Advances in stable isotope tracer methodology part 1: hepatic metabolism via isotopomer analysis and postprandial lipolysis modeling

Correspondence to Dr Melanie Cree-Green, University of Colorado Denver School of Medicine, Aurora, Colorado 80045-2581, USA; [email protected]

Introduction 

Isotopes have been used to investigate physiology for over 80 years.¹ Many studies in humans use stable isotopes, which are not radioactive and are well-tolerated when used in physiologic concentrations.² General methods for the use of stable isotopes to study physiology and pathology have been well described^{2 3} and the principal organs that drive overall whole-body metabolism include muscle, adipose, and the liver, with critical smaller roles played by the brain and skeletal system, among others.^{2 3} Classic isotope assessments quantify the rate of appearance (Ra) of glucose from the liver, the rate of lipolysis from adipose tissue, measures of protein turnover and rates of the oxidation of glucose, fats and proteins/amino acids.^{4 5} Continuous efforts have been employed to expand the types of physiologic pathways that can be measured and to improve the accuracy of existing models. Here, we describe two distinct advancements for the use of stable isotope tracers to assess human physiology.

Methods of sample analysis have traditionally involved the use of mass spectrometry to determine the isotopic enrichment of the compound of interest, paired with gas chromatography for better separation of compounds. Newer methods have replaced gas chromatography with high pressure liquid chromatography.⁶ With mass spectrometry, selecting different methods of derivatization of the sample or different ion fragments for selective ion monitoring in the mass spectrometer supports the analysis of specific atoms within the fragment. Additional insights can be gained by combining tracers that interact through metabolic processes of interest, such as the tricarboxylic acid (TCA) cycle flux⁷ or hepatic triglyceride synthesis⁸ to name a few. More recently, nuclear magnetic resonance (NMR) has been used to more precisely quantitate isotopomers.⁹ Herein we describe the application of this methodology combined with an oral glycerol tracer to assess relative hepatic pathway flux during the production of glucose.

Mathematical modeling of stable isotope data, such as the oral minimal model (OMM) used to describe glucose dynamics during an oral glucose tolerance test (OGTT),^10–12 can be critical for linking how the dynamics of a substrate are influenced by insulin sensitivity. Renewed interest has occurred in the...