Generalities and distribution in nature ô Biological value of Proteins ô Complementary Proteins

Generalities and distribution in nature

Proteins are fundamental constituents of living organisms and play a vital role in the structure and functions of living matter

They represent more than 50% of organic components and around 14-18% ( depending on age ) of total body weight.

They are composed of four chemical elements: C, H, O, N, which constitute amino acids, the building blocks of proteins.

Amino acids are quite numerous, however only 23 are frequently found in the more common proteins consumed as foods. Thus, they are classified as the "ordinary amino acids" while others are instead called the "occasional amino acids" since they are rarely evident (formed) 

Biological value of Proteins

The biological value of  a protein is based on the quantity of nitrogen contained within it, which is stored by the body for cell growth, tissue repair  and for the maintenance of vital functions. It is not excreted with faeces, urine and through the skin.

A protein that possesses a perfect balance of amino acids absorbed by 100% and stored for life activities of the organism has a bio-value of 100.  The protein of an egg has had this highest value conferred on it and is therefore considered a standard protein of reference. 

Complementary Proteins

The incomplete proteins existing in nature either lack one or more of the essential amino acids or may contain some in low quantities.  In other words, there are proteins which have either an "absolute" completeness of essential amino acids or a "partial" deficiency thereof.  The proteins derived from cereals, for example, have a partial deficiency of certain amino acids like lysine and tryptophan.  Therefore, by integrating the cereal proteins with those of milk, in appropriate quantities, a protein complex which is well balanced in essential amino acids is obtained.