You cannot accurately measure just that. But measuring calories you eat is a good enough approximation to help you control how much you eat. You can estimate you calories out by your weight, if you are gaining weight you are eating (and adsorbing) more then you are using, if you are losing weight then you are eating less - and that is the most important part.

There is also water weight to account for, but realistically there is an upper and lower bound to that and over several weeks you can get a pretty good idea for what level of calories you ingest leads to weight gain or loss. And if that changes for any reason you can adjust the amount you eat in correspondence. We are just looking for averages over time and the overall balance here, no need to be super accurate with exactly what you adsorb and what you have accurately used during an exercise. I never even measure calories burnt as it does not give much value vs just weighting your self over time.