Plants are the primary source of protein because they can synthesize protein by combining nitrogen and water from the soil and carbon dioxide from the air. Animals depend on plants to fulfill their protein requirement.
Most food stuffs contain protein but in widely varying amounts. Animal foods such as meat, fish and eggs are rich in protein and milk can also be considered to be rich in protein if due allowance is made for the large amount of water present in it. Among the vegetable foods, pulses and nuts are the richest sources of protein, with amounts often exceeding those present in animal foods. Soya bean is unique in this respect in that it contains over 40% protein. The common cereals such as rice and wheat are relatively poor sources of proteins. But considering the amount that is consumed daily, their contribution to the total protein intake by an individual is considerable. Leafy and root vegetables and fruits are very poor sources of protein.
Protein deficiency is one of the most common nutritional disorders not only in India but in the world at large. The reason being poverty, ignorance, lack of incentive to eat, chronic illness which reduced appetite and disorders of absorption during period of increased requirement.
Protein deficiency affects infants and adults in different ways. The infant looses weight, shows stunted growth and may succumb to infections. If there is severe deficiency of protein in the first two years of life, mental development, learning ability and behavior could be affected. In India and other developing tropical countries a very widely prevalent nutritional disorder called as protein-calorie malnutrition (PCM) occurs during childhood usually between ages one and four years (shortly after weaning). During this period, the child is not supplied with sufficient quantity of good quality protein, which results in deficiency diseases.
When an infant is fed with high carbohydrate and low protein foods, such as (Corn starch porridge) and tapioca preparation during weaning Kwashiorkor, (meaning the displaced child) occurs. This condition is characterized by growth failure, edema dry lusterless skin showing cracks and pigmentation, changes in hair colour and enlargement of the liver. When both protein and calories are deficient in the diet, nutritional Marasmus could occur. This condition is characterized by severe growth failure than Kwashiorkar but edema is usually absent.
In adults, deficiency of protein in diet causes (i) Loss of weight (2) Anaemia (3) Reduced resistance to infections (4) Oedema of the feet and accumulation of free fluid in the abdomen (Ascites).
The amount of protein needed varies according to age, physiological state, sex and activity. Growing children need a large amount in proportion to their weight than adults, because of growth. Infants need minimum of 3 grams of protein per kilogram of their body weight. The need increases till the age of 15 for girls, and 20 for boys, where girls need 70 grams of protein and boys 80 grams daily. Adults need about 60 to 70 grams of protein per day (1 gram per kilogram of body weight). This requirement is increased in times of physiological stress such as pregnancy, lactation, or convalescence.