Comment by usaar333

I got curious and did a bunch of research.

It relies on the fact that carbon dioxide is transmitted from tissues to lungs in blood as carbonic acid. The carbonic acid is formed by water and carbon dioxide.

It seems extremely accurate as long as you believe the distribution of labeled water equalizes in the body. You can readily see exactly how much labeled oxygen was lost via CO2 (via the carbonic acid reaction).

As long as metabolism and blood transport works the same in all animals, seems reliable.