If you think about it, the food we eat mostly ranges from glucose (CH*2O)n* to fatty acids (empirical formula CH*3(CH2)nCOOH. None of this has enough oxygen to get converted to CO2* without additional oxygen from somewhere.

It's true that it does not come directly from atmospheric oxygen, but indirectly it does. The atoms basically all get mixed around. For example, in glucose metabolism, oxygen reacts with reduced cytochrome c and H^+ to become water. Water hydrolyzes various phosphorylated substances to produce ADP and phosphate. The water and phosphate will donate this oxygen back in subsequent steps.

In glycolysis, two of the sugar's 6 oxygen atoms are eventually replaced by phosphate along the route to forming pyruvate. During decarboxylation of pyruvate, one of the lost oxygens is original to the sugar, while the other is the newly added one. So for the CO*2* molecule formed by pyruvate decarboxylation, it's half-and-half. One of the oxygens came from phosphate. The other came from the sugar.

In the citric acid cycle, another two CO*2* are produced by successive decarboxylations of isocitrate. Of course, it's a cycle, so more oxygen atoms have to be added back in. Of these, two come from water, one from phosphate, and one from pyruvate (originally glucose). So again, some of the oxygen in the CO*2* produced in the citric acid cycle comes originally from the sugar, but the majority comes from phosphate and water.