Causes of obesity: fat formation and energy balance

Chapter Two: The Causes of Obesity

I. Fat Formation

Humans obtain their energy from protein, fat, and carbohydrates in food.

Energy expenditure is used for basal metabolism, physical activity, and the specific dynamic action of food.

When calorie intake exceeds calorie expenditure, the excess calorie is converted into fat and stored. Excessive fat accumulation in the body leads to obesity.

To study the causes of obesity, namely the formation of fat, we use an analogy to represent the human body with a water tank, where water flows in through the top spout and out through the bottom spout.

Under normal circumstances, the amount of water flowing in and out is basically stable, that is, it maintains a certain volume.

When the amount of water flowing in from above increases, the water level in the tank rises, and the volume of water increases; when the amount of water flowing out from the faucet below decreases, the water level in the tank also rises, and the volume of water also increases.

If the amount of water flowing in represents the intake of heat energy, and the amount of water flowing out represents the consumption of heat energy, then the two situations that cause the water level in the tank to rise can lead to the accumulation of heat energy in the human body.

Having understood the above principles, we can better understand the energy balance formula below.

E input = E consumption = E storage

E_in: Energy taken in by the body; E_out: Energy consumed by the body; E_store: Energy stored by the body.

In order to maintain a stable body weight, the body should maintain a dynamic balance between energy intake and expenditure.

Therefore, theoretically speaking, if the body takes in more energy than it expends, it will cause energy accumulation and lead to obesity.

So how is energy formed and how is it consumed? What factors affect the body's energy intake and energy expenditure? This article will provide a brief introduction to these questions.

(a) Sources of heat energy

All human activities require the consumption of heat energy, which comes from carbohydrates, fats, and proteins in food. Therefore, these three nutrients are often referred to as heat-generating substances or heat-generating nutrients.

After food is digested and absorbed in the body, and then undergoes a series of oxidative decompositions catalyzed by enzymes, energy is gradually released from the three major nutrients to meet different physiological needs. If there is any surplus, the body will store it.

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 commonly used unit for heat energy is the calorie, which refers to the amount of heat required to raise the temperature of 1 milliliter of water from 15°C to 16°C, or to raise the temperature by 1 degree Celsius. In practical applications of nutrition, this unit is too small, so 1000 times that, the kilocalorie, is often used as the commonly used unit.

The international unit of measurement is the joule (which is also the legal unit of measurement in my country). This is equivalent to the amount of heat required to move 1 kilogram of matter 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 heat energy when oxidized in the body: each gram of carbohydrate produces 4 kilocalories; each gram of fat produces 9 kilocalories; and each gram of protein produces 4 kilocalories.

Knowing the amount of heat-producing substances ingested from the diet allows you to calculate the total calories consumed.

(ii) Consumption of thermal energy

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

For children who are in the growth and development stage, it is also necessary to increase the energy required for growth and development.

1. Basal metabolism: Basal metabolism is the energy requirement necessary to maintain the most basic life activities.

Specifically, this refers to a state in which the body is in a quiet and relaxed state, lying awake in a comfortable environment with a room temperature of 18-20°C, without physical or mental strain, and without the energy requirements of gastrointestinal and digestive activities.

At this point, metabolism is primarily used to maintain the minimum activity of the circulatory and respiratory systems, preserve the integrity of cellular function throughout the body, maintain body temperature and respiratory muscle activity, and maintain the tension of skeletal muscles throughout the body.

In short, the energy required for basal metabolism is the minimum energy needed to maintain human life.

Basal metabolism is affected by many factors, such as height, weight, age, sex, physiological state, and environmental changes.

Generally speaking, men have higher rates than women, children and adolescents have higher rates than adults, and pregnant women and breastfeeding mothers also have relatively higher rates.

The temperature is higher in cold climates than in warm climates, but the factors most closely related to basal metabolic rate are height and weight, especially body surface area.

2. Specific dynamic action of food: The specific dynamic action of food is also known as the thermic effect of food.

After the human body ingests food, energy metabolism increases in the body at rest, causing the body to lose more heat than before eating.

This extra increase in the body's energy metabolism caused by food intake is called the specific dynamic action of food.

The special dynamic action of food is generally believed to be due to the energy consumption caused by the digestion and intake of food.

The three major nutrients in food have different specific dynamic effects. Among them, protein has the strongest effect, accounting for about 30% of the heat produced by protein itself; carbohydrates are the second strongest, accounting for about 5% to 6% of the heat produced by carbohydrates themselves; and fat has the least effect, accounting for about 4% to 5% of the heat produced by fat itself.

When eating a mixed diet, the additional energy expenditure caused by the specific dynamic action of food is approximately 627.6–836.8 kJ (150–200 kcal), which is equivalent to 10% of the basal metabolic rate.

3. Resting metabolic rate: The sum of basal metabolic rate and energy expenditure from the specific dynamic action of food is the resting metabolic rate, which is relative to energy expenditure during physical activity.

For an individual, the metabolic rate is relatively constant during rest.

4. Energy consumption during physical activity: Unlike the metabolic rate at rest, the energy requirements of physical activity are variable and are greatly related to the intensity and duration of the activity.

Generally speaking, the greater the intensity of labor, the longer the duration, and the less skilled one is at the job, the more energy is consumed.

(Data from Tables 2-1 and 2-2 has been deleted)

(III) Factors Affecting Energy Balance

Theoretically speaking, if a person consumes too much energy, that is, eats too much, while energy consumption decreases (metabolic rate decreases during rest, and exercise decreases), it will cause energy surplus. However, the factors that cause this change are extremely complex.

It is influenced by both congenital genetic factors and acquired factors.

Acquired factors, also known as environmental factors, include dietary structure, lifestyle, physical activity and exercise, psychological factors, social factors, diseases, and medications. These factors can all cause energy metabolism imbalances and are important causes of obesity.