First of all, sports participants must understand that if the human body uses glucose as energy, it will produce 1 liter of carbon dioxide for every 1 liter of oxygen consumed, that is, the respiratory quotient (RQ) of glucose as energy is equal to1; RQ when fat is used as energy source is about equal to 0.7; When protein is used as the energy source, RQ is about 0.8. However, it is difficult to evaluate the respiratory state of tissues in human body. Therefore, by measuring the lung gas exchange state (respiratory exchange ratio, RER for short, the increase of carbon dioxide in lung gas exchange divided by oxygen consumption) when the human body participates in exercise, and protein only participates in providing energy in a small amount; Sports physiology researchers can evaluate the energy consumption characteristics of exercise process according to the gas exchange in the lungs.

Generally speaking, the RER of human body is about 0.82 at rest, but it will decrease (between 0.75 and 0.80) at very low intensity (walking, jogging and easy riding), and it is about 1 when approaching the maximum exercise. That is to say, in the state of low-intensity exercise, the proportion of fat participating in providing energy is high. With the increase of exercise intensity, RER also increases, and the proportion of glucose participating in providing energy also increases. In the state of maximum exercise, almost all of them are powered by glucose. When RER is equal to 0.85, glucose and fat each provide half of the energy needed by the body. In addition, with the rise of RER, the energy that the human body can produce for every 1 liter of oxygen consumed is also increasing. For example, when RER is equal to 0.8, the human body consumes 4.80 1kcal per liter of oxygen; When RER is equal to 0.9, every liter of oxygen consumed by human body can generate 4.924 kilocalories of energy; When RER is equal to 1, the human body consumes 5.047 kilocalories per liter of oxygen. Although the difference between the lowest energy and the highest energy is less than 1%, it is an indisputable fact that the energy consumption per kilogram of oxygen will gradually increase with the increase of exercise intensity.