Protein is a nutrient and a building material. There are animal and vegetable proteins. Animal proteins are mainly found in meat, fish, milk, cheese and eggs. Vegetable proteins are mainly found in bread, grain products, legumes and nuts. Adults need an average of 0.83 grams of protein per kilogram of body weight every day. Some groups need a little more. These are vegetarians, vegans, children, pregnant and breastfeeding women. People with certain conditions or wounds and strength and endurance athletes also need something more.
What are proteins?
Protein is a nutrient, just like carbohydrates and fat. Protein provides (energy) calories and amino acids. Amino acids are building blocks for protein in body cells. Another word for protein is protein. Almost all foods contain protein. It occurs in both plant and animal products.
Vegetable sources are:
- grains, such as whole-wheat pasta and couscous
- legumes, such as chickpeas, kidney beans, kidney beans, soybeans and lentils
- tofu and tempeh
Animal sources are:
- meat (meat is richest in protein: 20 to 30%)
- poultry, such as chicken
- milk (products)
Proteins and amino acids
Proteins consist of chains of smaller ‘chunks’: the amino acids. In total, protein in food can contain 22 different types of amino acids. The composition, sequence and structure of these amino acids differ. This makes each protein unique. An amino acid is made up of carbon (C), oxygen (O), nitrogen (N) and sometimes sulfur molecules (S).
You do not have to get all amino acids through the proteins from your food and drink. Some amino acids can be made by your body from other amino acids. These are called non-essential amino acids. The amino acids that you must ingest through your food and drink because you cannot make them yourself are called essential amino acids. In addition, there are semi-essential amino acids. That is, the body can make them on its own in most people, but in some conditions and diseases, the body can’t make enough of them. Then supplementation through food is necessary.
Why do you need protein?
Protein contains amino acids that the body cannot do without. For example, the amino acids are necessary for the construction of cells and for all kinds of regulatory processes in the body. Furthermore, a diet with relatively much protein helps to maintain weight. This is because protein saturates well and is important to maintain muscle tissue.
Protein as a building material
Tissues in the body are made up of cells. All cells contain protein, think of the cells in muscles, organs, the nervous system, bones and blood. An adult consists on average of 12 kilos of protein. The body builds this protein from amino acids. Especially in children, a lot of tissue is built up. But also during pregnancy and when breastfeeding.
Protein is also needed for existing cells. They are always renewing. The body breaks down protein from the cells to replace it with new protein. For example, the body removes damaged protein, which could lead to disrupted cell function and cell growth. In (burn) wounds, extra protein is needed to repair the tissues.
The body uses the amino acids from the broken down protein to build new protein. But amino acids are also lost. In addition, the body loses small amounts of protein with hair, nails, dander, sweat and urine. This must always be supplemented. This is especially true for conditions associated with increased protein breakdown or amino acid losses, such as burns.
Proteins are involved in many regulatory processes in the body.
- All enzymes are proteins. Enzymes, for example, enable digestion by helping to break down substances such as starch.
- Antibodies (antibodies) are proteins.
- Many hormones are proteins, such as insulin.
- Proteins play a role in the transport of substances in the blood and in the cell. Hemoglobin is an example of this. Hemoglobin carries oxygen from the lungs to the cells.
- Some cells contain ‘receptor proteins’. Certain substances can adhere to it. These proteins play a role in the transmission of signals.
Transfer of incentives
Some amino acids from protein are precursors of neurotransmitters. That is, they are needed to make neurotransmitters. Neurotransmitters are substances that play a role in nerve and brain cells and are involved in the transmission of stimuli. Examples are tryptophan, as a precursor of serotonin, and tyrosine, as a precursor of dopamine.
Protein provides energy: 4 kilocalories per gram. The body can convert amino acids from protein into food or from muscles into glucose. This happens especially if it has too little glucose. For example, if you have not eaten anything for a long time, or if you eat very few carbohydrates. But also if you eat more protein than your body needs.
The body uses the calories (energy) from protein slightly less well than from carbohydrates and fats. Some of the calories from protein are converted to heat immediately after a meal. This is the so-called ‘thermogenic effect’. Furthermore, protein is more satiating than carbohydrates and fats.
It seems that a high-protein diet has no effect on weight if the calorie count is the same as a low-protein diet. However, a diet with a lot of protein can help you not to get too many calories. People eat less if they consume a relatively large amount of protein, probably because protein is more satiating. This also explains the success of diets in which more than a quarter of the calories come from protein.
When losing weight, eating enough protein limits the loss of muscle tissue. This is beneficial, because muscle tissue uses more energy than fat tissue. For example, protein can help to maintain weight after weight loss.
Absorption and processing of protein by the body
The body breaks down protein from food into individual amino acids. This is done with the help of enzymes in the stomach and small intestine. Protein is digested faster when it is denatured. That is, the spatial structure of the protein has changed. This happens in the intestine under the influence of stomach acid, but also during cooking by heating. Protein that cannot be digested in the small intestine is further broken down or converted by bacteria in the large intestine. This can release hydrogen sulfide, the well-known rotten egg odor.
The amino acids released during digestion in the small intestine are absorbed by the body and transported via the blood to the liver and other tissues. They are then used for the production of body protein (on average about 70% of the amino acids from food). The rest is used as energy or converted to glucose or stored as fat.
Body proteins are constantly broken down and rebuilt. This mainly occurs in the cells of the liver and intestines, but also in muscle tissue. In total, about 200 to 300 grams of protein are replaced daily.