In theory, radioactive heavy isotopes of the elements could be used for such labeling; this was the case in many early applications of the method.
Advances in mass spectrometry during the 1970s and early 1980s reduced the amount of isotope required, which made it feasible to apply the method to larger animals, including humans.
A complete summary of the technique is provided in a book by British biologist John Speakman.
[5] The technique measures a subject's carbon dioxide production during the interval between first and last body water samples.
In addition, as CO2 travels from the site of respiration through the cytoplasm of a cell, through the interstitial fluids, into the bloodstream and then to the lungs some of it is reversibly converted to bicarbonate.
Measurement of 18O dilution with time gives the total loss of this isotope by all routes (by water and respiration).
This matter in practice is governed by the economics of buying 18O enriched water, and the sensitivity of the mass-spectrographic equipment available.
For example, the technique can measure the metabolism of animals in the wild state, with the technical problems being related mainly to how to administer the dose of isotope, and collect several samples of body water at later times to check for differential isotope elimination.
In both animals and humans, the test is made more accurate if a single determination of respiratory quotient has been made for the organism eating the standard diet at the time of measurement, since this value changes relatively little (and more slowly) compared with the much larger metabolic rate changes related to thermoregulation and activity.
Because the heavy hydrogen and oxygen isotopes used in the standard DLW measurement are non-radioactive, and also non-toxic in the doses used (see heavy water), the DLW measurement of mean metabolic rate has been used extensively in human volunteers, and even in infants[8] and pregnant women.
[10][11] A paper in 2021 summarized the results of over 6400 measurements using the technique in humans aged between 8 days and 96 years old.