Muscle fiber type and weight loss success or failure: How the ratio of fast and slow muscle fibers determines "gaining weight despite dieting" and personalized intensity selection.

In the practice of weight loss through exercise, some people achieve remarkable results, while others see little to no effect, or even experience the embarrassing situation of "gaining weight despite trying to lose it." This largely depends on the type of muscle fibers in the human body. From a physiological perspective, human skeletal muscle fibers are broadly divided into two types: fast-twitch (white) muscle fibers and slow-twitch (red) muscle fibers. The ratio between the two is determined by genetics and remains relatively constant throughout life. Slow-twitch muscle fibers contract slowly and are more sustained, have high mitochondrial and oxidase activity, are rich in surrounding capillaries, and are adept at aerobic metabolism. Therefore, people with a high proportion of slow-twitch muscle fibers are extremely efficient at using fat for energy and are usually lean, such as marathon runners.

Fast-twitch muscle fibers contract rapidly and with great force, but they fatigue very easily and primarily rely on anaerobic glycolysis for energy. People who engage in speed and explosive sports such as sprinting and weightlifting have a high proportion of fast-twitch muscle fibers. During exercise, fast-twitch muscle fibers mainly utilize carbohydrates rather than fat. In people with a high proportion of fast-twitch muscle fibers, as soon as the intensity of exercise increases slightly, energy supply quickly shifts to anaerobic glycolysis of carbohydrates, leading to a sharp drop in the proportion of energy supplied by fat. Due to this metabolic characteristic, people with a high proportion of fast-twitch muscle fibers have a relatively lower ability to utilize fat in daily life, and are therefore more prone to obesity.

The root cause of the "gaining weight despite dieting" phenomenon lies in uncontrolled intensity. People with a high proportion of fast-twitch muscle fibers who engage in high-intensity exercise not only fail to burn fat but also produce large amounts of metabolic byproducts-lactic acid. Lactic acid enters the liver. On one hand, it inhibits the activity of lipoxygenase, hindering fat breakdown; on the other hand, under certain conditions, lactic acid can even be used as raw materials to synthesize fatty acids in the liver, leading to fat accumulation. This is why some people with fast-twitch muscle fiber obesity struggle to lose weight despite strenuous exercise.

People with a high proportion of fast-twitch muscle fibers can still exercise to lose weight; the key is to keep their heart rate at a very low level, typically recommended to be between 100 and 120 beats per minute. At this low intensity, even with a fast-twitch muscle fiber type, the energy supply can still be primarily aerobic oxidation. Because energy expenditure is low per unit of time, these individuals must compensate by significantly extending their exercise time. How to determine your muscle fiber type? People with good explosive power and fast movements but who tire easily usually have a high proportion of fast-twitch muscle fibers; people with good endurance and the ability to sustain low-intensity activity for extended periods have a high proportion of slow-twitch muscle fibers.

In the initial stages of exercise for weight loss, skeletal muscle energy supply follows the "proximity principle." For the first 15-20 minutes, muscle glycogen is primarily used; fat mobilization requires a series of hormone-induced and enzymatic reactions. The activity of lipoxygenase only begins to increase after 20 minutes of exercise. Therefore, regardless of muscle fiber type, all aerobic exercise aimed at fat loss must be sustained for at least 30 minutes. Ideally, if physical condition permits, 60-120 minutes is optimal.

In summary, both excessive and insufficient exercise intensity will affect the results. Without producing lactic acid, higher intensity leads to higher energy expenditure per unit time, resulting in better effects. Obese individuals should first undergo an exercise function assessment to identify their muscle type and develop a personalized plan tailored to their metabolic characteristics. Blindly following others' high-intensity plans, especially for obese individuals with a high proportion of fast-twitch muscle fibers, will not only fail to reduce fat but may also exacerbate metabolic disorders.