For every liter of O2 you use, your body gets 5.1 kilocalories (kcals) from burning pure carbohydrates, and 4.7 kcal from burning pure fat. The ratio of O2/CO2 you expire signals how much fat versus carbohydrate you are using. If you are burning only carbohydrate, you produce exactly as much CO2 as O2, and if you are burning only fat, you produce 70 percent as much CO2 as O2. Most of the time, you burn a mixture of fat and carbohydrate, yielding an average of 4.8 kcal for every liter of oxygen.
