Portable indirect calorimetry devices may be accurate for VO2 but lack accuracy for VCO2, making them less suitable for estimating total energy expenditure or fat oxidation.

“I was under the impression and I'd love to be corrected here that these portable devices are probably decent for measuring V2 I.E the consumption of oxygen but their accuracy is sorely lacking for measuring vco2 the production of carbon dioxide and therefore it might be a good tool for estimating V2 Max but it's not a great tool for estimating total energy expenditure which is calculated by using V2 and V CO2 and nor is it a great tool for measuring uh fat oxidation because there you must be”