The biochemical logic of human energy supply during exercise: the synergy of the three major energy systems and the threshold of fat oxidation.

All human movement is accomplished through muscle contraction; in fact, any movement in the human body is accompanied by muscle contraction. Muscles are broadly classified into three types: skeletal muscle, smooth muscle, and cardiac muscle. Generally, when we talk about muscles, we are referring to skeletal muscle. Skeletal muscles are distributed throughout the body, and their contraction allows for various movements. The human body ultimately converts energy from food into energy needed for muscle movement through a complex physiological process. Muscle contraction is the physiological process of converting chemical energy into mechanical energy within the body. While there are various energy sources in nature, the direct energy source for muscle movement in the human body is adenosine triphosphate (ATP). When the body needs energy, the energy contained in carbohydrates, fats, and proteins is transferred to ATP through different pathways to provide energy.

Human muscle cells store very little ATP, enough to sustain muscle movement for only 1-2 seconds. To meet the body's energy needs, ATP is constantly being consumed and synthesized. Three energy metabolism pathways in the human body provide ATP synthesis: the phosphagen system, the glycolysis system, and the oxidative breakdown of carbohydrates, fats, and proteins (also known as the aerobic oxidation system). During exercise, all three energy pathways are involved, but which pathway(s) primarily provide the energy needed for muscle movement depends on the intensity and duration of the exercise.

Creatine phosphate (CP) is the fastest high-energy phosphate compound to provide energy for ATP resynthesis. However, skeletal muscle cells store very little CP, and the total amount of stored ATP and CP can only sustain muscle movement for less than 8 seconds. This energy supply mode, which does not require oxygen and does not produce lactate, is called the non-lactic acid anaerobic energy system. It is the backbone of high-power, short-duration explosive movements, such as 100-meter sprints or tennis swings. Due to its extremely low capacity, once the substrate is depleted, the body must find new ways to generate ATP.

Next is the lactate-based anaerobic energy system, namely anaerobic glycolysis of muscle glycogen. This process does not require oxygen and can rapidly release a large amount of energy. However, the lactate produced causes a decrease in the pH level within muscle cells, reducing the activity of glycolytic enzymes. Furthermore, the large amount of lactate generated diffuses from inside the muscle cells into the extracellular fluid, causing an imbalance in the body's acid-base balance, thereby reducing the body's exercise capacity. Therefore, anaerobic glycolysis of muscle glycogen provides energy for a relatively short period and cannot sustain prolonged, high-intensity exercise. This energy supply mode typically dominates during 2-3 minutes of high-intensity activity.

The aerobic oxidation system is the complete oxidative breakdown of carbohydrates, fats, and proteins under a sufficient oxygen supply. The main products of this process are carbon dioxide and water, and it does not inhibit athletic performance, thus allowing for sustained exercise. Although both carbohydrate and fat oxidation belong to the aerobic oxidation energy supply system, the energy ratio of carbohydrates and fats varies at different times during prolonged aerobic exercise. This is because fat oxidation is more complex than carbohydrate oxidation, requiring more metabolic enzyme systems and a longer time to mobilize enzyme activity.

In the initial stages of aerobic exercise, typically within 20-30 minutes, carbohydrates provide a higher proportion of energy than fat. As exercise duration increases, the proportion of energy from carbohydrates gradually decreases, while the proportion from fat gradually increases. Studies have shown that after three hours of continuous aerobic exercise, the proportion of energy from fat reaches its peak, approaching 100%. This is the core reason why exercise for weight loss emphasizes "long duration." Only when the aerobic system is deeply mobilized and the intensity does not reach the lactic acid production threshold can fat become the primary substrate.

During weight loss through exercise, the exercise method not only needs to reduce the proportion of anaerobic energy supply to a minimum, but also needs to be prolonged, low-to-moderate intensity aerobic exercise. Exercise intensity is the key to determining the proportion of energy substrates supplied. If the intensity is too high, lactic acid will accumulate in the body. The formation of lactic acid can reduce the activity of enzymes related to fat metabolism, and lactic acid can be further synthesized into fat in the liver, which is counterproductive for those who want to lose fat.

Therefore, the core physiological basis of exercise for weight loss lies in creating a negative energy balance by continuously increasing energy expenditure compared to energy intake. Scientific exercise, by controlling the intensity, keeps the body in the "pure aerobic" zone, which is 20% to 40% of heart rate reserve. Within this range, not only can the intense hunger caused by excessive glucose consumption be avoided, but the body can also maximize the use of fat stores, achieving a systematic reduction in body fat content.