Stable isotope tracers are molecules where one or more of the atoms of the molecule is/are substituted for an atom, such as hydrogen, nitrogen and carbons, of the same chemical element (same number of protons) but with an additional neutron, engendering a “stable isotope”; these are easily detected and precisely quantitated using mass spectrometry. Commonly used examples of this include: 1) for protein metabolism: 13C phenylalanine, where 1 of the carbons are of the 13C, rather than 12C motif, 2) for fat metabolism, 13C palmitate, or 3) 2H glucose for carbohydrate metabolism. There are a large number of such molecules available with distinct atomic labelling profiles, which may be applied to a vast number of applications; of particular relevance to this audience, the synthesis, release and oxidation of fat, carbohydrates and protein constituents. The application of stable isotope labelled essential amino acid tracers (particularly 2H or 13C motifs on phenylalanine or leucine) have been long used in exercise research and been successfully applied to increasing our understanding of how exercise regulates energy and protein metabolism. Yet, a major limitation to the application of these tracers has been their restriction to studies of a shorter-term nature (less than 24 h) and under controlled laboratory environments. This is in addition to the invasive requirements of these approaches e.g. the need for cannulation for I.V infusions and concurrent arterial cannulation or arterialization of blood (for arterio-venous balance techniques to quantify synthesis and breakdown across a vascular bed), as well as the need for multiple muscle tissue biopsies. Not to mention the sterility requirements for isotope I.V infusates and clinical consumables. Recent technological advances in analytical instrumentation (e.g. pyrolysis IRMS and LC-MS/MS) have opened up a number of new avenues of research application and in a less invasive manner. Firstly, the re-introduction of heavy water (deuterium oxide (D2O or 2H2O)) has made it now possible to simultaneously study longer term changes in protein metabolism that were previously implausible due to the restraints of I.V infusions. Although there remain few studies, we have shown that this methodology can be used to investigate links between longer-term muscle protein synthesis and muscle hypertrophy (1). Others have done the same in relation to mitochondrial biogenesis and skeletal muscle stem cell activity (2). We have also shown these methods to be equally effective for studying protein metabolism over hours or days (3) and for testing the efficacy of interventions, albeit without the need for tracer infusions. Interesting recent work using these D2O methodologies has also shown that these approaches can be combined with proteomics (4) to establish the synthesis rates of plasma proteins, such as muscle creatine kinase, that act as a proxy of muscle protein synthesis, and moreover, to determine which individual proteins are being synthesized in muscle in response to interventions. In addition to this, metabolomics approaches are now being used where elements of a specific labelled metabolite (e.g. 13C valine) can be tracked and its fate related to metabolic capacity; these fluxomics methods have been successfully applied to track the impact of exercise capacity upon branched chain amino acid oxidation (5). Finally, other recently developed tracer methods include the use of orally administered d3-methylhistidine (6) and d3-creatine (7), which are now being used: i) to determine the normally difficult to quantify muscle protein breakdown (typically through A-V balance techniques) and, ii) to quantify whole body muscle mass without the need for imaging (DXA, CT, MRI). Both of these entail minimally invasive means of sampling labelled metabolites (methylhistidine and creatinine, respectively) in subject’s urine. Combining contemporary stable isotope methodologies with established approaches heralds a new era for both biological insight and engendering less invasive approaches to studying exercise physiology and biochemistry.