Genetics, Metabolism, and Life Stages: Analyzing the Physiological Mechanisms Behind "Not Gaining Weight" and the Causes of Midlife Obesity

Obesity is generally considered to have multiple causes, resulting from the combined effects of genetic and environmental factors. Genetic factors play a significant role in obesity: if both parents are obese, the incidence rate in their children is as high as 70% to 80%; if one parent is obese, the incidence rate is 40% to 50%; and if neither parent is obese, the incidence rate is only 10%. This shows that heredity is an important factor, but not the absolute factor. Heredity only plays a predisposing role; the development of obesity is more closely related to an individual's acquired diet, activity levels, and habits.

Overeating and overeating are major causes of obesity in modern society. Among environmental factors, high-fat, high-calorie diets are closely related to obesity in modern people. The pressures of studying and working make people reluctant to exercise, leading to reduced physical activity, which is another major contributing factor. For adolescents in their growth and development stage, energy metabolism should be in a positive balance; however, if intake is significantly excessive, it can induce a lifelong risk of obesity.

Why do some people "never gain weight" while others "gain weight even from drinking water"? This is related to a person's basal metabolic rate and digestive and absorption capacity. Basal metabolism refers to the minimum energy required for the human body to maintain life activities; that is, the energy metabolism of the human body when awake, resting, fasting, and at an ambient temperature of 20°C. Men's basal metabolic rate is, on average, higher than women's, and higher in childhood than in adulthood. People with a high basal metabolic rate are less likely to become obese.

This is because the levels of hormones that regulate metabolism are related to basal metabolic rate. A high basal metabolic rate leads to increased thyroid secretion, significantly increasing the rate of metabolism, especially fat breakdown. Therefore, people with lower metabolic rates are more prone to obesity than those with higher metabolic rates when eating the same amount of food. Furthermore, the quantity and activity of enzymes that catalyze food breakdown vary greatly among individuals, resulting in different absorption rates of the three major nutrients in the digestive tract. These differences in absorption rates can lead to varying degrees of weight gain or loss when consuming the same amount of food. Excessive stress or gastrointestinal problems can also cause pathological "inability to gain weight."

People are prone to weight gain in middle age, mainly due to three factors. Physiologically: As people age, bodily functions gradually decline, and the metabolic rate decreases year by year. If intake remains at the level of youth without increased expenditure, energy will accumulate in surplus. Furthermore, fat distribution changes after middle age, shifting from a more even distribution throughout the body to a concentration in the abdomen, buttocks, and thighs. Physically: Reduced physical and mental activity, coupled with a lack of exercise, leads to the conversion of excess calories into fat. Mentally: Stable careers and families reduce stress, and a comfortable and relaxed lifestyle also contributes to weight gain.

Body weight is composed of two parts: fat mass and lean mass. Fat mass is the product of body fat percentage and body weight; bone, muscle, and non-fat components are called lean mass. People with a high lean mass have good physical fitness, strong muscles, and may be heavier but not considered obese. A person's athletic ability depends to some extent on the proportion of lean mass, and regular exercise is the core means of increasing lean mass. For adolescents, a relatively high body fat percentage within the normal range is acceptable; blindly controlling weight can affect health.

Among the factors influencing obesity, differences in muscle fiber type also play a role. Muscle tissue with a high proportion of fast-twitch muscle fibers has a lower capacity to utilize fat. In individuals with a high proportion of fast-twitch muscle fibers, even slightly higher exercise intensity leads to a rapid shift in energy supply to anaerobic glycolysis, resulting in a rapid decrease in the proportion of energy derived from fat. Therefore, people with a high proportion of fast-twitch muscle fibers are relatively more prone to obesity. When exercising to lose weight, this group must exert significantly less intensity than those with a high proportion of slow-twitch muscle fibers and compensate by extending the duration of their exercise.

In some regions, men are more likely to eat leftovers, resulting in a higher obesity rate among men than women. Ultimately, overeating is the most significant cause of increased obesity. For the sake of health, it is recommended to throw away leftovers rather than eat them.