Metabolism refers to the conversion of substances such as food and oxygen into intermediate and end products within the body. This conversion process releases energy, which the body therefore obtains from food. These building blocks are then assembled into new substances to construct, for example, cell walls, muscle fibers, nerve fibers, and bones. Metabolism enables the body to grow, maintain its functions, and gain or lose weight. Enzymes and hormones are essential for metabolism, as are environmental factors that accelerate and regulate the chemical reactions. The word "metabolism" is frequently used casually in various contexts – and often mistakenly equated with digestion, which is incorrect, as the liver, not the intestines, is the most important metabolic organ.

Our main energy sources, and therefore those associated with metabolism, are fat , protein , and carbohydrates/sugar .

Carbohydrate metabolism: During digestion, complex carbohydrates from food are broken down into simple sugars (for example, glucose or fructose). These then enter the cells via the bloodstream, where the actual metabolic process takes place. The body can obtain energy from the simple sugars. If this energy is not needed, the sugar is converted into starch and stored in the liver and muscles ( anabolism ).

Protein metabolism (amino acid metabolism): Proteins consist of amino acids, into which they are broken down during digestion before being transported to the cells. There, they serve for energy production, the building of muscle cells, hormones, and enzymes.

Fat metabolism: Fat serves as an energy source in cells and is the most important energy reserve. It also plays a role in the production of hormones and signaling molecules.

Generally, two different metabolic processes are distinguished – the anabolic (building) and the catabolic (consuming) metabolism, which ideally are in balance. While nutrients are stored during the anabolic phase, they are released again during the catabolic phase, thereby releasing energy.

Since daily energy consumption is constantly changing and each person has a different basal metabolic rate, an imbalance quickly arises, leading to a more anabolic and storage state in modern times. This is not necessarily due to a metabolic disorder, but in most cases to an unbalanced diet that is not adapted to individual needs.