Proteins are complex nitrogenous compounds. They are the essential constituents of all nutrients.

Functions of proteins :

1. Body building, repair and maintenance of tissues.

2. Maintenance of osmotic pressure.


3. Synthesis of antibodies, plasma proteins and hemoglobin.

Constituents of proteins

Proteins contain carbon, hydrogen, oxygen and nitrogen. Some proteins may contain sulphur, phosphorus, iron and also trace ele­ments. Proteins are made of smaller units called amino acids. The human body needs 24 amino acids.

Essential amino acids


They are isoleucine, leucine, methionine, lysine, phenylalan­ine, threonine, tryptophan, valine, arginine and histidine. These amino acids cannot be synthesised in the body and so they must be supplied in food.

Sources’ of proteins

The two main dietary sources of proteins are animals and vegetables.

Animal proteins


Milk, meat, egg and fish are important animal sources of proteins. Animal proteins contain all essential amino acids in ade­quate amounts. Of these, egg proteins are the best because they have high biological value and digestibility.

Vegetable proteins

Vegetable proteins are found in pulses, cereals, beans and nuts. They are poor in essential amino acids.

Biological value of proteins


It is the percentage of nitrogen retained from oujt of the nitrogen absorbed from the gut. Higher the biological value, greater the quality of protein. Animal proteins are rated better than plant protein. It is because they are biologically complete i.e. they con­tain all essential amino acids.

Protein metabolism

The important aspects of protein metabolism are:

1. Proteins are not stored in the body. They are replaced every day.


2. Body proteins are constantly broken down into individual amino acids. These are again reused for protein synthesis.

3. The amount and pattern of protein in the body is maintained constant.

4. For the effective utilisation of dietary proteins, the calorie intake should be adequate.

Protein requirement


The average daily requirement of protein is one gram per kg. body weight.