Metabolic investigations by tracer techniques

The following method is often counted separately and complementary to indirect calorimetry (see e.g. [3]). Nevertheless, we will introduce it here as it renders metabolic information in addition to direct calorimetry with more detail about metabolic pathways than the classical indirect approach. 

Radioactive or non-radioactive labelling of substrates offers an independent approach to study animal metabolism. Most common are C and radioactive isotopes and deuterium H, carbon nitrogen N and oxygen 0 as stable ones. The concentration of the latter can be increased artificially to facilitate their detection. The stable isotopes are preferable as they are harmless to the animals as well as to the scientist. In the case of nitrogen this isotope is the only choice for labelling. They are still rather expensive and need a lot of skill in mass spectrometry detection. 

Lifson and coworkers [48] used isotope dilution techniques in form of the double-labelled water method to evaluate the CO2 production and the origin of the o.xygen expired in CO2 In the carbon dioxide entry technique CO2 production is determined and transformed to energy units by means of the RQ value [49]. Both techniques were applied to larger animals, but they are not applicable to short time investigations. 

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