Essential Amino Acids and Where to Find Them

Your body uses 20 different amino acids to build its proteins. It can make eleven of them from other molecules, rearranging carbon skeletons and moving nitrogen around. The other nine it can’t make at all, or can’t make fast enough. These are the essential amino acids, and they must come from food. “Essential” doesn’t mean the others are unimportant; it means these nine are essential in the diet.

This list goes through all nine, explains what each does, and shows where to find them.

Why some amino acids are essential

Building an amino acid means making a particular carbon skeleton and attaching an amino group. Humans have lost, over evolutionary time, the enzymes needed to build several of these skeletons, especially those with branched chains, aromatic rings or particular side-chain groups. Plants and many microbes still have these enzymes, which is ultimately where all our essential amino acids come from, either directly from plants or via animals that ate them.

(For how amino acids are built and classified, see amino acids: structure and the 20 standard types.)

The nine essential amino acids

A popular memory aid uses the first letters: PVT TIM HALL (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Leucine, Lysine). Arginine is sometimes included in that phrase as the “A”, but for healthy adults it’s usually classed as conditionally essential; see below.

1. Histidine

• Roles: needed to make histamine (involved in immune responses and stomach acid secretion); its side chain can accept or release protons near body pH, so it’s often found in enzyme active sites and in haemoglobin, where it helps bind oxygen.
• Sources: meat, fish, poultry, dairy, soy, wholegrains.

2. Isoleucine

• Roles: a branched-chain amino acid (BCAA) used in muscle metabolism and energy production; helps make haemoglobin.
• Sources: meat, fish, eggs, dairy, lentils, nuts, seeds.

3. Leucine

• Roles: another BCAA; a particularly strong signal for switching on muscle protein synthesis.
• Sources: dairy (especially whey), meat, fish, soy, beans, lentils.

4. Lysine

• Roles: abundant in structural proteins; needed for collagen cross-linking, calcium absorption and making carnitine, which carries fatty acids into mitochondria.
• Sources: meat, fish, eggs, dairy, beans, lentils, quinoa. Lysine is often the limiting amino acid in cereal grains such as wheat, maize and rice.

5. Methionine

• Roles: the “start” amino acid for every new protein chain; provides sulfur for making cysteine; a key donor of methyl groups in metabolism.
• Sources: eggs, fish, meat, sesame and sunflower seeds, Brazil nuts, grains. Methionine is often the limiting amino acid in legumes (beans and lentils).

6. Phenylalanine

• Roles: converted into tyrosine, which is then used to make the neurotransmitters dopamine, noradrenaline and adrenaline, plus thyroid hormones and the pigment melanin.
• Sources: meat, fish, eggs, dairy, soy, nuts.
• Note: people with the inherited condition phenylketonuria (PKU) can’t convert phenylalanine properly and must follow a strict low-phenylalanine diet from birth. That’s why foods containing the sweetener aspartame (which is made from phenylalanine) carry a warning for people with PKU.

7. Threonine

• Roles: part of structural proteins such as collagen and elastin; important in the gut lining and immune function.
• Sources: meat, fish, dairy, eggs, lentils, cottage cheese.

8. Tryptophan

• Roles: used to make the neurotransmitter serotonin and the hormone melatonin, and can be converted into the vitamin niacin.
• Sources: poultry, dairy, eggs, oats, nuts, seeds, soy.
• Note: the idea that turkey makes you sleepy because of its tryptophan is mostly a myth; turkey contains no more tryptophan than many other meats. The drowsiness after a big meal has more to do with the size of the meal.

9. Valine

• Roles: the third BCAA; used in muscle metabolism, tissue repair and energy.
• Sources: dairy, meat, soy, beans, mushrooms, peanuts, wholegrains.

Conditionally essential amino acids

Some amino acids are normally made in adequate amounts but can become essential when the body is under stress, growing fast or ill:

• Arginine: important for growth; may be needed in the diet by infants and during recovery from injury.
• Cysteine and tyrosine: made from methionine and phenylalanine respectively, so they depend on those essential amino acids being available. Premature babies may need them directly.
• Glutamine: used heavily by the gut and immune cells; demand rises during serious illness or burns.
• Glycine and proline: needed in large amounts for collagen, especially during growth.

Complete and incomplete proteins

A food protein is often called complete if it contains all nine essential amino acids in adequate proportions:

• Complete: meat, fish, eggs, dairy, soy, quinoa, buckwheat.
• Incomplete (low in one or more): most grains (low in lysine) and legumes (low in methionine), nuts and seeds.

Protein combining solves this. Grains and legumes complement each other: grains supply methionine, legumes supply lysine. Classic combinations include rice and beans, hummus with pitta bread, and lentil dal with rice or chapati. Nutritionists now know that the combination doesn’t need to happen in the same meal; eating a variety of plant proteins over the day is enough.

How much protein do adults need?

A widely used reference intake for healthy adults is about 0.75 to 0.8 grams of protein per kilogram of body weight per day. For a 70 kg adult, that’s about 55 grams a day. Needs are higher during pregnancy and breastfeeding, in childhood and adolescence, during recovery from illness, and possibly for older adults and people doing heavy training.

Most people in high-income countries eat more than enough protein. What matters more for those on plant-based diets is variety, so that all essential amino acids are covered.

What happens to extra amino acids?

The body can’t store amino acids the way it stores fat or glycogen. Surplus amino acids have their amino group removed (deamination), and the nitrogen is converted to urea in the liver and excreted in urine. The remaining carbon skeleton is used for energy or converted to glucose or fat. Protein provides about 17 kJ per gram when used as fuel.

Measuring protein quality

Nutrition scientists rate food proteins by how well they meet the body’s needs for essential amino acids and how well they’re digested. The older PDCAAS score and the newer DIAAS score both compare a food’s essential amino acid content with a reference pattern, then correct for digestibility. By these measures, eggs, milk and whey rate highly, soy close behind, and wheat much lower because of its limited lysine. Scores for a whole diet matter more than for single foods, though, which is why a mix of plant proteins can meet requirements perfectly well. Cooking and processing can also affect quality: heat can make some proteins more digestible, while very high temperatures can damage lysine.

Key takeaways

• The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
• They must come from food, because the body can’t make them.
• Animal proteins and soy supply all nine; grains tend to be low in lysine and legumes in methionine, so combining them works well.
• Some amino acids are conditionally essential during illness, growth or stress.
• Excess amino acids are deaminated, with nitrogen excreted as urea. Read more about proteins in proteins: chains of amino acids.