Used as a stable isotope-labeled tracer in metabolic studies to track phenylalanine metabolism in humans and animals. Helps in diagnosing phenylketonuria (PKU) and other metabolic disorders by enabling precise measurement of metabolic fluxes using mass spectrometry or NMR. Employed in protein biosynthesis research to study amino acid incorporation and turnover rates. Also utilized in the development and validation of analytical methods for amino acid quantification in biological samples.