The technique measures a subjects carbon dioxide production during the interval between first and last body water samples. The method depends on the details of carbon metabolism in our bodies. When cellular respiration breaks down carbon-containing molecules to release energy, carbon dioxide is released as a byproduct. Carbon dioxide contains two oxygen atoms and only one carbon atom, but food molecules such as carbohydrates do not contain enough oxygen to provide both oxygen atoms found in CO. It turns out, one of the two oxygen atoms in CO is derived from body water. If the oxygen in water is labeled with O, then CO produced by respiration will contain labeled oxygen. In addition, as CO travels from the site of respiration through the cytoplasm of a cell, through the interstitial fluids, into the bloodstream and then to the lungs some of it is reversibly converted to bicarbonate. So, after consuming water labeled with O, the O equilibrates with the bodys bicarbonate and dissolved carbon dioxide pool (through the action of the enzyme carbonic anhydrase). As carbon dioxide is exhaled, O is lost from the body. This was discovered by Lifson in 1949. However, O is also lost through body water loss (such as urine and evaporation of fluids). However, deuterium (the second label in the doubly labeled water) is lost only when body water is lost. Thus, the loss of deuterium in body water over time can be used to mathematically compensate for the loss of O by the water-loss route. This leaves only the remaining net loss of O in carbon dioxide. This measurement of the amount of carbon dioxide lost is an excellent estimate for total carbon dioxide production. Once this is known, the total metabolic rate may be estimated from simplifying assumptions regarding the ratio of oxygen used in metabolism (and therefore heat generated), to carbon dioxide eliminated (see respiratory quotient). This quotient can be measured in other ways, and almost always has a value between 0.7 and 1.0, and for a mixed diet is usually about 0.8.
In lay terms:
* Metabolism can be calculated from oxygen-in/CO-out.
* DLW (tagged) water is traceable hydrogen (deuterium), and traceable oxygen (O).
* The O leaves the body in two ways: (i) exhaled CO, and (ii) water loss in (mostly) urine, sweat, & breath.
* But the deuterium leaves only in the second way (water loss).
From deuterium loss, we know how much of the tagged water left the body as water. And, since the concentration of O in the bodys water is measured after the labeling dose is given, we also know how much of the tagged oxygen left the body in the water. (A simpler view is that the ratio of deuterium to O in body water is fixed, so total loss-rate of deuterium from the body multiplied by this ratio, immediately gives the loss rate of O in water.) Measurement of O dilution with time gives the total loss of this isotope by all routes (by water and respiration). Since the ratio of O to total water oxygen in the body is measured, we can convert O loss in respiration to total oxygen lost from the bodys water pool via conversion to carbon dioxide. How much oxygen left the body as CO is the same as the CO produced by metabolism, since the body only produces CO by this route. The CO loss tells us the energy produced, if we know or can estimate the respiratory quotient (ratio of CO produced to oxygen used).