A Comprehensive Analysis of the Causes of Obesity and Effective Weight Loss Strategies

Aside from genetic predisposition, endocrine imbalances, or metabolic diseases (such as thalamic damage, decreased thermogenic capacity, and decreased K⁺, Na⁺ pump, and ATPase activity), the fundamental cause of obesity is an imbalance of energy expenditure, meaning that energy intake exceeds energy expenditure, and the excess energy is stored in the body as fat, causing weight and body fat levels to exceed normal levels. This type of obesity is the most common, accounting for 94% of obese individuals. A high-fat and high-sugar diet can also contribute to obesity. Some reports suggest that lipoprotein lipase (LPL) acts as the "gatekeeper" for fat entry into adipocytes, but the effect of a high-fat diet on LPL activity remains controversial. To address this, Lawrence et al. conducted an experiment force-feeding rats 5 ml of grain oil, observing a 2.4-fold increase in LPL activity in rat adipose tissue after 3 hours. They also observed that feeding weaned rats a high-fat diet (40% of their kcal from fat) significantly increased their body fat percentage, leading to obesity and exceeding normal weight. This was attributed to the low energy requirement, low thermogenesis, and high digestibility of fat. Dietary sugar is considered a fat promoter, but its effect on obesity is relatively smaller compared to fat. A study simulating a human diet high in sugar and fat was conducted, feeding rats a diet with 17%–25% kcal from sugar and 40%–45% kcal from fat. This study showed that animal body fat could increase to 61%, with a daily fat growth rate 8.9 times higher than that of lean mice (body fat level 18%). It also observed that a high-sugar, high-fat diet stimulated appetite and caused hyperphagia. However, one group of mice, fed the same diet, did not exhibit hyperphagia, yet their body fat still increased to 51%, more than double that of normal mice. There are differing opinions regarding whether inactivity or insufficient activity is a cause of obesity. Some suggest that a rich diet combined with a sedentary lifestyle is a step towards obesity, and low levels of physical activity may be a significant factor. Bullen used cinematography to observe that overweight girls (BMI 34.7) showed significantly less activity in volleyball and tennis compared to normal-weight children (BMI 21.3). Obese children are often discriminated against due to restrictions on participation in strenuous exercise, causing psychological damage, which exacerbates obesity, creating a vicious cycle. Maxfield et al. used a center-of-gravity shift table to calculate the time spent on lower limb activity in obese women. The results showed that obese individuals spent 15% less time on physical activity than those of normal weight; however, other reports suggest that obesity is unrelated to activity levels, with obese children not necessarily less active than normal-weight children at school. These conflicting data points are attributed to inaccurate methods of observing activity levels, short observation periods that fail to accurately reflect the relationship between behavior and obesity, and a lack of consideration of the correlation between physical activity expenditure and dietary intake. Furthermore, research on the relationship between physical activity and the degree and type of obesity is insufficient. Therefore, whether insufficient physical activity is a significant cause of obesity remains controversial.

For a long time, a weight exceeding the average weight of normal individuals by 10% or 20% has been used as the basis for diagnosing or classifying obesity. However, recent data suggests that body weight includes both fat and lean tissue (or lean body mass). After systematic exercise training, the increase in muscle tissue (i.e., lean body mass) can significantly increase weight compared to individuals of the same height. Moreover, while lean body mass increases, body fat percentage decreases, leading to improved athletic performance. Therefore, it is believed that body fat percentage (BF%) should be used as the basis for diagnosing or classifying obesity; and that men with >20% body fat and women with >30% body fat are considered obese. This standard is determined by adding a standard deviation to the average body fat percentage of young male and female adults with a certain level of activity. Body fat percentage is age-related, increasing with age. Considering the health risks of excessive body fat in middle-aged and elderly individuals, Lamb proposed that middle-aged and elderly individuals should still refer to the standards for young adults.

Body Mass Index (BMI), also known as Ouetelet's index, is a commonly used index for assessing the degree of obesity and has been used since 1869.

While there are reports linking BMI to mortality rates, the scientific basis for using BMI as a basis for obesity classification is insufficient, but it can help in assessing treatment effectiveness and prognosis.

Obesity is associated with a range of diseases and health problems and can shorten lifespan. Wcinhaus reported that for every 5 pounds exceeding ideal weight, lifespan is shortened by one year. Simmonson identified at least 26 medical conditions associated with obesity, including hypertension, hyperlipidemia, coronary heart disease, diabetes, gallbladder disease, osteoarthritis, kidney disease, and malignant tumors. Furthermore, obesity can cause severe psychological damage; obese individuals are emotionally unstable, especially those who are obese during childhood and adolescence. Limited participation in strenuous exercise can sometimes lead to social isolation, creating a vicious cycle. Obesity itself is a risk factor for cardiovascular disease. Considering obesity as a whole, it is associated with a 15%–20% mortality rate. Based on the benefits of weight loss in reducing morbidity and mortality and improving health, weight loss has become an important research topic in sports medicine, rehabilitation medicine, and nutrition.