It takes exercise to produce oxygen demand, so as to decompose enough fat and produce enough oxygen demand. It is necessary to exercise and break down sugar.
In other words, you need to have energy to exercise. When you exercise to a certain extent, when you have more oxygen demand and intake, you can make your body mainly break down fat. This is the reason why anaerobic weight loss is poor, because glycogen is consumed quickly and less oxygen is obtained.
In fact, your body will have enough blood sugar and glycogen. Sometimes, it is almost impossible for you to carry out seemingly ordinary training. Similar to ketogenesis. Don't eat carbonated water for a long time. Be careful not to eat before training, but this kind of injury is difficult to evaluate. There is also literature showing that there is no significant difference in the efficiency of reducing fat between fasting and postprandial.
Simply put, you need the energy of sugar to start training, and it is almost enough for your body to break down fat for energy supply. Sugar is not something you won't have if you don't eat before training, but something you will store on an empty stomach. When you don't have enough yesterdays, you will be in a bad situation.