In vivo research with stable isotopes in biochemistry, nutrition and clinical medicine: an overview.

Tracers and kinetic modelling provide the opportunity to follow the movement and to quantify the metabolic fates of biological compounds in vivo. For studies in children and for repeated studies in adults, the use of stable isotopically labelled substrates are preferable and safe. Measurement of isotopic enrichment in biological molecules is highly specific and can be extremely precise. This allows the development of models of biological system dynamics in cells and organs that are otherwise inaccessible for sampling. Applications in biochemistry, nutrition and clinical medicine show the potential of stable isotopes in vivo. Methodology is of paramount importance and includes the choice of bolus studies, (primed) continuous infusions, use of multiple tracers and use of isotopomer information and intrinsic labels. There is no limit to the number and kind of compounds that can be traced. Topics include intermediate metabolism (lipids, proteins and carbohydrates) using hydrogen, carbon and nitrogen labels. In clinical medicine, 13C-breath tests are available for detection of Helicobacter pylori ([13C]urea breath test), and for assessment of a variety of gastro-intestinal and hepatic functions (e.g. [13C]octanoate and mixed [13C]triglyceride breath tests for studies of gastric emptying and fat absorption, respectively). Because theoretical, methodological, physiological and practical aspects are intertwined, in vivo research with stable isotopes demands close collaboration between the clinical researcher and the chemist responsible for the mass spectrometry.