Water is pretty cool stuff. It keeps you hydrated, helps you flush waste materials from your body, and when combined with healthy muscle glycogen levels from a high-carbohydrate diet, makes your muscles look nice and full and vascular and all-round swole.
But the physiological usefulness of water hardly ends there.
If you take some water, partially or completely remove the hydrogen and oxygen and replace them with the elements deuterium and oxygen-18, you end up with what is known as doubly labeled water.
At this point you’re no doubt thinking, “Why on Earth would you do that to water?”
Because the use of doubly labeled water has proved itself to be a rather nifty way of measuring energy expenditure in free-living humans (and animals). And it offers a greater degree of accuracy than the aforementioned energy expenditure formulas.
In spite of AC's deeply held belief in the significance of immature science, I think this is a helpful description of what doubly labelled water is, and what it does.
It's a powerful tool to understand the interplay between caloric intake and expenditure, and how they interact with differing macronutrient intakes and differing individuals.