The carbon in the CO2 does come from food. The oxygen comes from... well, it varies, it could be food, or water you've drunk, or O2 you've breathed and that's been converted into water.

The overall reaction is (for glucose):

⅙ C6H12O6 + O2 → CO2 + H2O

but this overall reaction actually spans a large number of molecular steps. there isn't a single step where CO2 and O2 both appear

this is a similar reaction as for the burning of wood or fossil fuels. O2 reacts with reduced carbon to form CO2