Proteins are one of the most important food constituent for the following reasons:

  1. In the growing animal an abundant supply of protein is required to provide the building stones (amino acids) out of which body proteins are constructed in the process of growth.
  2. In the diet of an adult, protein is needed to maintain the proper level of body proteins, to make good tissue loss by wear and tear and to build up new tissue protein after a wasting illness.
  3. The proteins also supply amino acids essential for the synthesis of enzymes, proteinous hormones, etc and also essential non-protein nitrogenous substances such as purine, pyrimidines, coenzymes, choline hormones, etc

   Essential and Non-essential Amino Acids

As amino acids are the building blocks of Protein, the nutritional value of any particular protein depends to a large extent upon the nature of its constituents amino acid and their relative concentration. Some of the amino acids can be readily synthesized within the body and their presence in the diet is not essential. These are called non-essential or dispensable amino acids.

Other amino acids cannot be synthesized in the body or can be synthesized in only small amounts and must be present in the diet ingested by man, these are called indispensable or essential amino acids. These includes ten amino acids which are valine, lysine, leucine, isoleucine, tryptophan, methionine, phenylalanine, threonine, arginine and histidine.

The fact that some amino acids are essential was shown by experiment which involved observing the effects of their removal from the diet of growing animals. For example young rat were fed on an otherwise adequate diet but which contained only one protein namely zein (derived from maize) which is deficient in both lysine and tryptophan. On this diet, the rats not only stopped growing but also lost weight. Tryptophan was then added to this diet and now the fall in weight is stopped but the growth still did not start. When finally lysine was also supplied then the normal growth started.

In more recent studies, instead of a whole protein, different mixtures of known amino acids have been used in feeding experiments which have shown that only 10 amino acids whose names have been given above are essential and the rest are non essential.

Because the adult animal has reached peak of body growth, therefore increases in body weight cannot serve as a useful criterion for the presence of all essential amino acids in food. But it has been shown that when certain of amino acids are absent in an otherwise adequate diet, the output of N in the urine and feces exceed its intake. This negative N balance can be changed to a positive one when the missing amino acid is added to the diet. The maintenance of body weight may also be used as an index of a positive or negative N balance. Thus it was shown by an experiment that the absence of lysine in the diets of adult rat is followed by a steady fall in their body weights.

To maintain a normal  N equilibrium the requirements of essential amino acids are not the same at different ages. For example, infant need 9 of the 10 essential amino acids (do not need arginine) while adults need only 8 (do not need arginine and histidine). However argninine lack results in a sever oligospermia. Arginine and histidine are therefore also called semi-essential amino acids. Some authorities include cystine and tyrosine also amongst semi-essential amino acid because these amino acids can spare (although cannot completely replace) methionine and phenylalanine respectilvey.

   Biological Value of a Protein

It is also called the net protein utilization and is defined as
Dietary N retained/Dietary N absorbed x 100
Higher the biological value of protein, higher will be its nutritive value. The biological values of the proteins of some common foodstuffs are given below:

Fish        -                              88%
Milk       -                              84%
Egg Albumin -                    82%
Liver      -                              75%
Meat     -                              74%
Soyabean  -                        72%
Wheat  -                               69%
Oats  -                                   66%
Maize  -                                59%
Peas  -                                   46%

The above mentioned data shows that the food derived from animal sources are better in the quality of proteins than plant foods.