Simple nutrients such as lactate, fatty acids, and glucose enriched with 13C are used to measure the rate through different pathways. The 13C is incorporated into intermediates as a result of chemical transformation, and the location and amount of 13C present in the carbon backbone can be determined using 13C NMR.
Analysis of the spectra allows one to determine rates of substrate oxidation (the flux through the citric acid cycle), net uptake of carbon (anaplerosis) that feeds processes such as gluconeogenesis, and flux through substrate cycles (for example, pyruvate cycling).
Incorporation of 13C into glutamate. A much simplified view of the incorporation of 13C into an intermediate (in this case, glutamate via the citric acid cycle) to determine the metabolism of 13C-labeled nutrients.