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The Human Body’s Energy Metabolism: Understanding the Basics
The human body requires energy to carry out various physiological processes. These processes can be active processes that you perform, such as working and moving. But the body also needs energy for functions like your heartbeat and breathing.
In this article, we delve deeper into the body’s energy metabolism. Topics we’ll discuss include how the body handles various macronutrients such as carbohydrates, proteins, and fats, and why some people may have a faster metabolism than others.
Table of Contents
Metabolism
As mentioned in the introduction, the human body needs energy to provide for its physiological processes. Basal metabolic rate, or BMR, is the amount of energy the body needs without any additional activities. So, imagine if you were to lie in bed all day, how much energy would your body need?
This varies from person to person. Some people have a fast basal metabolic rate, while others have a slow metabolism. This is influenced by factors such as age, gender, weight, height, and body composition. Body composition is particularly important for people with higher muscle mass. Muscle mass is living tissue and requires energy at rest. In contrast, fat tissue is stored energy. Therefore, people with higher muscle mass also have a higher basal metabolic rate.
When discussing basal metabolic rate, we cannot overlook active metabolism. Active metabolism refers to the activities you undertake that also require energy. We distinguish between obtaining energy through the breakdown of substances, known as catabolism, and obtaining energy through the building of certain substances, known as anabolism.
The human body obtains its energy from food, which can consist of three macronutrients: proteins, fats, and carbohydrates. The body breaks these down in different ways. Therefore, it’s important to understand how your body handles these macronutrients and their roles in your body.
Protein Metabolism
When the body breaks down proteins, the liver comes into action. The liver breaks down proteins into amino acids, distinguishing between two types of amino acids:
- Essential amino acids
- Non-essential amino acids
The human body cannot produce essential amino acids on its own. It relies on nutrients for the production of these amino acids.
The body starts digesting proteins in the stomach. Proteins leave the stomach in the form of proteoses, peptones, and polypeptides. These peptides are broken down into amino acids and assist in the building and breakdown of body cells. If the body doesn’t need the amino acids, they can be deaminated and used for energy by the liver. While carbohydrates are the preferred energy source for the body, in the absence of carbohydrates or fats, protein can serve as fuel instead of a building block.
Carbohydrate Metabolism
Carbohydrates are the body’s preferred fuel source, primarily providing energy. Some carbohydrates serve as dietary fiber and play a crucial role in gut flora, but mostly, they act as fuel. Within carbohydrates, we further distinguish three types:
- Simple carbohydrates
- Complex carbohydrates
- Multiple carbohydrates
Similar to proteins, the breakdown of carbohydrates starts in the stomach. Sugar molecules are broken down into their new form as monosaccharides. Upon reaching the liver, the metabolism of these carbohydrates begins. Fructose and galactose are converted into glucose, which is then stored as glycogen in the liver and muscles. If necessary, this glycogen can be converted back into glucose. This process is called glycogenolysis.
As mentioned in the discussion on proteins, protein can also serve as fuel. This occurs when the body lacks sufficient carbohydrates or fats for energy provision. Conversely, if there is excess glucose in the liver, it is converted into fat tissue. This fat tissue can store a large amount of energy in the form of fat. This is evident in the body as an increase in weight and a higher percentage of body fat.
In addition to the glycogen reserve in the liver, muscles can also store glycogen. This is stored as energy for when a heavy workout is performed. The energy in the muscles allows you to sustain a certain effort for a longer period. Endurance athletes benefit from keeping these glycogen stores as high as possible by eating a carbohydrate meal before exercise. This is seen in cyclists and soccer players.
Finally, carbohydrate metabolism is regulated by a number of hormones. The insulin hormone regulates blood glucose levels. This insulin is responsible for better absorption of glucose by the muscles and fat tissue. If blood glucose levels drop, it comes at the expense of energy supply to the brain. For their energy needs, the brain cannot use proteins or fats, resulting in dizziness and decreased concentration.
Fat Metabolism
Finally, fats. Like proteins and carbohydrates, fats come in various forms. Firstly, they can be divided into:
- Unsaturated fats
- Saturated fats
Fats, like carbohydrates, can be used for energy production. They are broken down into water and carbon dioxide and provide twice as much energy as a gram of protein or carbohydrates. When fat is used as energy by the body, it first needs to release fats from adipose tissue. This occurs under the action of lipase, after which the released fats are converted into fatty acids and glycerol. These enter the bloodstream where they are absorbed by the body.
As with carbohydrates, hormones also play a significant role in fat metabolism. In fat synthesis, also known as lipogenesis, insulin plays a major role, as does lipoprotein lipase, which is an enzyme.
In fat breakdown, also known as lipolysis, other hormones such as glucagon, growth hormones, and noradrenaline come into play.
If you consume few carbohydrates, your body will use adipose tissue to obtain its energy. Fatty acids
often do not fully oxidize, resulting in an intermediate product: acetone. This is often the case when people follow a low-carbohydrate diet.
