Protein from food is broken down in the gastrointestinal tract and absorbed into the bloodstream as free amino acids.
With the help of these various amino acids, the body can then build its own proteins.
- The initial chemical breakdown of protein occurs in the stomach with the help of hydrochloric acid.
- The enzyme pepsin, which is produced in the stomach, breaks down protein into peptides. Pepsin continues to break down protein even in the small intestine if the pH is low enough.
- The largest and final stage of protein breakdown takes place in the small intestine.
- The amino acids are then absorbed into the intestinal lining and transported to the liver. There, they are converted back into proteins, carbohydrates, or fat. They can also be released into the bloodstream as free amino acids.
- Branched-chain amino acids are metabolized in the muscle. Insulin stimulates their uptake and thus has anabolic effects. Cortisol and glucagon have catabolic effects.
Protein metabolism
- Neither proteins nor amino acids are stored in the body. Any excess is converted into energy or transformed into fat and glucose, which may then be further converted into glycogen. Proteins can therefore be used as a source of energy.
- Proteins can be used as an energy source when we either have an excess of protein or when the amount of glucose available for metabolism is insufficient. As soon as blood sugar levels drop, the liver begins to compensate by first breaking down glycogen stored in the liver. Once this process is complete, proteins are converted into glucose.
- When proteins are needed as an energy source—for example, during a low-carbohydrate diet—their other functions take a back seat, and muscle proteins may be broken down.
- The liver can break down proteins into their smallest components and produce glucose.