[neutronicus]

The trouble is that your digestive process isn't necessarily time-independent, so the same piece of food could represent different energy fluxes into your body if eaten at different times, and the calorie expenditure of your body isn't necessarily time-independent, so the same exercise (including just sitting around) could represent different energy fluxes out of your body if performed at different times.

Without actually measuring energy fluxes, "a kcal is a kcal" is meaningless.