There are 23 amino acids. Eight of them are strictly dietary essentials and cannot be synthesized at all by the body. These are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. In addition to these strictly essential amino acids, two other amino acids, namely, arginine and histidine, are essential for infants and young persons, as the capacity for their synthesis in the body is not adequate to meet the needs for growth.

In addition to the ten essential amino acids, there are thirteen non-essential amino acids. They are proline, carnitine, tyrosine, glutamic acid and glutamine, cysteine and cystine, glycine, alanine, b-alanine, aspartic acid, taurine, ornithine, citrulline, and gama-aminobutyric acid (GAMA). Under special conditions, the demand for some of the non-essential amino acids may be greater than the synthetic capacity, and therefore, they too can become dietary essentials.

About 75% of the amino acids entering the bloodstream through the liver from the gastro-intestinal tract after a meal, are metabolized in the liver, and only about 25% enter the general circulation. Out of the intake of amino acids in excess of immediate requirements, upto 50% are catabolized for energy-yielding metabolism, and non-essential amino acids that have not been ingested in adequate amounts are synthesized for transport to other tissues. The nitrogen from this amino acid catabolism is not excreted immediately, but a considerable amount of urea is recirculated through the gastro-intestinal tract and reabsorbed as ammonia.