Every reaction is accompanied by a change in the energy content of the system. The oxidation of 1 mole of glucose, for example, is accompanied by the libration of 673,000 kcal. In order to express more completely the change occurring in the oxidation of glucose, we should accordingly express our equation as

C ₆H ₁₂O ₆ + 6O _{2 =} 6CO ₂ + 6H ₂ O + 673,000 kcal

However, this heat of the reaction is not necessarily the same as the maximal possible work or free energy change which may be obtained from the oxidation of a mole of glucose. For if all the energy of the reaction where utilized to perform work, one could at constant temperature and pressure realize 688,160 kcal, which represent the free energy change of the reaction. The difference between the heat of the reaction and the free energy example in the example cited is 15,160 kcal, which represent the heat that would have flowed into the reaction mixture from the environment.

Part of the energy released in the oxidation of the glucose appears as heat; the fraction of the total energy appearing in this from varies the pending upon the manners which the conversion of the glucose to CO ₂ and H ₂O occurs.

The fact that the energy (released as heat) the rived from the oxidation of foodstuffs by the body is the some as that of obtained when these materials are burned in a calorimeter has been demonstrated by the classic experiment of Rubber, Atwell , Benedict, Lusk, others. Although utilized by a much more devious pathway, foodstuffs yield the same amount of energy as is obtained by their oxidation outside the body.