This is due to the following reasons: the total sugar reserve in the human body is only a few hundred grams, and the fat reserve is 10 kilograms. This is because sugar has a low energy storage capacity. Complete oxidation of 1 g sugar can only release 4 kilocalories, and complete oxidation of 1 g fat can release 9 kilocalories. Therefore, when the human body has excess energy, it will convert excess sugar into fat for use in case of food shortage, which has two advantages. One is to store the same energy. Fat gains less weight. Second, subcutaneous fat can keep warm and reduce the energy consumed to maintain body temperature. It can be seen that excess sugar is converted into fat, which is beneficial to human survival. On the contrary, fat is difficult to convert into sugar. In metabolism, only 5~ 10% of fat can be directly converted into glucose, and the rest can only be directly oxidized into energy.
However, sugar has a characteristic that its energy supply speed is much faster than that of fat, and it can supply energy for lactic acid through anaerobic metabolism, so the energy supply ratio of sugar is above 80% in such strenuous exercise as 100-meter sprint.
In aerobic exercise, because the energy supply speed is not so great, in order to save valuable sugar reserves, the human body will actively increase the proportion of fat energy supply. Generally, when the heart rate is about 70% of the maximum heart rate, the energy supply of fat and sugar accounts for 50% respectively (5% of energy supply in protein is ignored here, because the energy supply ratio in protein is low, so it is usually ignored by default for the convenience of calculating the fat-reducing efficiency, and only considered when calculating the daily supplement quality of protein).
This is actually an instinct formed in tens of thousands of years of human evolution. If 80% of energy is spent on sugar during non-strenuous exercise, then the sugar reserve will be consumed quickly. When you need anaerobic exercise, such as avoiding the enemy's harm, it is very likely that sugar is not enough to run, which is very dangerous, so the human body will not do such stupid things.
It is precisely because of the regulatory mechanism of the human body that the less strenuous the exercise, the higher the proportion of fat energy supply, and the fat energy supply can reach more than 70% during meditation. However, because meditation itself consumes too little energy, don't expect to lose fat by it. Jogging for an hour consumes more than ten times as much energy as sitting for an hour, so although the proportion of fat energy supply during jogging is only 50%, and sitting for an hour is over 70%, the fat lost by jogging for an hour is still far higher than that by sitting for an hour, which is not contradictory to what we call aerobic exercise to reduce fat.
And even under the same exercise intensity, the human body will dynamically adjust the fat energy supply ratio according to the total sugar reserve in the body. After running at the same speed for an hour, it is assumed that the fat supply accounts for 50% in the first half hour. After the human body detects a decrease in sugar reserves in the second half hour, it will continue to increase the proportion of fat energy supply to about 55%.
So doing aerobic exercise on an empty stomach is also the same principle. On an empty stomach, the body's sugar reserves are low, so when doing aerobic exercise, the body will actively increase the proportion of fat energy supply in order to reduce sugar consumption, but it should also be noted that people with hypoglycemia should not do it. People with hypoglycemia may faint if they squat on an empty stomach for a long time, let alone do aerobic exercise.