table isotopes (¹³C, 15N, 180) form a safe medium for non-invasive diagnosis. In their natural abundance, they participate with our elemental consistence anyway. Non- radioactive, they can be used without any radiation hazard with everybody, including children and pregnant women. organ function, substrate oxidation, metabolic function or dysfunction, bacterial colonisation, and bacterial overgrowth can be assessed, and even quantified, by the 13(/12C isotope ratio in C02 in breath, after administration of the related 13C-Iabeled substrate. In metabolisation procedures, Carbon Dioxyde often is the final product, quickly resorbed and exhaled. For a 13C-substrate, undergoing such metabolisation, the 13C02 exhaled in breath will describe speed and efficiency of the metabolisation process, respectively the capacity of organ function or the intensity of bacterial colonisation. With its quick time response, 13C is available for the use in exact kinetic studies. In comparison, the other stable isotopes ( 15N, 180, or Deuterium}, naturally available for non-invasive diagnosis, end up in the urine and provide their signal only cumulated over time.