Doubly Labeled Water

Doubly labeled water is water in which both the hydrogen and the oxygen have been partly or completely replaced for tracing purposes (i.e., labeled) with an uncommon isotope of these elements.

In practice, for both practical and safety reasons, almost all recent applications of the "doubly labeled water" method use water labeled with the heavy, non-radioactive forms of the elements deuterium and oxygen-18 (O-18 or 18O), or deuterium oxide-18 (D₂18O). In theory, radioactive forms of hydrogen and oxygen could be used for such labeling, and this was the case in many early applications of the method.

In particular, use of the doubly labeled water method (or DLW method) yields a particular type of measurement of metabolic rate, in which the average metabolic rate of an organism is measured over a period. This is done by administering a dose of doubly labeled water, and then measuring the elimination rates of deuterium and O-18 in the subject over time through the regular sampling of heavy isotope concentrations in the body water (by sampling saliva, urine, or blood). The minimum number of samples required is two—an initial sample after the isotopes have reached equilibrium in the body, and a second sample some time later. The time between the collecting of these samples depends on size of the animal involved. In small animals the period may be as short as 24 hours, and in larger animals like adult humans, the period may be as long as 14 days. In animals this average daily metabolic rate measured by the DLW method is often also called the Field metabolic rate or FMR. The method was invented in the 1950s by Nathan Lifson and colleagues at the University of Minnesota; however, its use was restricted to small animals until the 1980s because of the high cost of the oxygen-18 isotope. 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 humans. The first application to humans was in 1982, by Dale Schoeller, over 25 years after the method was initially discovered. A complete summary of the technique is provided in a book by British biologist John Speakman.