Genetic factors, muscle fiber type, and causes of obesity: How the ratio of fast and slow muscle fibers determines fat loss efficiency.

It is generally believed that obesity has multiple causes, but its occurrence and development are mainly the result of the combined effects of genetic and environmental factors. Genetic factors play a significant role in obesity. If both parents are obese, the incidence of obesity in their children is as high as 70% to 80%; if one parent is obese, the incidence is 40% to 50%; and if neither parent is obese, the incidence is only 10%. Clearly, "heredity" is a crucial factor in obesity. This similarity between parents and children in obesity is the result of the combined effects of genetic and environmental factors. Parents' behavioral habits have a significant impact on their children in many ways; children show clear similarities to their parents in their food choices and calorie intake.

Genetic factors are an important but not absolute cause of obesity. Genetics only plays a predisposing role; the development of obesity is more closely related to an individual's diet, activity level, and lifestyle habits. If calorie intake is properly controlled and physical exercise is increased, even the presence of obesity genes will not lead to obesity. Whether fat accumulates in the body and leads to obesity is closely related to the body's ability to utilize fat for energy. From a physiological perspective, skeletal muscle fibers can be broadly classified into fast-twitch and slow-twitch muscle fibers. The ratio of fast-twitch to slow-twitch muscle fibers in the body is genetically determined and does not change with training or lifestyle habits.

Fast-twitch muscle fibers contract quickly but cannot sustain their contractions, primarily relying on anaerobic metabolism for energy. Individuals engaged in speed-based sports, such as elite sprinters and weightlifters, have a predominance of fast-twitch muscle fibers. In individuals with a high proportion of fast-twitch fibers, the proportion of energy derived from anaerobic glycolysis increases rapidly with exercise intensity. Since anaerobic glycolysis relies primarily on carbohydrates rather than fats, individuals with a high proportion of fast-twitch fibers are less efficient at utilizing fat for energy compared to those with a high proportion of slow-twitch fibers. Slow-twitch muscle fibers contract slowly but can sustain their contractions, primarily relying on aerobic metabolism for energy. Individuals with a high proportion of slow-twitch fibers are adept at aerobic oxidation for energy, and their muscle cells contain abundant mitochondria, resulting in a stronger aerobic oxidation capacity.

Aerobic oxidation primarily relies on carbohydrates and fats for energy. Therefore, individuals with a higher proportion of slow-twitch muscle fibers have a stronger ability to utilize fat for energy, making it less likely for body fat to accumulate and thus less prone to weight gain. Conversely, individuals with a higher proportion of fast-twitch muscle fibers have a weaker ability to break down fat for energy during exercise, making them more susceptible to weight gain compared to those with a higher proportion of slow-twitch muscle fibers. Athletes with high explosive power, meaning those with a higher proportion of fast-twitch muscle fibers, are more likely to gain weight after retirement. For example, the well-known "Alien" Ronaldo, who dominated the football field with his exceptional speed and explosiveness during his professional career, suffered greatly from weight gain after retirement.

It cannot be denied that the phenomenon of gaining weight despite dieting does exist. The main reason is likely the failure to choose a scientifically sound exercise method based on one's muscle type. Fast-twitch muscle fibers have a strong glycolytic energy supply capacity. People with a high proportion of fast-twitch muscle fibers will experience an increase in anaerobic glycolysis as exercise intensity increases, meaning their energy source will be primarily carbohydrates rather than fat. Therefore, if people with a high proportion of fast-twitch muscle fibers engage in high-intensity exercise, they will not only fail to burn fat but will also experience a significant increase in lactic acid, a byproduct of anaerobic glycolysis of muscle glycogen, in their bodies. Lactic acid can be oxidized and broken down in the liver, but it can also be used as a raw material to synthesize fatty acids, leading to fat accumulation in the body.

This is the fundamental reason why some people gain weight despite trying to lose it. So, does this mean that people with a high proportion of fast-twitch muscle fibers are unsuitable for exercise to lose weight? Of course not. The key is to control the intensity of the exercise. When engaging in low-intensity exercise, people with a high proportion of fast-twitch muscle fibers primarily rely on aerobic oxidation for energy. However, if the intensity is too high, the proportion of energy supplied by anaerobic glycolysis increases, leading to a significant rise in lactic acid levels and subsequently fat accumulation. Therefore, people with a high proportion of fast-twitch muscle fibers can achieve weight loss through prolonged low-intensity exercise (heart rate approximately 100 to 120 beats per minute).

Generally speaking, people with good explosive power who can engage in fast-paced movements but tire easily have a higher proportion of fast-twitch muscle fibers. Conversely, people with good endurance who can exercise for extended periods and are less prone to fatigue have a higher proportion of slow-twitch muscle fibers. The higher the degree of obesity, the lower the anaerobic threshold, and the lower the intensity and duration of exercise needed for weight loss. This significant difference in energy supply makes people with a high proportion of fast-twitch muscle fibers relatively more prone to obesity. Therefore, obese patients with a high proportion of fast-twitch muscle fibers must exercise at a significantly lower intensity than those with a high proportion of slow-twitch muscle fibers for weight loss. Because the exercise intensity is lower, less energy is consumed per unit of time; only by extending the exercise time can a good weight loss effect be achieved.