It's the krebs cycle. If you look at how sugar is broken down by the cells to make atp, the actual energy your cells use, it takes C6H12O6 (sugar) and makes it into a few CO2 and H2O, getting energy (atp) from breaking it down into the components. That CO2 is then exhaled, which is I think what OP is referring to.

Note: it's been a long time since I learned that in class so it is most likely somewhat incorrect