Genetic differences in muscle fiber type: How fast-twitch and slow-twitch muscle fibers determine body type and fat loss strategies

In the practice of exercise for weight loss, the same plan often produces drastically different results in different people. This difference largely stems from the type of muscle fibers in the human body. From a physiological perspective, human skeletal muscle fibers can be broadly divided into fast-twitch (white) and slow-twitch (red) muscle fibers. The ratio of fast-twitch to slow-twitch muscle fibers in the body is determined by genetics, and they usually do not fundamentally change with training or lifestyle habits. This genetic trait determines a person's athletic talent and, to a large extent, their "fat or thin" physique.

Slow-twitch muscle fibers contract slowly and sustain for extended periods, primarily relying on aerobic metabolism for energy. Individuals with a high proportion of slow-twitch muscle fibers have muscle cells rich in mitochondria and oxidative enzymes, and a dense capillary network, ensuring a continuous supply of oxygen. Since fat oxidation mainly occurs within mitochondria, slow-twitch muscle fibers have an exceptionally high capacity for utilizing fat for energy. Many elite long-distance runners and marathon runners tend to be lean, and even with high calorie intake, they don't easily accumulate fat, precisely because their bodies' "fat-burning factories"-slow-twitch muscle fibers-are predominantly dominant.

Fast-twitch muscle fibers contract rapidly and generate great explosive power, but they fatigue very easily and primarily rely on anaerobic glycolysis for energy. People engaged in speed-based sports, such as sprinters and weightlifters, have a predominance of fast-twitch muscle fibers. In individuals with a high proportion of fast-twitch fibers, as soon as exercise intensity increases slightly, energy supply quickly shifts to anaerobic glycolysis, and the substrate for glycolysis can only be carbohydrates, not fats. Therefore, people with a high proportion of fast-twitch fibers are significantly less efficient at utilizing fat for energy than those with slow-twitch fibers. Fast-twitch muscle fibers are very sensitive to weight training and are prone to hypertrophy adaptation. This is why some athletes with great explosive power may rapidly gain weight after retirement if they do not strictly control their diet.

The phenomenon of "gaining weight despite dieting" is also closely related to muscle fiber type. People with a high proportion of fast-twitch muscle fibers, when engaging in high-intensity exercise, not only fail to burn fat but also experience a significant increase in lactic acid produced by anaerobic glycolysis of muscle glycogen. Lactic acid not only inhibits the activity of lipoxygenase, hindering fat breakdown, but may also be used as raw material to synthesize fatty acids in the liver, leading to secondary fat accumulation in the body. This explains why some people gain weight instead of losing it when trying to lose weight through vigorous exercise, and their muscles become increasingly bulky.

So, are people with a high proportion of fast-twitch muscle fibers destined to be unable to lose weight through exercise? The answer is no; the key lies in precise heart rate monitoring. This group should adopt a "very low intensity, very long duration" strategy, strictly controlling their exercise heart rate at 100-120 beats per minute. Under this low-level load, even fast-twitch muscle fibers can maintain a certain proportion of aerobic metabolism without producing significant lactic acid. Because their energy output per unit time is low, they must significantly extend their exercise time (e.g., more than 90 minutes each time) to obtain the total energy deficit.

Weight loss individuals can easily assess their muscle fiber type based on their daily performance: If you have good explosive power and fast movements, but easily feel fatigued or have leg soreness when engaging in prolonged low-intensity activities, you usually have a higher proportion of fast-twitch muscle fibers; if you have good endurance and can jog or walk slowly for a long time without feeling tired, you usually have a higher proportion of slow-twitch muscle fibers.

The ideal solution is to undergo a sports function assessment at a professional institution. Weight loss through exercise is a highly personalized process. Understanding your own muscle tone and choosing an exercise intensity that matches it is crucial to avoid blindly following others' high-intensity plans. For those with fast-twitch muscle mass syndrome (LTMS), steady, slow, and sustained aerobic exercise is the true solution. Attempting to lose fat quickly by "competing in speed and strength" often only leads to metabolic disorders.