Final answer:
The doubly labeled water method uses isotopes oxygen-18 (18O) and deuterium (2H) to measure energy expenditure in individuals. a. Oxygen-18 (18O)
Step-by-step explanation:
The isotopes used in the doubly labeled water method are oxygen-18 (18O) and deuterium (2H). This technique involves the consumption of water labeled with these two stable isotopes to measure energy expenditure in individuals.
Since deuterium and oxygen-18 are non-radioactive, they provide a safe way to track the body's production of carbon dioxide, which correlates with energy expenditure.