Causes of obesity: nerves, endocrine system, and fat formation

**24. What factors easily lead to obesity?**

To date, the etiology and pathology of obesity are not fully understood. Multiple factors can lead to obesity, and a single patient may have multiple factors present simultaneously. Generally speaking, these factors leading to obesity can be divided into two categories: internal and external factors. Internal factors are abnormalities in bodily regulation (including genetic, neuropsychiatric, and endocrine factors), while external factors are imbalances in diet and activity (including dietary habits, lifestyle, occupation, and social environment).

Studies have shown that increased fat synthesis is the material basis for obesity, and excessive calorie intake and reduced expenditure are prerequisites for obesity. Currently, most scholars indicate in their research that obesity is the result of the combined effects of genetic and environmental factors. The main causes and pathologies of obesity are briefly described below.

(1) Higher Nervous Activity and Endocrine Factors. Higher nervous activity occurs in the cerebral cortex and affects the hypothalamic appetite center through neurotransmitters. Influenced by the metabolism of sugars, fats, and amino acids in the body, simple obesity often presents with functional changes in the hypothalamus, but not necessarily pathological changes. The mental state manifested by higher nervous activity in the cerebral cortex often affects appetite; as the saying goes, "a cheerful heart leads to a plump body," meaning that a happy mood naturally leads to a hearty and satisfying diet. At this time, the vagus nerve is excited, insulin secretion increases, and appetite is heightened. When the mind is depressed, the adrenergic nerves are stimulated, leading to sympathetic nerve excitation and suppression of appetite. Over time, this can result in anorexia and weight loss.

(2) In addition to the hypothalamus, other endocrine disorders in the body can also cause obesity, among which insulin is the most critical, followed by adrenocortical hormones.

Insulin is a hormone secreted by pancreatic β cells. It can lower blood sugar or promote the synthesis of liver and muscle glycogen, inhibit gluconeogenesis, promote the uptake of glucose by adipocytes to synthesize fat, and inhibit fat breakdown. Clinically, examinations of obese individuals have revealed that their basal fasting insulin levels are higher than normal. When their weight is reduced to a normal level, plasma insulin levels and insulin receptors can return to normal, indicating that this change is secondary.

In addition, hypopituitarism and hypothyroidism can both cause obesity. A protein secreted by the anterior pituitary gland called growth hormone promotes protein synthesis, mobilizes stored fat, and has anti-insulin effects. When this hormone level decreases, insulin becomes relatively dominant, leading to increased fat synthesis and obesity. Estrogen is also related to fat synthesis and metabolism; women have more fat than men, and the thickness of their fat is twice that of men, which is due to the influence of estrogen. Pregnancy, menopause, and women taking oral contraceptives are all prone to obesity. Many eunuchs in ancient palaces were obese. Clinically, it is also common to see patients who have had their testes removed to treat benign prostatic hyperplasia become obese. This is because after testicular removal, the pancreatic islets enlarge, increasing insulin secretion and promoting fat synthesis.

**25. How is fat formed?**

The human body's energy sources are proteins, fats, and carbohydrates from food; energy expenditure is used for basal metabolism, physical activity, and the specific dynamic action of food. When energy intake exceeds expenditure, the excess energy is converted into fat and stored. Excessive fat accumulation in the body leads to obesity.

To study the causes of obesity, namely fat formation, we can use an analogy: a water tank to represent the human body. Water flows in through a spout at the top and out through a spout at the bottom. Under normal circumstances, the amount of water flowing in and out is relatively stable, maintaining a certain volume. When the amount of water flowing in increases, the water level in the tank rises, and the volume of water increases; conversely, when the amount of water flowing out decreases, the water level in the tank also rises, and the volume of water also increases. If we represent the amount of heat energy ingested as the amount of water flowing in and the amount of heat energy consumed as the amount of water flowing out, the two situations that cause the water level to rise can lead to the accumulation of heat energy in the human body.

To maintain a stable weight, the body needs to maintain a dynamic balance between energy intake and expenditure. Therefore, theoretically, if energy intake exceeds energy expenditure, energy accumulation will occur, leading to obesity. So, how is energy generated and how is it consumed? What factors affect energy intake and expenditure? This article will provide a brief overview of these questions.

(1) Source of heat energy

All human activities require energy, which comes from carbohydrates, fats, and proteins in food. These three nutrients are often referred to as energy-producing substances or energy-producing nutrients. After food is digested and absorbed in the body, it undergoes a series of oxidative breakdowns under the catalysis of enzymes, gradually releasing energy from these three nutrients to meet different physiological needs. Any excess energy is stored by the body. The human body can only store a small amount of energy in the form of glycogen; the main form of energy storage is fat.

The unit of heat energy used to be calories, with 1 calorie representing the amount of heat energy required to raise the temperature of 1 milliliter of water from 15°C to 16°C (i.e., a 1°C increase). However, in practical nutritional applications, this unit is too small, so 1,000 times that, kilocalories, are commonly used. Currently, the international unit of measurement is the joule (J), and my country has also adopted it. One joule is equivalent to the amount of heat energy required to move 1 kilogram of material 1 meter with a force of 1 Newton. The conversion formula between the two is:

1 calorie = 4.184 joules; 1 kilocalorie = 4.184 kilojoules

The three major nutrients produce different amounts of energy through oxidation in the body: each gram of carbohydrates produces 16.7 kilojoules (4 kilocalories); each gram of fat produces 37.7 kilojoules (9 kilocalories); and each gram of protein produces 16.7 kilojoules (4 kilocalories). Therefore, if the amount of energy-producing substances ingested from the diet is known, the total energy intake can be calculated.

(2) Consumption of heat energy

For adults, the body's energy consumption is mainly used to meet the following three types of energy needs: basal metabolic energy needs, physical activity energy needs, and the energy needs of the specific dynamic action of food.

①Basal Metabolism: Basal metabolism is the energy requirement necessary for maintaining the most basic activities of life. Specifically, it refers to the energy needed when the body is in a resting and relaxed state, awake and lying in a comfortable environment with a room temperature of 18℃～20℃, without physical or mental exertion, and without gastrointestinal or digestive activities. At this time, metabolism is primarily used to maintain the minimum activities of the circulatory and respiratory systems, maintain the integrity of cellular function, maintain body temperature, respiratory muscle activity, and maintain the tension of skeletal muscles. In short, the energy required for basal metabolism is the minimum energy needed to maintain human life.

Basal metabolic rate (BMR) is influenced by many factors, such as height, weight, age, sex, physiological state, and environmental changes. Generally, men have a higher BMR than women, children and adolescents have a higher BMR than adults, and pregnant and lactating women also have a relatively higher BMR. BMR is higher in cold climates than in warm climates, but the factors most closely related to BMR are height and weight, especially body surface area.