Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

Here is a more detailed explanation of the “doubly labeled water” method. Most water, H2O, is made from hydrogen with a molecular weight of 1 (1H, which has 1 proton) and oxygen with a molecular weight of 16 (16O, which has 8 protons and 8 neutrons). It is also possible to make a harmless form of water using “heavy” hydrogen (2H, which has an extra neutron) and oxygen (18O, which has two extra neutrons). Regardless of what kind of hydrogen and oxygen is in the water, the H and O leave the body in different ways: the H and O leave as water when we urinate, breathe, and sweat, whereas the O also leaves the body when we breathe out carbon dioxide (CO2). Because we can accurately and precisely measure 2H and 18O in urine, the difference in the rate at which H versus 18O remains in your urine over several days allows us to calculate how much CO2 you breathed out, thus allowing a reasonably precise calculation of how much energy you spent. The method also allows us to calculate your fat-free body mass and how much water goes in and out of your body.