Doubly Labeled Water - Mechanism of The Test

Mechanism of The Test

The technique measures a subject's 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 CO₂. It turns out, one of the two oxygen atoms in CO₂ is derived from body water. If the oxygen in water is labeled with 18O, then CO₂ produced by respiration will contain labeled oxygen. In addition, as CO₂ 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 18O, the 18O equilibrates with the body's bicarbonate and dissolved carbon dioxide pool (through the action of the enzyme carbonic anhydrase). As carbon dioxide is exhaled, 18O is lost from the body. This was discovered by Lifson in 1949. However, 18O 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 18O by the water-loss route. This leaves only the remaining net loss of 18O 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/CO₂-out.
• DLW ('tagged') water is traceable hydrogen (deuterium), and traceable oxygen (18O).
• The 18O leaves the body in two ways: (i) exhaled CO₂, and (ii) water loss in (mostly) urine, sweat, & breath.
• BUT: the deuterium leaves only in the second way (water loss).

SO: from deuterium loss, we know how much of the tagged water left the body as water. And, since the concentration of 18O in the body's 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 18O in body water is fixed, so total loss-rate of deuterium from the body multiplied by this ratio, immediately gives the loss rate of 18O in water.) Measurement of 18O dilution with time gives the total loss of this isotope by all routes (by water and respiration). Since the ratio of 18O to total water oxygen in the body is measured, we can convert 18O loss in respiration to total oxygen lost from the body's water pool via conversion to carbon dioxide. How much oxygen left the body as CO₂ is the same as the CO₂ produced by metabolism, since the body only produces CO₂ by this route. The CO₂ loss tells us the energy produced, if we know or can estimate the respiratory quotient (ratio of CO₂ produced to oxygen used).