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 O), or deuterium oxide-18 (D2O). 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.