Used as a stable isotope-labeled internal standard in mass spectrometry and metabolic studies to accurately quantify leucine and related metabolites in biological samples. Enables precise tracking of protein synthesis, amino acid metabolism, and nutrient utilization in research involving humans, animals, or cell cultures. Its isotopic enrichment allows clear differentiation from endogenous leucine, improving measurement accuracy in pharmacokinetic and metabolic flux studies. Commonly applied in clinical diagnostics, proteomics, and nutritional science.