muscle glycogen synthesis. Finally, this paper will discuss the utilization of ¹³C NMR spectroscopy in the noninvasive measurement of hepatic glycogen content for direct quantification of hepatic glycogenolysis and indirect quantification of gluconeogenesis in normal subjects and subjects with Type II diabetes mellitus.

This paper will provide only a brief overview of the basic principles of NMR spectroscopy, which are reviewed in detail elsewhere (Gadian, 1982; Jardetzky and Roberts, 1981). The technique of NMR spectroscopy relies on the spin properties of certain atomic nuclei, which make them behave like tiny bar magnets. Within molecules, these nuclei are usually oriented randomly in space. One might expect then that, when placed in a magnetic field, these nuclei will behave like a compass needle and line up with the field. However, as a result of the laws of quantum mechanics, these nuclei do not behave like conventional bar magnets but instead they tend to line up either with or against the field with the two different orientations having slightly different energies (Gadian, 1982; Jardetzky and Roberts, 1981). When subjected to an oscillating magnetic field, the nuclei can be made to move between these transition states. Under the applied magnetic field, the nuclei will precess (resonate) at a particular frequency. The higher the magnetic field, the faster the frequency of precession and the greater the difference between the two energy states. The electromagnetic frequency at which precession occurs depends on the particular nucleus being analyzed and its molecular environment (Gadian, 1982; Jardetzky and Roberts, 1981). Table 11-1 shows some of the nuclei that can be studied using NMR spectroscopy. Hydrogen nuclei, in the form of protons (¹H), when placed in a magnetic field of 2.1 Tesla (T) (the unit of magnetic field intensity) will precess at 89.5 megahertz (MHz). Overall, ¹H NMR spectroscopy is the most sensitive