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Proteins are made up of smaller units called amino acids. There are about 20 different amino acids, eight of which must be present in the diet. These are the essential amino acids. Unlike animal proteins, plant proteins may not contain all the essential amino acids in the necessary proportions.
Structure & Functions

Proteins are highly complex molecules comprised of linked amino acids. Amino acids are simple compounds containing carbon, hydrogen, oxygen, nitrogen and occasionally sulfur. There are about 20 different amino acids commonly found in plant and animal proteins. Amino acids link together to form chains called peptides. A typical protein may contain 500 or more amino acids. Each protein has it's own unique number and sequence of amino acids which determines it's particular structure and function.

Proteins are broken down into their constituent amino acids during digestion which are then absorbed and used to make new proteins in the body. Certain amino acids can be made by the human body. However, the essential amino acids cannot be made and so they must be supplied in the diet. The eight essential amino acids required by humans are: leucine, isoleucine, valine, threonine, methionine, phenylalanine, tryptophan, and lysine. For children, histidine is also considered to be an essential amino acid.

Proteins are essential for growth and repair. They play a crucial role in virtually all biological processes in the body. All enzymes are proteins and are vital for the body's metabolism. Muscle contraction, immune protection, and the transmission of nerve impulses are all dependent on proteins. Proteins in skin and bone provide structural support. Many hormones are proteins. Protein can also provide a source of energy. Generally the body uses carbohydrate and fat for energy but when there is excess dietary protein or inadequate dietary fat and carbohydrate, protein is used. Excess protein may also be converted to fat and stored.
Dietary Sources

Most foods contain at least some protein. Different foods contain different proteins, each with their own unique amino acid composition. The proportions of essential amino acids in foods may differ from the proportions needed by the body to make proteins. The proportion of each of the essential amino acids in foods containing protein determines the quality of that protein. Dietary proteins with all the essential amino acids in the proportions required by the body are said to be a high quality protein. If the protein is low in one or more of the essential amino acids the protein is of a lower quality. The amino acid that is in shortest supply is called the limiting amino acid.

Protein quality is usually defined according to the amino acid pattern of egg protein, which is regarded as the ideal. As such, it is not surprising that animal proteins, such as meat, milk and cheese tend to be of a higher protein quality than plant proteins. This is why plant proteins are sometimes referred to as low quality proteins. Many plant proteins are low in one of the essential amino acids. For instance, grains tend to be short of lysine whilst pulses are short of methionine. Combining plant proteins, such as a grain with a pulse, leads to a high quality protein which is just as good, and in some cases better, than protein from animal foods. Soy is a high quality protein on its own which can be regarded as equal to meat protein.

The limiting amino acid tends to be different in different proteins. This means when two different foods are combined, the amino acids in one protein can compensate for the one lacking in the other. This is known as protein complementing.

Previously, it has been thought that protein complementing needed to occur within a single meal. However, it is now known that this is not necessary as the body keeps a short-term store of the essential amino acids.

What are Proteins and Why They Are Important