Suppose I have some material that came from or was made from a plant or animal. I will assume that its molecular structure, along with carbon, will contain some oxygen as well as other elements. If I combust that materiel in pure oxygen-18 ($\ce{^{18}O2}$),the products will, of course, be $\ce{CO2}$, $\ce{H2O}$ and others. Question: Will the $\ce{CO2}$ contain only the oxygen-18 or will it contain some of the oxygen-16 that was part of the molecular structure of the material?
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Of course, even if the combustion is made in pure $\ce{(O-18)2}$ the $\ce{CO2}$ produced by this combustion contains some of the oxygen-$16$ atoms coming from the material to be burned.
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$\begingroup$ To substantiate this answer: sources of oxygen by the organism can be all the carbohydrates (short sugars, glycogen, chitin), or carboxylic acids and their derivatives (lipids, proteins) to mention a couple of examples. $\endgroup$ Commented Aug 9, 2023 at 7:19