Nutrition Facts

The sources of protein can be divided into two broad categories animal source and plant or vegetable source.
Both of the sources are important source for supply of protein in our diet, depending on the cultural practice, religious belief, availability and locality.

Animal sources:

The important sources of animal protein include eggs, meat, fish, cheese and milk. The animal proteins contain all the essential amino acids and the quantity of these essential amino acids is adequate in animal protein. Essential amino acids have to be supplied in our diet as they can not be synthesized by our body.

The egg proteins are considered best among all the food proteins as egg protein has highest “biological value” and digestibility. For this quality the egg protein is used as “reference protein”.

Vegetable sources:

The rich sources of vegetable protein are pulses (lentils etc), cereals (rice, maize, wheat, millets etc), nuts, beans etc. In developing countries the vegetable proteins are the main source of protein in the diet. The vegetable source proteins are poor in essential amino acids. They are deficient in one or more essential amino acids. These are known as “limiting” amino acids. But combination of two or more vegetable protein compensates the deficiency of the essential amino acids. For example cereal protein (rice, wheat etc.) is deficient in lysine and threonine and pulse protein is deficient in metheonine. Here lysine, threonine and metheonine are the “limiting” amino acids. But when these two sources (here rice/wheat and pulse) are combined in the diet they compensates each other and make the whole of the protein “biologically complete”.

Proteins are made up of smaller units known as amino acids. The protein molecules are made up by joining many amino acids and forming a chain or tree like structure. Many amino acids are there which are required by human body. The total number of amino acids required by human body is 24. And out of the 24 amino acids 9 of them are known as essential amino acids as they are required by our body but our body can not synthesize them and if they are synthesized also the quantity is not sufficient for our need. That is why the essential amino acids have to be supplied in our diet.
The essential amino acids are leucine, isoleucine, lysine, methionine, phenylalanine, threonine, valine, histidine and tryptophan. For synthesis of protein for our body tissues both essential and non essential amino acids are required. The essential amino acids must be supplied in our diet, but the non essential amino acids can be synthesized by our body if the required building blocks are present.
Functions of essential amino acids:

1. Essential amino acids are required for the synthesis of body proteins that include important proteins like enzymes, antibodies, immunoglobulins or Ig (IgG, IgM, IgE etc.), hemoglobin etc.
2. Essential amino acid tryptophan is required for the formation of niacin which is a B complex vitamin. Niacin is synthesized from tryptophan. Niacin deficiency causes pellagra.
3. Essential amino acid methionine acts as a donor of methyl groups for synthesis of choline, folic acid (a vitamin) and nucleic acids.
4. New tissue can not be formed if all the essential amino acids are present in our diet.
5. Essential amino acids cystine and tyrosine are essential for premature babies.

A protein is “biologically complete” if all the essential amino acids are present in that protein in amounts corresponding to our needs. If one or more essential amino acid is absent or lacking in amounts corresponding to our needs, the protein is said to be “biologically incomplete”. The quality of protein or biologically complete or incompleteness is closely related to pattern of the amino acids. In general, animal proteins are of better quality than vegetable proteins because they are biologically complete. Egg proteins and milk proteins have amino acid pattern which is most suitable for human needs. Vegetable proteins lack in one or more amino acids, so they are biologically incomplete.
But if two or more vegetable proteins are combined they compensate the deficiency of each others (deficiency of essential amino acids) and become more biologically complete.

The word protein is derived from – and the meaning of protein is “of first importance”. Proteins are indeed the most important in human nutrition. Proteins are complex organic substances containing nitrogen in its molecule. Proteins differ from carbohydrates and fats form the fact that they contain nitrogen. The nitrogen content in protein is approximately 16%. Proteins contain carbon, hydrogen, oxygen, nitrogen and sulphur in its molecule in varying amount. Some of the proteins contain phosphorus, iron (hemoglobin is classical example of protein containing iron) and sometimes other elements. Proteins are very large molecules with very high molecular weight (largest organic molecule) and made up of many smaller units called amino acids. About 20% of adult body weight is made up of protein.
Functions of Proteins:
Proteins are required for the following major functions:

1. Proteins are required for growth and building of body and proteins are body building materials. This is very important for the growing children and infants. But this is very small component of function of protein in compare to the maintenance proteins do everyday continuously.
2. Repair and maintenance of body tissue is the most important and extensive function of proteins. Proteins are required for healing of wounds, any damage of tissue. Any ware and tear is repaired by proteins which is a constant process.
3. Maintenance of plasma oncotic pressure is also function of proteins. Without normal plasma oncotic pressure there will be no perfusion and diffusion of chemicals.
4. Proteins can supply energy of 4 kcal of energy per gram if there is inadequate supply of calorie, but supply of energy (calorie) is not primary function of protein.
5. Synthesis of important substances like antibodies, immunoglobulin, hemoglobin, hormones, enzymes, plasma proteins, coagulation factors and many more. All of these are proteins. They perform very important functions. Antibodies and immunoglobulins fight with bacteria and other foreign invaders and protect body from diseases. Hemoglobin carries oxygen and supply to tissues which is vital for survival. Hormones are very important for normal functioning of humans. Enzymes are required for all the chemical reactions that take place in the body. Plasma proteins maintain oncotic pressure and also supply proteins where it is required. Coagulation factors are required for control of blood loss from injury or cut surface.

Other than the above mentioned functions, proteins also have some minor functions. The bactericidal (killing of bacteria) action of white blood cells is hampered if there is protein deficiency. Cell mediated immunity is also hampered in protein deficiency states. Both of the above conditions are seen in protein energy malnutrition (kwasiorkar and marasmas).