Energy metabolism, metabolism is one of the most basic characteristics of life, which includes two aspects of material metabolism and energy metabolism. The body absorbs nutrients from the outside world through material metabolism, and at the same time, it decomposes and absorbs the chemical energy contained in the body to convert it into energy that can be used by tissues and cells. The human body uses energy to maintain life activities. The energy release, transfer, storage, and utilization associated with material metabolism is often referred to as energy metabolism.
Figure 1. Energy metabolism pathway diagram.
The nutrients the body takes from the outside include carbohydrates, fats, proteins, trace elements, water, and vitamins, among which carbohydrates, fats, and proteins are the main energy sources of the body. The process of oxidizing energy substrates in the body (such as proteins, carbohydrates, lipids, etc.) to generate energy is called energy expenditure. Total daily energy expenditure (TEE) of the human body is divided into three parts: Basal Energy Expenditure (BEE), Diet-induced thermogenesis (DIT) and physical activity energy expenditure (Activity induced energy expenditure (AEE). Among them, basic energy metabolism (BEE) determines the most basic energy requirements of the human body. It plays a series of important roles in life survival, such as cell functions in the body and protein synthesis. It accounts for 60% -75% of daily energy needs. Compared with DIT and PAEE, BEE is more stable, and the daily change is small. It is an important indicator to distinguish the energy needs between different people. BEE is calculated through the Basal metabolic rate (BMR), which is the body's basic energy expenditure per minute (BEE/1440). Basal energy metabolism rate (BMR) refers to a neutral and mild environment (20-25 ° C), waking up in the morning, the body is in a state of digestion and absorption (that is, the digestive system is inactive and requires a minimum of 12 hours overnight) Food), the energy expended by the body per minute when breathing normally and completely relaxed.
Figure 2. Energy metabolism in healthy heart muscle.
About 50% of the energy released by the body's various energy substances when oxidized in the body is quickly converted into thermal energy, which is mainly used to maintain the body's body temperature. Thermal energy can no longer be converted into other forms of energy and therefore cannot be used for work. The remaining less than 50% of the energy is "free energy" that can be used for work. This part of the free energy carrier is adenosine triphosphate (ATP), and energy is stored in the high-energy phosphate bond of ATP. In the process of body energy conversion, ATP is both an important energy storage material and a direct energy supply material. When the tissue cells of the body perform various functional activities, the direct source of energy is the reserve energy in ATP. In addition to ATP, there is another energy storage compound in the body that contains high-energy phosphate bonds, namely creatine phosphate (CP). When the energy generated by the decomposition of substances in the body increases and the ATP concentration increases, ATP will transfer high-energy phosphate bonds to creatine to generate CP. Store energy; conversely, when the energy consumption of tissue cells increases and the ATP concentration decreases, the stored energy is transferred to adenosine diphosphate (ADP) to generate new ATP. Therefore, CP is often seen as a reservoir of ATP. From the perspective of the whole process of energy metabolism, the synthesis and decomposition of ATP is the key link of energy conversion and utilization in the body.
Figure 3. Structure of adenosine triphosphate (ATP).
1. Ravussin E.; et al. Relationship of genetics, age, and physical fitness to daily energy expenditure and fuel utilization. Am J ClinNutr. 1989, 49(5): 968-975.