Proteinogenic amino acids, also known as standard, normal, or primary amino acids, are those 20 amino acids that are found in proteins
An essential amino acid
or indispensable amino acid is an amino acid that cannot be synthesized de novo by the organism, and therefore must be supplied in the diet.
Recommended daily amounts:
Estimating the daily requirement for the indispensable amino acids has proven to be difficult; these numbers have undergone considerable revision over the last 20 years. It was known for years that infants were unable to synthesize histidine, but only relatively recently has it been determined that adults are also unable to do this. The following table lists the WHO recommended daily amounts currently in use for essential amino acids in adult humans, together with their standard one-letter abbreviations.
Use of essential amino acids
Foods that lack essential amino acids are poor sources of protein equivalents, as the body tends to deaminate the amino acids obtained, converting proteins into fats and carbohydrates. Therefore, a balance of essential amino acids is necessary for a high degree of protein utilization.
Complete proteins contain a balanced set of essential amino acids for humans. Proteins found in plant sources such as quinoa, buckwheat, hemp seed and amaranth, are considered complete. Soybeans are considered by some to be a source of complete protein although this is contested.
The limiting amino acid
The net protein utilization is profoundly affected by the limiting amino acid content (the essential amino acid found in the smallest quantity in the food), and somewhat affected by salvage of essential amino acids in the body. It is therefore a good idea to mix foodstuffs that have different weaknesses in their essential amino acid distributions.
This limits the loss of nitrogen through deamination and increases overall net protein utilization. For vegans eating cereals and legumes in the same day will provide enough high quality protein.