Protein is an essential nutrient whose name comes from the greek work protos, meaning “first”. A molecule of protein can be seen as a chain with the links consisting of amino acids, the building blocks of protein.
Usage of Protein
Our body uses protein to maintain tissues, build new cells and synthesize new proteins that make it possible to perform basic bodily functions. The human body is full of proteins:
- In the inner/outer membranes of every living cell.
- Muscle tissue contains myosin, myoglobin, actin.
- The bone’s outer layer is hardened with minerals like calcium, but the inner structure is protein.
- Red blood cells contain hemoglobin, which carries oxygen throughout our body.
About half of the dietary protein that is consumed each day is used to make enzymes, the specialized “worker” proteins that perform specific jobs such as digesting food and dividing or assembling molecules to make new cells and chemical substances.
How Proteins Move Into Cells
The cells in the digestive tract (see digestive system) can absorb only single amino acids or very small chains. Proteins from food are broken into their component amino acids by digestive enzymes. Other enzymes in body cells build new proteins by reassembling the amino acids into specific compounds that our body needs, a process called protein synthesis.
During protein synthesis:
- Amino acids connect with fats to form lipoproteins, molecules that transport cholesterol around our body.
- Nucleic acids combine with proteins to create nucleoproteins, which are essential components of the cell nucleus.
- Proteins combine with phosphoric acid to produce phosphoproteins, e.g. ovovitelline in eggs.
The carbon, oxygen and hydrogen that remain after the protein synthesis are converted to glucose and used for energy.
Essential and Non-Essential Proteins
To create all the proteins that our body needs, 22 different amino acids are required. Ten of these are considered essential. This means that the body cannot synthesize them, i.e. we must obtain them from food. The rest are non-essential – the body can manufacture them from carbohydrates, fats and other amino acids.
- Essential amino acids. Arginine (for children), histidine (for children), leucine, isoleucine, lysine, methionine, threoniwne, phenlyalanine, tryptophan and valine.
- Non-essential amino acids. Alanine, asparagine, citrulline, aspartic acid, cysteine, hydroxyglutamic acid, glutamic acid, glycine, norleucine, proline, serine and tyrosine.
High-Quality and Low-Quality Proteins
Many animals contain similar combinations of amino acids as our human body. That is the reason why proteins from foods of animal origin – fish, meat, poultry, eggs and dairy products – are called high-quality proteins. Our body absorbs these proteins efficiently, i.e. they can be used without much waste to synthesize other proteins.
Proteins from plants – fruit, grains, beans, vegetables, nuts and seeds – have often limited amounts of some essential amino acids, i.e. their nutritional content is lower than animal proteins. Soybeans are an exception as they have complete proteins with sufficient amounts of all amino acids essential for humans.
The basic standard against which the value of proteins are measured is the egg. Nutrition scientists have arbitrarily given the egg a biological value of 100 percent, meaning that, gram for gram, the egg is the food with the best supply of complete proteins. Other foods that have more protein may not be as valuable as the egg due to lacking of sufficient amounts of one or more essential amino acids.
How Much Protein is Needed?
A healthy adult woman requires 40 – 55 grams of protein daily and a healthy adult man needs 50 – 70 grams per day.