Protein Quality, Sources and Dificiency

Protein quality: It is the digestibility and quantity of essential amino acids for providing the protein in correct ratio for human consumption. There are various methods that rank quality of different types of protein. Some of the popular methods are Protein digestibility corrected amino acid score (PDCAAS) and Digestible indispensable amino acid score (DIAAS).

Complete protein: Complete protein is a source of protein that contain adequate proportion of all essential amino acids necessary for dietary need of human. Meat, fish, poultry, egg, milk, cheese, yoghurt are source of complete protein.

How a typical Nepali meal rice and lentil give a complete protein?Cereals like rice are deficient in lysine, threonine. In other hand, pulses are high in lysine and low in sulfur containing amino acid. Thus deficient is balanced while we eat these food together.

Supplementary value of protein: one way of improving quality of dietary protein with limiting amino acid is by adding another protein containing missing amino acid. Some protein are poor quality protein as such due to limiting amino acid but they are complementary in limiting essential amino acid composition i.e.  a limiting essential amino acid in one protein is present in excess amount in another protein and vice versa. So they supplement each other and make good quality protein in diet. This is known as supplementary value of protein. For example; rice and lentil are complementary protein as such, being deficit in certain amino acid but when they are consumed together, they make good quality protein in diet by supplementing one another.

Good source of protein: Good source of protein includes most of the animal sources of protein such as milk and milk products, meat, fish and poultry. Plant source such as lentils, beans and pulses are also good source of protein accounting to give the highest protein content (Soybean 43%).

Poor source of protein: Cereals, fruits and vegetables are most often classified as poor source of protein.

Recommended Dietary Allowances (RDA): Generally, RDA of protein is 1g/Kg body weight. This means, if a person is 50 Kg, he/she should intake 50 g of protein in a day in his/her diet. It is generally regarded as 10% of his /her total daily calorie for adult. RDA for protein is a modest 0.8 g of protein per kilogram of body weight for adult and adolescent (mainly teenagers 13-19 years) requires relatively more amount as their body is developing.

Effect of protein deficiency: Without protein, human body cannot work normally. Proteins are made up of amino acids and deficient of even single essential amino acids would cause disorder in body. When the body lacks protein, our body breaks down protein with rich tissue (muscle) to excess them. Therefore, initial effect of low protein intake can be muscle wasting accompanied by increasing weakness, low immunity and thus vulnerable to diseases easily.

Kwashiorkor is a severe form of malnutrition caused by deficiency of dietary protein. The extreme lack of protein causes swelling of gut diagnosed as an edema or retention of water. Marasmus is another severe form of protein energy deficiency observed usually in children of famine region.

Effect of heat on protein:

When heat is applied in protein, protein denatures (protein break down its original structure). Applied heat generally break the connecting hydrogen bonds between amino acids, thus changing its physical properties and characteristics irreversibly. Example includes boiling or frying of egg, fried blood, and gelatin. Denaturation destroys biochemical properties and functions of protein but the nutritional value of protein is not lost.

Heat does not destroy the protein. However, overcooking food containing protein can destroy heat sensitive amino acid (for example lysine) or make the protein more resistant to digestive enzymes.