[thope]

Correct me if I'm wrong, but maybe the carbon dioxide you exhale comes from the air you inhale. No air in, no carbon dioxide out.

[jjoonathan]

Your metabolism looks like a combustion process: O2 + hydrocarbons -> CO2 + H2O. The carbon comes from food you ate. If you aren't breathing out CO2, you're either not metabolizing or you're storing the "burned" carbon in an undesirable metabolic byproduct (e.g. lactic acid).

[jdblair]

Correction: https://en.wikipedia.org/wiki/Respiration_(physiology)

[thope]

Ok thank you, I also found a short answer[0]. It seems the carbon comes from the conversion of glucose; that is, from food.

[0] http://www.smh.com.au/news/big-questions/we-breath-in-oxygen...