When a study reports that athletes oxidised 60 g of an ingested carbohydrate per hour, it sounds as simple as measuring heart rate. It is not. Measuring exogenous carbohydrate oxidation, a method sometimes referred to as 13C carbohydrate oxidation or carbon-13 breath tests, requires a deeper understanding of biochemistry, physiology, and stable isotope tracer methodology. Carbon-13 testing services are now also offered commercially, but they all use shortcuts to make them eas