for BCAA imbalances and may aid the development of rapid and more precise methods to estimate BCAA requirements in pigs.     Objective The objective is to identify metabolites by metabolomics in blood and urine that are unique in the catabolic pathways of BCAA. The identified markers will be validated in pigs fed diets with a predefined deficiency of one of the BCAAs. The markers will provide the scientific basis for developing novel and improved methods to determine BCAA requirements in pigs.   Implications and perspectives The currently used methods are based on measurements of daily gain and feed intake during several weeks. The variation within these response parameters often makes it difficult to draw conclusions. A panel of markers will contribute with several physiological response parameters improving the empirical strength of estimating BCAA requirements that is needed for further optimization of protein utilization in pigs. Novel intensive short-term methods can also be developed using the markers as response parameters. The results can serve as the scientific background for identifying markers of other essential amino acids. This will in turn result in more precise AA recommendations, improved animal performance and health, improved utilization of dietary protein, lower crude protein concentrations and less nitrogen emissions into the environment. Other commercial aspects of the project include a potential of evaluating protein quality under experimental and practical conditions.