Fat is mobilized from the reservoir, enters the blood, and then enters the skeletal muscle cells. The mobilization process needs the participation of some hormones, and the enzyme system involved in its oxidative decomposition is very complicated and has many steps. Many processes need to enter mitochondria to eventually produce acetyl coenzyme A, and then enter the tricarboxylic acid cycle to finally produce water and carbon dioxide. There is a "proximity principle" for skeletal muscle energy supply, and the energy supply system is convenient to use. Obviously, in the exercise process in which the aerobic oxidation energy supply system participates, skeletal muscle first uses the oxidative decomposition of muscle glycogen to provide energy because it is convenient.
The mobilization speed of fatty acid oxidative decomposition is very slow, and the proportion of fat participating in oxidative energy supply begins to increase gradually after 20 ~ 30 minutes of exercise. The three major energy-supplying substances of the human body: sugar, fat and protein (protein's participation in energy supply is very small and neglected) all work in a mixed way, that is to say, there are almost no activities powered only by sugar or fat. Only the energy supply ratio is slightly different.
In low-intensity activities, the proportion of fat energy supply is relatively high, and the proportion of sugar energy supply is relatively low; In high-intensity activities, the proportion of fat energy supply is relatively low, and the proportion of sugar energy supply is relatively high. This is also the reason why we advocate low-intensity exercise to lose weight, because low-intensity exercise fat has a high energy supply ratio.