Proteins

Dietary proteins are used by the body to build and repair cells, tissues and organs.   About 50% of the human body is protein and every cell contains some protein.   Protein is in bones, muscle, cartilage, skin, blood, and lymph.   All enzymes and many hormones are protein.   Only bile and urine contain no protein.   Different proteins perform a wide variety of biological functions.   Some proteins are enzymes, which catalyze chemical reactions.   Other proteins provide structure, e.g., joints of the cytoskeleton of cells.   Other functions of proteins include immune response and the storage and transport of various ligands. ^{Wiki n.p.}

Amino Acids

Dietary proteins are composed of 100 to 300 of several of 20 amino acids, which are compounds containing carbon, hydrogen, oxygen, nitrogen and, in some cases, sulphur.   Dietary protein is not used directly by the body, but must be broken down in its components parts, called amino acids.   These are carried by the blood into the body cells.   Cells reconstitute the amino acids into proteins for use by body organs.   All amino acids have an acid group and an amino group attached to a carbon atom.   The amino groups contains nitrogen.   Amino acids differ in the "R-group".

The amino group of one amino acid can link with the acid group of another amino acid to form a chain of amino acids.   This link is called a peptide bond.   Two amino acids joined together form a dipeptide.   Many amino acids join to form a polypeptide.   A typical protein may contain 500 or more amino acids joined together by peptide bonds.   Each protein has its own specific number and sequence of amino acids.   The chains of amino acids making up a protein are also held together by other bonds, sometimes containing sulphur.   The shape of the protein molecule is important, since it often determines the function of the protein.   Each animal species, including humans, has its own characteristic proteins. ^{BNF n.p.}   When required, the body can convert amino acids to glucose for energy in a chemical reaction called gluconeogenesis.

Essential Amino Acids

Sometimes the amino group of an amino acid is transferred to another molecule by a process called transamination.   In this way the human body is able to make some amino acids for itself.   These are known as non-essential amino acids.   However, it is not possible to do this for every amino acid, so a certain number must be supplied by protein in food.   They are known as the essential amino acids and for the human adult there are 8: Leucine, Isoleucine, Valine, Threonine, Methionine, Phenylalanine, Tryptophan, and Lysine.   In children, Histidine is also considered to be an essential amino acid, since children are unable to make enough to meet their needs. ^{BNF n.p.}

Protein usually is obtained from animal sources, i.e. meat, fish, eggs and dairy foods.   It also is obtained from grains, nuts and pulses.   After a protein is eaten it is broken down by digestion into amino acids, which are then absorbed and used to make other proteins in the body.   All animal and plant cells contain some protein but the amount of protein present in food varies widely.   Both the amount of protein and the presence of specific amino acids are important.   If a protein contains the indispensable amino acids in the approximate proportion required by humans, it is said to have a high biological value.   If it is comparatively low in one or more of the essential amino acids, it is said to have a low biological value.   In general, proteins from animal sources have a higher biological value than proteins from plant sources, but the limiting amino acid varies.   Lysine is the limiting amino acid in wheat protein, Tryptophan in maize protein, and Methionine and Cysteine in beef protein.   Among the vegetable sources, soya protein is the most complete.   The limiting amino acid tends to be different in different proteins, so when two foods providing vegetable protein are eaten at a meal, such as a cereal (e.g. bread) and a pulse (e.g. beans), the amino acids of one protein may compensate for the limitations of the other, resulting in a combination of higher biological value.   This is known as the complementary action of proteins. ^{BNF n.p.}

Dietary proteins cannot be stored in the body, so they must be a part of every diet.   Excess protein is excreted or stored in the body as glycogen after the amino acids are synthesized into glucose in a chemical reaction called gluconeogenesis.   The excess nitrogen in protein is excreted from the body as urea in urine.   Children need about 3 times more protein as adults. ^{Brody 34-35}