In 1838, a Dutch chemist, G.J. Mulder, described a certain organic material as "unquestionably the most important of all known substances in the organic kingdom. Without it, no life appears possible on our planet. Through its means the chief phenomena of life are produced." This complex nitrogen-bearing substance was called protein from the Greek word meaning "take the first place." Protein in now a group name signifying the principal nitrogenous constituents of the protoplasm of all plant and animal tissues.

Proteins are extremely complex organic compounds of the elements carbon, hydrogen, oxygen, nitrogen, and,with some exceptions, sulphur. Most proteins also contain phosphorous, and some specialised proteins contain iron, iodine, copper and other inorganic elements. The presence of nitrogen distinguishes proteins from carbohydrates and fats.

Proteins are thus vital substances, which form important constituent of muscles, tissues, and the blood. Proteins supply the building material for the body and make good the wear and tear of tissues. Several substances concerned with vital life processes such as enzymes, which help in digestion of food, are chiefly protein in nature.

There are several varieties of protein. Each type contains a specific number of "building blocks" known as amino-acids. Before they can be absorbed by the body, all proteins must first be broken down into amino-acids. When food stuffs are ingested, the nutrients and amino-acids do not immediately diffuse into all the different tissues. There are a series of biochemical reactions in the digestive tract which collect these proteins, break them down and then utilise them as needed. Any interference with the normal digestive process causes in-complete protein digestion resulting in gas, bloating etc.

There are about 22 amino acids needed for the normal functioning of the body. The body can manufacture many amino acids if it has no adequate nitrogen source, but it cannot produce certain others in sufficient amounts to meet its needs. The amino acids that the body cannot synthesis is in adequate amounts are called essential or indispensable because they must be supplied by the diet in proper proportions and amounts to meet the requirements for maintenance of growth of tNon-essential or dispensable amino acids are those thatissue. the body can synthesize in sufficient amounts to meet its needs if the total amount of nitrogen supplied by protein is adequate. The essential and non-essential amino acids are listed in table A. ? TABLE A

Classification of Amino Acids with respect to their essentiality

EssentialNonessential Histidine*Alanine IsoleucineArginine Leucine        Asparagine Lysine        Aspartic acid MethionineCysteine PhenylalanineCystine TheronineGlutamic acid TrypophanGlutamine Valine        Glycine Hydroxyproline? Proline? Serine? Tyrosine

It will be seen from this statement that nine amino acids are essential for maintenance of nitrogen equilibrium in human bodies. The estimated requirements of essential amino acids for infants, children and adults are given in Table B. Men in the older age group appear to differ in their requirements. Studies seem to suggest an increase need for methionine and lysine for them. Infants and children have proportionally greater demands for essential amino acids than adults. In addition, infants require histidine as an essential amino acid.

Factors in addition to the age, sex and physiological condition of an individual influence the requirements for specific amino acids. If total protein intake is low, small surpluses of certain amino acids can increase the need for others. The non-essential amino acids in protein also affect the quality of protein. For example, the amount of sulphur - containing essential amino acid methionine required may be somewhat reduced if cystine, a sulphur-containing nonessential amino acid,is supplied in the diet. Likewise, the presence in the diet of tyrosine, a non-essential amino acid similar in structure to phenylalanine, may reduce the requirement for phenylalanine.