211 © Springer Nature Switzerland AG 2019 A. J. Krentz et al. (eds.), Translational Research Methods in Diabetes, Obesity, and Nonalcoholic Fatty Liver Disease, https://doi.org/10.1007/978-3-030-11748-1_9 Isotopic Tracers for the Measurement of Metabolic Flux Rates Carine Beysen, Thomas E. Angel, Marc K. Hellerstein, and Scott M. Turner Summary Background Stable isotopes have been used for more than 75 years to assess rates of synthesis, degradation, fuxes and turnover of proteins, lipids, and carbo- hydrates in animals and humans. Advances in the sensitivity and sophistication of mass spectrome- try and modeling and interpretation of tracer data have made possible the development of numerous useful tools for determining the turnover or many pathways that are important targets in disease development and treatment with pharmacological agents for cardiometabolic disease. Stable isotope tracers are safe and simple to administer in a clinical research setting and now allow – using minimally invasive techniques – the activity of synthetic and catabolic pathways to be quantifed. The activity of agents that target the synthesis or degradation of fatty acids, triglycerides, glucose, or proteins can be directly assessed in individual subjects before and after treatment and in diseases compared to health. The sensitive and quantitative nature of these measurements generally provides clear signifcant results with fewer than 20 sub- jects, allowing early assessment of pharmacologic activity in clinical studies. Because these measure- ments are quantitative, they can be very useful for defning dose response and pharmacokinetic/phar- macodynamic (PK/PD) relationships. Key Methods There are numerous publications describing iso- tope tracer methods. Our aim here is to provide methods and approaches that have worked well in our hands and with collaborators for investi- gations of pathophysiology and therapeutic interventions in early clinical trials. In many instances these isotope methods can be per- formed concurrently, providing a “multiplexed” approach to metabolic fux analysis. Several examples of multiplexed study designs will be provided. Methods are divided in the table below according to the metabolic pathway assessed. C. Beysen (*) ProSciento, Chula Vista, CA, USA e-mail: carine.beysen@prosciento.com T. Angel GlaxoSmithKline, King of Prussia, PA, USA e-mail: thomas.e.angel@gsk.com M. K. Hellerstein University of California Berkeley, Berkeley, CA, USA e-mail: marc@berkley.edu S. M. Turner Pliant Therapeutics, South San Francisco, CA, USA 9