Doubly labeled water can measure CO2 production to back-calculate energy expenditure when combined with diet information.

“the difference in the rate of excretion of the so-called Oh 18 water the heavy isotope of water which is oxygen is borne out both in urine but also in the expired air of an individual as carbon dioxide and the h2o which comes out by non-respiratory means you can actually get a measure of how much co2 the individual is producing”

“EE labeled water is a technique which uses two isotopes of water which is h2o in one the oxygen is a heavier version of the normal or the more more prevalent oxygen molecule and the other the hydrogen is a heavy atom as opposed to the more prevalent kind of hydrogen in the in the environment”

“you drink this you basically get to exploit a trick which is by collecting your urine you're measuring the hydrogen that's coming out without measuring the carbon that's coming out so you can impute what your vco2 is effectively and you collect urine serially and over the course of 10 or 12 days you get an estimate”