Tracer Techniques for the Study of Metabolism
THE AUTHORS IN THIS SECTION discuss various tracer techniques, stable isotopes, positron emission tomography, and nuclear magnetic resonance, for studying metabolic processes. Chapters 8 and 9 focus on stable isotopes, which can be used to study turnover of protein, carbohydrate, and fat, thus monitoring changes in energy expenditure, relative fuel utilization, gluconogenesis, and other aspects of metabolic substrate oxidation. For in vivo nutritional studies, stable isotope tracers are injected into accessible compartments in order to compile data on inaccessible compartments from the tracer dilution curve. Mass spectrometry is the analytical method used for measuring stable isotope samples. The problem of the natural abundance of stable isotopes, which differs from place to place, must be overcome if any meaningful data are to be collected, especially in the field.
Positron emission tomography (PET), a noninvasive technique for the evaluation of protein metabolism that traces radioactive atoms incorporated into biological fuels, is discussed in Chapter 10. This technique can be used in conjunction with stable isotopes to evaluate the contribution of individual organs and body areas to whole-body protein metabolic processes. Unfortunately, widespread use of PET is not possible at this time due to the limited number of specialized laboratories that are needed and the high cost of operation.
Chapter 11 describes the use of nuclear magnetic resonance (NMR) spectroscopy for the investigation of both liver and muscle glycogen synthesis as well as the pathogenesis of Type II diabetes mellitus. This technique is based on the action of nuclei in the presence of a strong magnetic field, which in the end generates an image from released energy. For the purposes described here, NMR is noninvasive and safe but is relatively insensitive and expensive when the costs of the magnet and staff are considered. The use of NMR imaging for body composition analysis is described in Chapter 5 of this volume.