Most people consider protein to mean meat. Althoug h this is true to a certain extent it is definitely not the full picture. Most plant foods contain protein and in fact it would be very difficult to design a vegetarian diet that is short on protein.

Excess dietary protein from a meat source may have been linked to health problems and it is now thoug ht that one of the benefits of a vegetarian diet is that it contains adequate but not excessive protein.

What is it?
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. However, a varied vegetarian diet means a mixture of proteins are consumed, the amino acids in one protein compensating for the deficiencies of another.

Structure & Function
Proteins are highly complex molecules comprised of linked amino acids. Amino acids are simple compounds containing carbon, hydrogen, oxygen, nitrogen and occasionally sulphur. 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 too.

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. Good sources of protein for vegetarians include nuts and seeds, pulses, soya products (tofu, soya milk and TVP), cereals (wheat, oats, and rice), free-range eggs and some dairy products (milk, cheese and yoghurt). 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. This does not mean that vegetarians or vegans go short on essential amino acids. 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. Soya 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. Vegetarians and vegans eating a well-balanced diet based on grains, pulses, seeds, nuts and vegetables will be consuming a mixture of proteins that complement one another naturally without requiring any planning.

Beans on toast, cheese or peanut butter sandwich, muesli with milk (soya or cow's), and rice with peas or beans are all common examples of protein complementing.

Previously, it has been thoug ht 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. A well-balanced vegetarian or vegan diet will easily supply all the protein and essential amino acids needed by the body.

Increased protein needs during pregnancy and breast feeding are usually met simply by the extra calories from more food. Because infants and children are growing they require more protein than adults (proportional to their body weight). Children on a balanced diet usually get enoug h protein as long as they are getting enoug h energy (or calories).

Contrary to popular belief athletes and those who engage in a lot of exercise do not necessarily need extra protein as the extra energy required for strenuous activity is best supplied by carbohydrates. The extra protein needs of a body builder can usually be supplied by an increased energy intake from more food. Whilst vegetarian diets usually meet or exceed protein requirements, they are typically lower in total intake of protein than non-vegetarian diets. This lower protein intake may well be beneficial as high protein intake has been associated with osteoporosis and aggravating poor or failing kidney functioning when consumed without balancing nutrients to help buffer the acidity and a high fluid intake to keep the kidneys functioning efficiently.