It seems as if everybody has an opinion about proteins. How much protein you should eat and out of which sources. High protein diets are very popular right now and the supplementation market for proteins is booming. But do you even know what happens to the proteins you eat in your body? Here’s how protein digestion works and what the body does with all the protein you eat.
Protein is one of the big three macronutrients, along with fat and carbohydrates. Proteins you eat are made of building blocks called amino acids. There are 20 different amino acids your body uses. You could imagine amino acids as different colored beads on a thread. Your body combines those beads through an infinite amount of combinations. There are over 100.000 different protein combinations in your body. All made out of only 20 basic building blocks – amino acids.
The structure of protein
All amino acids have a similar structure. They are molecules which have a central carbon atom which is connected to four other groups. Including an amine group, an acid group, a hydrogen atom, and a side chain.
The 20 different amino acids differ only in the side chain (R). The simplest amino acid glycine has a hydrogen atom in place of the side chain. Other amino acids have more complicated side chains attached to them. The side chain also determines the properties of the amino acids. If a protein is going to be hydrophilic or hydrophobic depends on the nature of the side chain.
The name amino acid comes from the amine group. Amine means nitrogen-containing. This amine group the main difference between protein and the other macronutrients. Only protein contains this special form of nitrogen which is made available to the body during the digestive process. Nitrogen is necessary for many important metabolic processes. Protein is the main source of nitrogen that fuels our bodies. There is another big difference between protein and the other macronutrients. Unlike fat and carbohydrates, the formation of proteins depends on DNA.
Peptides are the bonds between two amino acids. When two amino acids bind they form a dipeptide. Three amino acids form a tripeptide and a chain of more than 10 amino acids form a polypeptide. Most proteins used in our bodies are polypeptides. These are very long chains of more than thousands of amino acids that form complex proteins.
All the amino acids in the body
There are 20 different amino acids our body uses to form proteins. Nine of these amino acids must be part of our diet because our body can’t make them. That’s why they are called essential amino acids. Non-essential amino acids are the other 11 amino acids. They can be synthesized from the essential amino acids. When you eat protein-rich food you’re in fact fueling your body with different mixtures of amino acids. The amino acids from dietary protein end up in the bloodstream, forming a “pool” of different amino acids. The body takes any amino acids it needs from this pool and forms various protein chains for many different functions.
These amino acids must be part of your diet.
These amino acids can be synthesized from essential amino acids.
- Aspartic acid
- Glutamic acid
Proteins have shapes
The long polypeptide chains can be formed into different shapes. The shape of a protein directly influences its function. For example, the protein chains in muscles are much long and narrow. This kind of structure is important as it allows the muscle to contract and relax. In contrast, hemoglobin has a globular in shape. That’s why red blood cells have a flat disc shape with deflated centers. This structure allows the red blood cells to be flexible and enables them to change shape. Thos is important as red blood cells must get to the tiniest blood vessels to supply the whole body with oxygen.
The process of protein digestion
To absorb protein, our body needs to convert polypeptides into individual amino acids. This process happens during digestion in the small intestines.
1. The digestion of protein begins in the stomach. Unlike with carbohydrates, the protein digestion does not start in the mouth. The stomach is full of hydrochloric acid which makes the protein unfold. In presence of hydrochloric acid, the inactive enzyme pepsinogen is converted into his active form called pepsin. Pepsin starts to break down the long protein chains into smaller polypeptides. These smaller chains of protein are then traveling to the duodenum.
2. The absorption of single amino acids into the bloodstream happens entirely in the small intestines. Only a small part of undigested protein ends up in the colon. After leaving the stomach chyme enters the small intestines. That’s when the pancreas secretes enzymes for protein digestion into the small intestine. These are the enzymes trypsin and chymotrypsin. It’s their job to break down the polypeptides into dipeptides and tripeptides. But the protein digestion is not complete. Protein is still not converted into individual amino acids.
3. Enzymes called carboxypeptidase and aminopeptidase are present in the lining of the small intestines. They finally break the left peptide bonds into individual amino acids. The amino acids end up in the bloodstream through the wall of the small intestines. They then travel to the liver through the portal vein.
What happens to digested protein
The liver uses large amounts of protein to form liver proteins, including the main liver protein albumin. You can think of the liver as the main distributor of amino acids for our body. The liver decides what happens to the individual amino acids. Depending on what our body needs the liver:
- converts amino acids to glucose or fat,
- combines amino acids into new proteins,
- uses amino acids for energy,
- releases amino acids into the bloodstream to reach other cells as needed
For example one of the biggest consumers of protein in your body are the muscles. They need the amino acids to form muscle tissue.
When food reaches the colon protein digestion is already done. Only a small amount of protein reaches the colon undigested. About 5% of proteins travel through the colon and are excreted through stool.
As mentioned above there are more than 100.000 proteins in our body. They all have important functions to perform, inside and outside of cells. These functions are vital and that’s why it’s important to consume both, a variety and a sufficient amount of proteins in our diet.
Good sources of protein
Generally speaking, the best sources of protein are grass-fed beef and chicken. They contain all the necessary protein in large amounts. Good sources are also eggs and other animal protein. People consuming a plant-based diet should take care to get enough proteins. Since people who consume a monotone plant-based diet can be deficient in essential amino acids. This can be solved by consuming a balanced diet with plenty of beans and grains.
Can you supplement individual amino acids?
Special absorption sites for dipeptides and tripeptides exist in the cells of the intestinal wall. They specialize in transporting amino acids into the bloodstream. When you consume a protein supplement containing a high amount of single amino acids, these amino acids compete for the same absorption sites with di- and tripeptides. This effect is even worse if you consume the supplement on an empty stomach.
This sudden rush of proteins on the absorption sites can block the absorption of other amino acids into the bloodstream. This causes an imbalance of amino acids in the amino acid pool and can lead to various amino acid deficiencies. That’s why it’s best to consume amino acids through a rich and varied diet.