Amino Acids Explained: The Building Blocks of Protein

Amino acids are the small molecules that link together to form peptides and proteins. The human body uses twenty standard amino acids, and every protein in the body, from muscle fibres to enzymes, is built from different combinations of them.

Essential and non-essential

• Essential amino acids. Nine cannot be made by the body and have to come from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
• Non-essential amino acids. The body can make these itself from other nutrients.
• Conditionally essential amino acids. A few are normally made in sufficient amounts but can run short during rapid growth, illness or injury.

What they do beyond building protein

• Signals. Several of the brain's chemical messengers are made directly from amino acids.
• Hormones. Thyroid hormones and adrenaline are both made from the amino acid tyrosine.
• Energy. When needed, amino acids can be converted into glucose or burned directly as fuel.
• Triggers. Leucine acts as a signal that tells muscle cells to start building new protein.

Complete and incomplete proteins

A food is called a complete protein when it supplies all nine essential amino acids in useful amounts. Meat, fish, eggs, dairy and soy are complete. Most other plant foods are low in one or two essential amino acids, but eating a variety across the day covers the gaps.

Constant turnover

The body does not keep a large reserve of amino acids. Proteins are continually broken down and rebuilt, and the amino acids released are recycled alongside those arriving from food. This is why a regular supply matters more than an occasional large amount.