Quantitative Isotope Tracer Methodologies for System-Level Metabolic Flux Analysis and Clinical Translation

Isotope tracer methodology provides a quantitative framework for investigating metabolic flux, substrate turnover and pathway compartmentalization in vivo. Tracer approaches enable dynamic examination of biochemical systems with minimal physiological perturbation, this can be achieved by harnessing the mass distinction and chemical equivalence of stable (e.g., ¹³C, ¹⁵N, ²H) and radioactive (e.g., ¹⁴C, ³H, ¹⁸F) isotopes. This review synthesizes the physicochemical basis of isotopic labeling, tracer dilution principles and nuclear decay mechanisms underlying scintillation detection and positron emission tomography (PET). Emphasis is placed on isotopologue distribution analysis using mass spectrometry (MS) (e.g. natural abundance correction, isotopic impurity adjustment, and resolution of spectral overlap: analytical considerations essential for accurate metabolic flux analysis). Applications in biomolecule (glucose, lipid, protein and glutamine) metabolism are examined, as well as ¹⁸F-fluorodeoxyglucose PET and hybrid PET/computed tomography (translational imaging strategies). The safety, sensitivity, and spatial resolution levels of stable and radioactive tracers are comparatively evaluated. Critical methodological limitations (such as tracer recycling, steady-state assumptions, compartmental modeling limitations, and cost considerations) are addressed to define its limits in terms of interpretation. Emerging advances in multi-isotope tracing, high-resolution MS platforms, and integrated systems level modeling are discussed as future directions. When applied with rigorous quantitative discipline and transparent analytical correction, isotope tracer methodologies remain essential tools for revealing metabolic regulation, disease pathophysiology and therapeutic response.