In chemical composition, the proteins differ from carbohydrates and fats as they contain nitrogen addition to carbon, hydrogen and oxygen, and very few contain sulphur also. Most of the proteins also contain phosphorus. Where as some specialized proteins contain iron, iodine, copper and other organic elements also.

Contribution of proteins to the energy value of well-balanced diets is usually between 10 and 15 percent of the total. But for the body building functions they are more important because every cell in the body of composed of proteins which are subjected to continuous wear and tear replacement.

Fat in the body can be derived from dietary carbohydrates and the carbohydrates from proteins, but the proteins of the body are totally and entirely dependent for their formation and maintenance on the proteins supplied in the food.

Most food proteins are composed of 12 to 22 amino acids.


Classification of Proteins

Complete Proteins:

Complete proteins are found in those foods which have all the essential amino acids in significant quantities. These foods are referred to as proteins having a high biological value.

Partially Incomplete Proteins:


Partial proteins are found in those foods which contain amino acids in the proportion that may maintain life but do not support growth.

They may essential amino acids but the proportionate amount may be low or a food may not contain one or more essential amino acids at all.

Incomplete Proteins:

These are found in those foods in which amino acids content is such that it is incapable either of maintaining life or of supporting growth.


Amino acids are classified as:

(a) Essential amino acids, and (b) Non-essential amino acids.

Essential amino acids are those which can not be synthesized by the body in sufficient quantities for growth and maintenance and therefore, must be supplied as a part of the daily diet. There are 8 essential amino acids and 12 non-essential amino acids found in proteins.

Important Functions of Proteins

Approximately 15 to 20 percent of the human body is protein which exists in many forms. It is present in every cell as well as it is an essential component of enzymes, hormones and body secretions.


It is an important constituent of blood. In the blood, the protein is haemoglobin which transports oxygen from lungs to the tissues and brings carbon dioxide from the tissues back to the lungs. Proteins also exist in the blood in the form of antibodies which function as defense against diseases.

The proteins also maintain the fluid balance within the body. Protein is required for the growth of infants and children and also for the maintenance of tissues at all ages.

Energy value of proteins is 4 calories personality gram, the same as that of carbohydrates. Thus, if the diet does not have sufficient carbohydrates, tissue proteins gets metabolized to provide the needed energy.

Protein-Energy Malnutrition


The term Protein-Energy Malnutrition (PEM) is used to identify a complex group of related nutritional problems. Since energy intake is regarded by many authorities as more important problem in child-malnutrition, the term energy-protein malnutrition is also used to describe these conditions.

Children with Protein-Energy Malnutrition are always retarded in their growth and development. However, other clinical symptoms vary with the age and conditions of the child.

An acute deficiency of both calorie and protein results in nutritional Marasmus.Whereas a severe protein deficiency with adequate energy intake may result in Kwashiorkor. Both of these conditions are serious.

The Kwashiorkor is more serious disease because; it causes a quick death of child if deficiency continues. But if timely medical care is provided, the recovery is also faster.


Now we shall discuss some of the serious consequences of protein deficiencies:

Nutritional Marasmus:

Marasmus is more likely to develop in children under one year of age when breast feeding fails or is not carried on for a sufficient length of time and suitable foods for weaning are either not available, or feeding practices discourage their use. The declining tendency of breastfeeding among mothers in developing countries contributes greatly to the high proportion of deaths due to nutritional deficiency and diarrheal diseases.

Symptoms of Marasmus

(a) Absence of subcutaneous fat and muscle wasting.

(b) Diminishing of height or length.

(c) Child looks like a little old man with a big head and huge protruding eyes.

(d) Wrinkled face and tiny body.

(e) Dehydration, due to acute watery diarrhea.

(f) Subnormal temperature.

(g) Dull and dry hair.

(h) Continued feeling of hunger.

(i) Low pulse rate.