Amino acids are nitrogen-containing substances that are made up of carbon, hydrogen and oxygen. These four elements mutually bind in a variable number and manner to form the amino acid molecule. Amino acids are the “bricks” of the protein molecule. There are about one hundred amino acids in the human body, but only 20 of them are involved in protein synthesis.
These 20 "proteinogenic" amino acids differ both in terms of nutritional power and plasticity. 9 of the 20 amino acids cannot be synthesised by the body; hence the need for their dietary intake. They are called indispensable or essential. However, protein synthesis requires all the amino acids to be present; therefore, they are all essential to maintain biological functions.
Proteins are extraordinarily important and are essential for the normal performance of vital processes; indeed, they are the major component of tissue weight in our body.
Nutritional classification of amino acids
Indispensable amino acids (essential)
• Lysine, methionine, threonine
• Leucine, isoleucine, valine
• Phenylalanine, tryptophan, histidine*
Semi-essential amino acids (they spare their corresponding indispensable amino acids)
• Tyrosine (synthesised from phenylalanine)
• Cysteine (synthesised from methionine)
Non-essential amino acids
• Glycine**, alanine, serine, proline**
• Glutamic acid, glutamine, aspartic acid, asparagine
• Arginine*
A major request for protein supply occurs in case of:
• rapid growth
• intensive physical activity
• poor dietary intake of proteins
• increased energy demand
• elderly age
• loss of muscle mass
• anorexia
Moreover, the protein demand can be higher in vegetarians, especially in vegans, because by avoiding eggs, milk and cheese, they are at risk of not meeting the demand for essential amino acids.
(*essential only for children because they do not synthesise enough of them)
(**conditionally essential)