The word “protein” by derivation means that which is of first importance. Indeed proteins are of the greatest importance in human diet. Proteins are complex organic nitrogenous compounds. They are composed of carbon, hydrogen, oxygen, nitrogen and sulphur in varying amounts. Some proteins also contain P and Fe and occasionally other elements too.

Essential amino acids:

Proteins are consist of smaller units, called amino acids. Some 20 amino acids are stated to be needed by the human body, of which 9 are called “essential” because the body cannot synthesize them in amounts corresponding to its needs, and therefore, they must be obtained from dietary proteins. These are as follows leucine, isoleucine, lysine, methionine, phenylalanine, threonine, valine, tryptophan and histidine. Non-essential amino acids include arginine, asparaginic acid, serine, glutamic acid, praline and glycine.

Some of the essential amino acids have important biological functions, e.g., formation of niacin from tryptophan; the action of methionine as a donor of methyl groups for the synthesis of choline, folates and nucleic acids.

A protein is said to be “biologically complete” if it contains all the EAA in amounts corresponding to human needs. When one or more of the EAA are lacking, the protein is said to be “biologically incomplete”. If we think of diet animal proteins are more superior compare to vegetables proteins.

Functions:

Proteins are needed by the body for:

body building, – this component is small compared with the maintenance component, except in the very young child and infant
repair and maintenance of body tissues; (c) maintenance of osmotic pressure;
synthesis of certain substances like antibodies, plasma proteins, hemoglobin, enzymes, hormones and coagulation factors
Proteins are connected with the immune mechanism of the body. The cell mediated immune response and the bactericidal activity of leucocyte have been found to be lowered in severe forms of protein energy malnutrition.
Proteins can also supply energy (4 kcal per one gram) when the calorie intake is inadequate, but this is not their primary function.

Sources:

ANIMAL SOURCES:

Proteins of animal origin are found in milk, meat, eggs, cheese, fish and fowl. These proteins has all essential amino acid in correct amount. Egg proteins are considered to be the perfect among food proteins because of their high biological value and digestibility. They are used in nutrition studies as a “reference protein”.

VEGETABLE SOURCES:

Pulses (legumes), cereals, beans, nuts, oil-seed cakes, etc are major source of vegetable proteins. They are poor in EAA.

lysine and threonine are deficient in cereal proteins; and pulse proteins in methionine. These are known as “limiting” amino acids. To provide proteins of same quality of animal proteins we have to eat combination proteins in our diet. Thus with proper planning, it is possible for a vegetarian to obtain a high grade protein, at low cost, from mixed diets of cereals, pulses and vegetables. This is known as supplementary action of proteins.

Protein metabolism:

There are three features of protein metabolism:

Since proteins are not stored in the human body they have to be replaced every day.

the body proteins are constantly being broken down into their constituent amino acids and then reused for protein synthesis.
it is not only the amount of protein that is maintained constant, but also the pattern of specific protein in body. For maximum utilization of dietary proteins, the calorie intake should be adequate.

Evaluation of proteins:

The net protein utilization (NPU) is considered of more practical value because it is the product of biological value and digestibility coefficient divided by 100. In exact terms, it is the “proportion of ingested protein that is retained in the body under specified conditions for the maintenance and/or growth of the tissues”.

Assessment of protein nutrition status:

At the present time the best measure of the state of protein nutrition is probably serum albumin concentration. It should be more than 3.5 g/dl, if level is 3.5g/dl it is called as mild malnutrition while level 3.0g/dl is severe malnutrition.

Protein requirements:

1.0 g protein/kg body weight for an Indian adult is required amount as per The Indian Council of Medical Research in 2010.

Relative protein value of some foods :

Food	Proteins per 100gm
fish	20.0
milk	3.2
dhal	21.0
rice	7.0
potato	1.6
banana	1.0
tapioca	0.7

Proteins: functions, sources& requirement.