Doubly Labeled Water (DLW) is a scientific method used to determine Energy Expenditure. DLW uses two natural isotopes of hydrogen and oxygen. Isotopes are the atoms with the same number of protons (the same atomic number – denoted by capital Z) but different number of neutrons. DLW is the water with the isotopes 2H (hydrogen – 2 aka deuterium) and 18O (oxygen – 18). These isotopes are traceable and stable so that it allows scientists to track it down.
DLW method traces back to 1950s and was applicable to small animals due to the expense of DLW. The use has begun spreading since 1982. Nowadays, scientists use DLW to measure the calorie a person burns each day for a normal daily life. This approach is quite accurate and safe. The participants in experiment are required to drink to the last bit of the labeled water. Their urine is collected for analysis.
So how can we understand the inner chemical process of this method?
The isotopes deuterium and oxygen-18 are labeled (tracked) through the metabolic pathway of the participants. Oxygen-18 will be lost as a form of water and carbon dioxide; where as the deuterium will be lost only in the form of water. Using mathematics, we can know how much deuterium is lost, which is also the same amount of oxygen-18 is lost in the form of water. Then the loss oxygen-18 in the form of carbon dioxide (CO2) is equal to the difference between the total of oxygen-18 and the loss of deuterium. Since CO2 is a production of carbohydrates, it can be used as an index of Energy Expenditure. Remember that the total oxygen-18 and deuterium are also measured (or labeled) before they enter the metabolic pathway and they are traceable, which makes it easier for scientists to know how much the deuterium is lost in the form of water.
“The History and Theory of the Doubly Labeled Water Technique.” www. ajcn.nutrition.org October. 1998 vol. 68 no. 4 932S-938S.
“Energy Expenditure Using Doubly Labeled Water”. www.iso-analytical.co.uk.