I was reading an interview with Rudolph L. Leibel and doubly-labeled water was mentioned. This sort of stuff fascinates me. Using doubly-labeled water, a mass spectrometer, loads of measurements and some mathematics, it's possible to work out how many kcals someone is burning. This method costs an arm & a leg. So, how does doubly-labeled water work?
Water has the formula H2O. H stands for Hydrogen and O stands for Oxygen.
Elements have isotopes. Hydrogen has two isotopes, Deuterium and Tritium. Deuterium oxide, or D2O is known as heavy water and one use for heavy water is the manufacture of atomic bombs, if you recall the film "The heroes of Telemark". Deuterium is non-radioactive, as is heavy water. Tritium is radioactive.
Oxygen has three stable non-radioactive isotopes one of which is O-18. This can be used to make labeled water H2O-18. Mix D2O with a bit of H2O-18 and you have doubly-labeled water. Now what? To quote Leibel:-
"The interesting thing is that when you give somebody water like this, the deuterium comes out of the body which is determined by water turnover in the individual. The O-18 is in equilibrium with carbon dioxide, so the O-18 comes out by two mechanisms: first with normal water by transpiration, perspiration and urine, but also in the breath.
The difference between those two decay curves (the O-18 comes out faster), which we obtain by getting urine from these patients every day for 10 days-that gap is proportional to carbon dioxide production in that individual. By doing this, we can figure out how much carbon dioxide this person made over a period of 10 days. Knowing that, and knowing what the so-called diet quotient is - in other words, what the ratio of carbohydrates to fat in their diet is - you can back-calculate the amount of oxygen used to produce that amount of carbon dioxide.
So by some simple algebra using the rate of carbon dioxide excretion, you can actually calculate how much oxygen their body used in the process of oxidative metabolism. That is a very critical number because it tells you how much energy they burned. Oxygen consumption can be immediately converted into calories.
So we measure caloric expenditure both by figuring out how many calories it takes to make their body weight absolutely stable, and checking that number by also using this double-doped water excretion technique using mass spectroscopy. It's quite expensive: the isotopes to do such a study cost about $500, not including the spectroscopy."
See A comparative study of different means of assessing long-term energy expenditure in humans.
Ain't science wonderful?