While the tenth lesson’s look at water as the source of oxygen needed to release carbon dioxide in the citric acid cycle may have seemed a bit tedious, the twelfth lesson brings the exact same principle to life with striking dietary relevance. Because carbohydrate is richer in oxygen than fat, it consumes less water during its metabolism and releases more carbon dioxide. Carbon dioxide puts stress on the lungs and its generation should be restricted in the case of lung injury to allow healing. This calls for a low-carbohydrate, high-fat diet. On the other hand, carbon dioxide is needed to support the action of vitamin K and biotin, and to promote delivery of oxygen to tissues during exercise. Thus, many other situations may call for more carbohydrate. The fact that carbs and fat generate different amounts of carbon dioxide is also the basis for the respiratory quotient (RQ), an index of carbon dioxide generated per oxygen consumed. Exercise scientists use the RQ to estimate how different conditions affect the relative utilization of different macronutrients for fuel.