Physiologic Effects of Substrate Metabolism
The Determinants of Energy Consumption
For a wider review of the present topic, see the paper by Iapichino et al. [27, 28].
The complete oxidation of one mole of CHO, lipids, or proteins requires very different amount of O2 (134, 515, 114 L, respectively), so that to obtain 1 kcal oxidizing CHO, lipids, and proteins, we need 200, 212, and 239 mL of O2, respectively. It means that cardiac work, demanded for oxygen supply, is greater burning lipids and minimal burning CHO. By contrast, 1 kcal from CHO produces 200 mL of CO2, less from lipids (157 mL) and proteins (191 mL). It means that the respiratory work is minimal for lipids.
Moreover, each mole of produced ATP costs 3.7 L O2 for CHO, 3.9 L O2 for lipids, and 5 L O2 for protein. Then, the ATP yield per liter of O2 is maximal oxidizing CHO. Therefore, the most efficient way to use the available oxygen is burning CHO, while the most efficient way to reduce VCO2 is lipid oxidation.
However, the energy released per gram of substrate oxidized is also different: 4.18 kcal/g for mixed CHO (3.72 kcal/g glucose, 4.31 kcal/g for glycerol); 9.44 kcal/g for mixed lipids (8.34 kcal/g for medium chain triglycerides (MCT)); 4.69 and 3.48 kcal/g for p-hydroxybutyric and acetoacetic acid, respectively [29]; and 4.7 kcal/g for protein oxidation.
