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Exercise: Oxygen Consumption & Carbon Dioxide Output Indirect Calorimetry, Respiratory Exchange Ratio and Determination of Fuel Type |
Humans can briefly extract energy from fat and carbohydrate without needing oxygen in a process called anaerobic metabolism, but this process can only continue for a very short time before it causes so much lactic acid to accumulate in tissue that further metabolism becomes impossible. We need oxygen to do it right and ultimately our ability to burn fat and carbohydrate is totally dependent upon oxygen. This dependency requires that oxygen consumption be directly proportional to calories burned in exercise such that if we can accurately measure oxygen consumed, we can determine calories burned. This is known as indirect calorimetry. Another useful parameter is carbon dioxide output. When fat or carbohydrate is "burned" it ultimately invloves combining carbon atoms in the fuel with oxygen to produce carbon dioxide that is then breathed out from the lungs. In a very general way therefore, carbon dioxide output is proportional to oxygen consumption, but with a caveat (and this is VERY important): Carbon dioxide output per liter of oxygen consumed is slightly greater when we are burning carbohydrate than when we are burning fat . This difference means that if we carefully measure how much oxygen is going into and how much carbon dioxide is coming out of a person who is exercising, we can, from the ratio of carbon dioxide out and oxygen in, get a pretty good idea of exactly what percent of the calories burned are coming from fat and what percent from carbohydrate. This ration of carbon dioxide-out to oxygen-in is called the "Respiratory Exchange Ratio" or RER. |
Armed with indirect calorimetry through oxygen consumption and the RER, we can then set out to determine what types and intensities of exercise work best to burn the most calories and fat. Instead of guessing or theorizing, we can use real data to get real answers. We'll explore that data and those answers in the next section. |
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