Proteins are, along with carbohydrates and fats, complex molecules that play many critical roles in the body. Whereas carbs and fats are essential energy carriers, proteins do most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs. They are the true body-building nutrients, made up of smaller units called amino acids. All cells need amino acids that are combined into specific proteins necessary for the structure and repair of muscles, bones, hair, nails and so on.
There are 20 different types of amino acids our body needs to produce proteins. Twelve of those amino acids can be made in sufficient amounts in the body itself. The other eight amino acids, also called essential amino acids, must be supplied in the food we consume. Essential amino acids are lysine, phenylalanine, tryptophan, methionine, threonine, leucine, isoleucine and valine.
Nitrogen is a naturally occurring element that is essential for growth and reproduction. It is found in amino acids that make up proteins. The amount of nitrogen in protein is approximately 16%. Some amino acids also contain sulfur (S), phosphorus (P), iron (Fe) or iodine (J).
Biological value
When a protein contains the essential amino acids in a proportion similar to that required by humans, we say that it has a high biological value. The biological value (BV) of a protein is a value that measures how well the body can absorb and utilize a protein. Proteins in certain foods are worth more than in others. The more a protein resembles our own protein (e.g. milk protein), the higher the value. The biological value of human protein was given the value 100.
Foods that lack one or more essential amino acids are given a lower value. For example: the animal protein gelatin is low in tryptophan and methionine (essential amino acids). It is therefore considered incomplete and of low biological value
Proteins with a high level of essential amino acids that help our body build its own proteins are called complete proteins. Animal protein sources such as eggs, milk, meat and fish usually have high biological values. Egg white protein is considered to have one of the best amino acid profiles for human nutrition (so not the protein from cow milk as is often said). Plant proteins, with the exception of potatoes, rice and soy, generally have lower biological value because they lack one or more essential acids.
Proteins and our diet
A nutritionally balanced diet provides adequate protein, as some proteins will have an excess of essential amino acids to replenish the deficits of other proteins. When we eat bread with cheese or potatoes with vegetables, we consume the proteins of all these different foods together. Peas in combination with wheat increase the protein value of food. Legumes have high lysine and very low methionine levels, whereas the opposite is true for grains. It is not surprising then that pea soup is often served with bread, as the proteins in both foods are complementary. For the same reason, it is recommended to drink milk with your bread or sandwiches as the deficit of lysine in wheat bread is replenished with the high lysine levels in milk. To increase the protein value of the foods you eat, simply stick to a nutritionally balanced diet.