Why it is possible to estimate, to a good approximation, the energy derived from a sugar cube in the body, by doing laboratory experiments ?

The sugar cube experiment in the laboratory  gives us a good approximation of the amount of energy that can be derived from the sugar cube because the amount of energy is neither created or destroyed, it is just converted to another form. If the energy from the sugar cube is converted to other form in the lab then, it is possible that same amount of energy will be derived.