Why can CO2 (exhaled in expiration) be used as an indirect measure of catabolism?

Catabolism is the break down of large molecules for energy production. During any production of energy in the citric acid cycle (or during anaerobic metabolism) CO2 is produced. This, unlike the measurement of O2 for metabolism, represents the fuels that are broken down, and not the energy that is produced. Thus, CO2 can determine how much fuel is being utilized (especially in a starved state) while O2 can determine how much of it is used for energy.