Article 37. Causes of Simple Obesity and Etiology of Pathological Obesity

IV. Causes and Pathogenesis of Obesity

1. Causes of simple obesity

(1) Genetic factors: As the saying goes, "Even drinking cold water can make a fat person gain weight." Of course, we know that drinking cold water cannot make you fat, as water does not produce energy. However, some people eat little and are not very active, yet they are obese; while others eat a lot and are not very active, yet they are thin. Studies have shown that heredity is partly due to the inheritance of body shape and partly due to the inheritance of enzyme activity. The conversion of carbohydrates, proteins, and fats into energy requires the participation of many enzymes. The activity of enzymes varies among different people, and fats form much faster in obese people than in normal people. Metabolic studies on obese people show that their basal metabolic rate is lower and their energy utilization efficiency is higher. Therefore, even if they do not eat much, their energy storage still exceeds their consumption. Another factor is that obese people are slower and generally do not like to exercise, resulting in less energy consumption. Therefore, children of obese parents need to pay attention early on, control their diet, and increase their physical exercise.

(2) Absorption Exceeds Consumption: The smallest unit of human tissue is composed of cells and intercellular matrix. Cells and their intercellular matrix are composed of substances such as carbohydrates, proteins, fats, water, inorganic salts, and vitamins. The various foods consumed by humans are composed of these nutrients, only the proportions of each component vary in different foods. When the nutrients absorbed by the body are equal to those consumed, the body will gain weight; conversely, it will lead to emaciation or obesity. Due to the continuous development of the social economy and the increasing improvement of people's living standards, people's dietary structure has been greatly improved, and the consumption of high-energy substances has greatly increased, while physical activity has gradually decreased. Naturally, absorption exceeds consumption, resulting in obesity.

(3) Metabolism: The metabolism of obese people differs significantly from that of normal people:

① With the same diet, obese people have a higher rate of anabolism (the process by which antibodies absorb external nutrients and process them into their own body cells to store energy) than normal people.

②Obese people "save" energy when resting, standing, or walking compared to people of normal weight;

③ Obese people are less responsive to cold stimuli when inactive (this may be due to the cold-resistant function of a certain thickness of subcutaneous fat and ischemic skin), while normal people are more responsive and have an increased metabolic rate. The metabolic characteristics of obese people are like a "good housekeeper," adhering to the principle of "thrift," storing the calories they ingest in a timely manner (hypersynthesis, turning ingested substances into their own cells, and converting excess calories into fat for storage), but saving on "expenditure" (to maintain the same physical activity, obese people have a low metabolic rate and consume less energy). In this way, "income" is high and "expenditure" is low, and over time, they become "rich" and "fat."

(4) Digestive and absorptive functions of the gastrointestinal tract: After food enters the human gastrointestinal tract, it is broken down by various digestive enzymes, eventually becoming absorbable components that enter the bloodstream and are transported throughout the body for utilization or tissue formation. If gastrointestinal function is good, food is thoroughly digested and absorbed, resulting in good health. However, in middle age, as the basal metabolic rate decreases, obesity is more likely. Conversely, if digestive and absorptive functions are weak, some nutrients from food are excreted in feces, leading to a generally thinner physique. Furthermore, the amount of various digestive enzymes secreted can affect the digestion of different nutrients. For example, people with higher levels of lipase and bile secretion digest fat more easily, absorbing a larger proportion of it, thus making them more prone to weight gain.

2. Etiology of pathological obesity

Pathological obesity is a group of diseases caused by endocrine disorders or metabolic disturbances; obesity is only one of the main symptoms in these patients. Common types of pathological obesity include:

(1) Hypercortisolism-induced obesity: This type of obesity is characterized by a swollen face, neck, and trunk, while the limbs are not swollen, and may even be slender. Common physical signs include a "moon face," a "buffalo hump," and "central obesity" where the abdomen is as large as if the patient is pregnant. In addition to obesity, it is also accompanied by symptoms such as purple striae on the skin, hirsutism, hypertension, and sexual dysfunction. This disease is caused by hyperfunction of the adrenal cortex due to adrenal cortical hyperplasia, adenoma, or adenocarcinoma, resulting in excessive secretion of cortisol. Because Cushing first described this disease in 1932, it is also known as "Cushing's syndrome."

(2) Pituitary obesity: The appearance of obesity is basically the same as that of Cushing's syndrome, and it is caused by pituitary basophilic cell tumors. These cell tumors often cause hyperfunction of the adrenal cortex and increased secretion of cortisol.

(3) Brain obesity: This type of obesity, if it occurs in adolescence, will cause underdeveloped reproductive organs; if it occurs in adulthood, it will cause loss of sexual organ function, lack of sperm, decreased libido, amenorrhea, and infertility. The lower abdomen and the area around the external genitalia are particularly obese, while the face and limbs are not obese. This type of obesity is caused by damage to the hypothalamus-pituitary region due to infection, tumors, or trauma, which leads to abnormal appetite, fat metabolism, and sexual function.

(4) Gonadal obesity: Obesity is a common symptom of hypogonadism, whether in postmenopausal women or men with anorchia or hypogonadism, and may be related to disordered fat metabolism. Clinically, obesity is more common in women (especially multiparous women or those who take oral contraceptives long-term), indicating that estrogen is related to fat synthesis and metabolism. The accumulation of body fat increases with age, which may also be related to hypogonadism or hypothyroidism.

(5) Obesity due to Polycystic Ovary Syndrome (PCOS): This disease is characterized by obesity, oligomenorrhea or amenorrhea, infertility, and hirsutism. Pathologically, there is bilateral ovarian enlargement, thickening of the ovarian capsule, and multiple enlarged follicles forming cysts. Symptoms and signs include: ① Obesity. It has been reported that the incidence of obesity in PCOS patients is 40%. ② Hirsutism. There is often a lot of hair on the face, around the lips, buttocks, and lower legs, and the eyebrows and pubic hair are also thick. ③ Infertility. According to relevant reports, the incidence of infertility in patients with this disease is about 75%, with long-term anovulation and a monophasic basal body temperature. ④ Bilateral ovarian enlargement. Follicle-stimulating hormone levels are low, while serum androstenedione or testosterone is higher than normal. ⑤ Prolonged progressive oligomenorrhea and amenorrhea.

(6) Neuropsychiatric factors: Both animal experiments and clinical studies have found that the hypothalamus of the brain has two types of nerve nuclei that regulate appetite. The ventral nucleus is the satiety center; when it is excited, a feeling of fullness occurs, leading to food refusal. When it is damaged, appetite increases significantly. The ventral nucleus is the feeding center; when it is excited, appetite increases. When it is damaged, anorexia and food refusal occur. These two centers regulate and restrain each other. Under normal circumstances, they are in a dynamic equilibrium, keeping appetite within a normal range, neither excessive nor refusal, thus maintaining a normal and stable body weight. However, when a disease causes lesions in the hypothalamus, whether it is inflammation or a sequela of inflammation (such as encephalitis or meningitis), head trauma, or intracranial tumors, the ventral nucleus may be damaged (causing increased appetite), while the ventral nucleus may be relatively hyperactive (causing increased appetite). In this case, the patient will overeat due to the abnormal regulatory effect of the two nuclei, leading to obesity. Conversely, if the ventral nucleus is damaged (causing anorexia and refusal to eat), while the ventral nucleus is relatively hyperactive (causing refusal to eat after a full meal), the combined effect of the two nuclei will result in anorexia and refusal to eat, leading to emaciation.