A comprehensive analysis of fat growth and the causes of obesity, and the physiological basis of scientific weight loss.

How does fat grow?

Fat is a unique tissue in the human body, distributed in various subcutaneous areas and near internal organs. Adipose tissue is mainly composed of adipocytes, a small number of fibroblasts, and a small amount of intercellular collagen (Figure 5). Adipocytes contain 80% fat, 18% water, and 2% protein. The average diameter of an adipocyte is 80 μm. Each adipocyte contains 0.6 μg of fat. A normal adult has 30 × 10⁹ adipocytes.

Under normal circumstances, between 2 and 5 years of age, adipocyte precursors, similar to fibroblasts, can differentiate into adipocytes. Some cells differentiate into adipocytes, while others become "stem cells," i.e., the preadipocyte pool. During this period, some currently unclear factors promote the final differentiation of preadipocytes into adipocytes (Figure 6). After sexual maturity between 11 and 14 years of age, the number of adipocytes no longer increases; subsequent weight gain or loss is due to the increase or decrease in the size of adipocytes. Because the number and rate of fat cell enlargement vary in different areas, the degree of obesity often differs across a person's body.

Subcutaneous fat is typically divided into three layers: the subcutaneous intrinsic fat layer, the superficial fat layer (areolar layer), and the deep fat layer (lamellar layer). When a person gains weight, it is mainly the deep fat cells that increase in size, which can differ significantly in volume from fat cells in other layers. In the human body, some areas only have the subcutaneous intrinsic fat layer and lack the other two layers, while other areas have both the subcutaneous intrinsic and deep fat layers but lack the superficial fat layer.

How do people gain weight?

While a person's weight does not increase at an average rate throughout their life, the majority of people do experience a gradual increase in body weight. There are many reasons why people gain weight. For adults, the main reasons are:

(1) Overeating and insufficient exercise;

(2) Insufficient exercise;

(3) Endocrine disorders and certain diseases;

(4) Genetic factors and family history;

(5) Mental factors, such as a relaxed and easygoing attitude.

It's not always the case that obesity is caused by a single factor; it's often the result of several factors working together. At different stages of life, various factors contribute to obesity in different ways. Simply suppressing any one factor cannot completely stop the weight gain process, which is the main reason why simple weight loss methods are not always effective.

Regarding diet, the main source of fat in the body is carbohydrates, which are absorbed into the bloodstream after digestion and are converted into fat in the liver. Animal fat, after digestion in the intestines and absorption into the bloodstream, is only present in very small amounts and is not a major cause of obesity.

Furthermore, dietary habits are a major factor. Overeating and binge eating are the primary culprits for obesity.