Amino Acid Requirements and Essentiality

Amino acids are the building blocks of protein, and only some of them can be synthesised by the human body. Those that cannot, the indispensable or essential amino acids, must be supplied by the diet, which is why amino acid requirements, not protein alone, ultimately define the nutritional adequacy of a diet.

Definition

An essential (indispensable) amino acid is one whose carbon skeleton cannot be synthesised by the human body and which must therefore be obtained from the diet; amino acid requirements are the dietary intakes of these indispensable amino acids needed to support protein synthesis and metabolic function in a healthy individual.

Scope

This topic covers the classification of amino acids as indispensable (essential), dispensable (non-essential), and conditionally indispensable, the basis and expression of individual amino acid requirements, and how amino acid patterns underlie protein-quality evaluation. It is reference-educational and does not provide individualised intake advice or supplementation guidance.

Core questions

• Which amino acids are essential, and why can the body not make them?
• What distinguishes essential, non-essential, and conditionally essential amino acids?
• How are individual amino acid requirements determined and expressed?
• How do amino acid requirement patterns underlie protein-quality scoring?
• When can a normally non-essential amino acid become conditionally essential?

Key concepts

• Indispensable (essential) amino acids
• Dispensable (non-essential) amino acids
• Conditionally indispensable amino acids
• Amino acid requirement pattern (scoring pattern)
• Limiting amino acid
• Indicator amino acid oxidation method

Mechanisms

The human body can synthesise some amino acids from other substrates but lacks the pathways to make the carbon skeletons of the indispensable amino acids (such as lysine, methionine, threonine, tryptophan, and the branched-chain amino acids), which must come from dietary protein. Dispensable amino acids can be produced endogenously, while conditionally indispensable amino acids become dietary requirements when synthesis is limited by physiological state or precursor supply (wu-2009). Requirements for individual indispensable amino acids are estimated by methods including nitrogen balance and amino acid oxidation studies, then expressed as a requirement pattern that also serves as the reference for protein-quality scores (faowhounu-2007, iom-2005). The amino acid in shortest supply relative to this pattern is the limiting amino acid for a given protein.

Clinical relevance

Amino acid requirement patterns are the reference against which dietary protein quality is judged and are used in evaluating mixed and plant-based diets and in nutrition education about complementary proteins (young-pellett-1994, faowhounu-2007). This entry is reference-educational, describing how essentiality and requirements are defined and measured; it does not recommend specific amino acid intakes or supplements for individuals.

History

The classification of amino acids as essential or non-essential was established through mid-twentieth-century human and animal feeding studies, notably the work of William Cumming Rose defining the indispensable amino acids and their requirements. Requirement estimates were later refined by stable-isotope and amino acid oxidation methods and consolidated by expert consultations, which set the reference patterns used in protein-quality evaluation (faowhounu-2007, iom-2005, wu-2009).

Debates

How precisely are individual amino acid requirements known?

Estimates derived from nitrogen balance and from amino acid oxidation methods have at times differed, and reference patterns have been revised as newer tracer methods refined requirement values, so the exact figures for some indispensable amino acids remain subject to ongoing evaluation.

Key figures

• William Cumming Rose
• Vernon Young
• Guoyao Wu
• Peter Pellett

Seminal works

• faowhounu-2007
• wu-2009
• young-pellett-1994

Frequently asked questions

What makes an amino acid essential?

An amino acid is essential, or indispensable, when the human body cannot synthesise its carbon skeleton at a rate sufficient to meet needs, so it must be supplied by dietary protein; the remaining amino acids are non-essential because the body can make them.

What is a conditionally essential amino acid?

A conditionally essential amino acid is one that is normally non-essential but becomes a dietary requirement under particular circumstances, such as immaturity, illness, or limited precursor availability, when the body's own synthesis cannot keep pace with demand.