Essential Amino Acids

 Essential Amino Acids

An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life forms, the nine amino acids humans cannot synthesize.

Six other amino acids are considered conditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe distress. These six are arginine, cysteine, glycine, glutamine, proline and tyrosine.

Six amino acids are non-essential (dispensable) in humans, meaning they can be synthesized in sufficient quantities in the body (considered the 21st amino acid).  The limiting amino acid is the essential amino acid found in the smallest quantity in the foodstuff.

All the amino acids are required by the body but some of which are termed indispensable in the diet as these cannot be synthesized by the animal organism and others are termed dispensable in the diet as these amino acids can be synthesized in the body. Absence of the former class of amino acids in the diet results disturbances in the nitrogen equilibrium, growth, maintenance, nutrition and life span, hence this class of amino acids is otherwise termed as nutritionally essential amino acids.

Varieties

Following amino acids have been found to be essential

·        Phenylalanine.

·        Tryptophan.

·        Methionine.

·        Threonine.

·        Valine.

·        Isoleucine.

·        Lysine.

·        Leucine and

·        Histidine.

Some are semi-indispensable and they are:

·        Arginine.

·        Cysteine.

·        Tyrosine.

·        Glycine.

These six are non essential amino acids-

·        alanine 

·        aspartic acid 

·        asparagine 

·        glutamic acid 

·        serine 

·        selenocysteine 

Apart from its other functions, phenylalanine gives rise to tyrosine in the body, which acts as the precursor for melanin, thyroxin and epinephrine (adrenaline). The first six amino acids are essential for the maintenance of nutrition, growth and life. Lysine and leucine are also essential for growth.

Arginine helps in growth also, but is synthesized in the body. But the rate of synthesis cannot keep up pace with the requirement and hence it has to be supplied exogenously to supplement that which is already synthesized quantitatively. Hence, it is known as semi-indispensable