Complete protein: the 9 essential amino acids, one by one
A complete protein contains all nine essential amino acids in the amounts you need and is well digested. Your body can't make these nine on its own, so they have to come from food. Here you'll meet the nine essential amino acids one by one and see why your body needs protein every day.
In short
- The proteins in food and in your body are built from 20 amino acids. Nine of them the body can't produce in sufficient amounts, so it has to get them from food.
- A complete protein contains all nine in the amounts needed and is well digested.
- If one of the nine runs short, it limits the building of new proteins, and the body breaks down the leftover amino acids.
- The body has no store for protein the way it has fat tissue. Its free amino acids add up to only about 100 g.
- Grains are low in lysine, legumes are low in methionine. Together, in a varied diet, they fill each other's gaps.
20 building blocks, 9 of them from food
Every protein is a chain of amino acids lined up in an exact order. The proteins in living things are assembled from 20 kinds of amino acids. The body can make most of them out of other substances.
Nine, however, it can't produce fast enough to cover its needs. According to EFSA, they are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. They're called essential, and you have to get them from food.
The story starts with dogs. In 1912, the scientist Abderhalden noticed that without tryptophan in their food, the animals lost nitrogen, in other words protein, while without proline they didn't. From 1924 on, Rose and his colleagues ran a series of experiments with rats and people that pinned down which amino acids are essential.
There's also a middle group. Arginine, cysteine, glutamine, glycine, proline and tyrosine are usually made by the body itself. In some special situations, for example in premature babies, the need goes beyond what the body manages to produce. That's why EFSA calls them conditionally essential.
What “complete” means
Say you're spelling the word MOUSSAKA with fridge magnets. You have ten each of M, O, U, S and K, but only one A. However many other letters you have, you can't spell a single MOUSSAKA, because each one needs two A's.
Amino acids work the same way. If one essential amino acid runs short, it limits how all the others can be used for new proteins. The body breaks down the leftover amino acids.
A complete protein is one in which all nine essential amino acids are present in amounts close to our needs and are well digested. The FAO measures this with the DIAAS score. A score of 100 and up means excellent quality, 75 to 99 means good. How eggs, milk, meat, soy, beans and bread score is covered in our article on animal and plant proteins.
One caveat
The FAO makes another important point: quality doesn't replace quantity. An excellent protein in a small amount is still a small amount of protein.
The nine, one by one
Besides being building blocks for proteins, most essential amino acids have a second job in the body. Here's a quick look at what each one is known for.
- Histidine. In hemoglobin and myoglobin, it helps hold the iron in place. The body makes histamine from it, and in the muscles, the dipeptide carnosine.
- Isoleucine. One of the three branched-chain amino acids. Most amino acids start breaking down in the liver, while the three branched-chain ones start in the muscles.
- Leucine. The best known of the nine. Scientists study it as a signal that starts the building of new proteins. It's the one we need most of: 39 mg per kilogram of body weight a day.
- Lysine. Once the body starts breaking it down, there's no going back. That's why EFSA calls lysine and threonine strictly essential. Grains are low in lysine.
- Methionine. The cell uses it to start assembling a new protein. Legumes are low in methionine.
- Phenylalanine. The body turns it into tyrosine. From tyrosine it makes hormones, signaling substances in the brain and the pigment melanin, which colors hair and skin.
- Threonine. The gut holds on to 50 to 60% of the threonine from food, mainly to make mucins. That's the mucus that lines the gut walls.
- Tryptophan. The body has less tryptophan than any other amino acid. It's the only raw material for serotonin. The body also makes niacin (vitamin B3) from it: 1 mg of niacin takes about 60 mg of tryptophan.
- Valine. The third branched-chain amino acid. Like isoleucine and leucine, it starts breaking down in the muscles.
How much of each do you need? Here are the average adult requirements from the 2007 report by the WHO, the FAO and the United Nations University, as cited by EFSA. In the last column, we've worked them out for someone who weighs 65 kg.
| Amino acid | mg per kg per day | Per day at 65 kg |
|---|---|---|
| Histidine | 10 | 650 mg |
| Isoleucine | 20 | 1 300 mg |
| Leucine | 39 | 2 535 mg |
| Lysine | 30 | 1 950 mg |
| Methionine + cysteine | 15 | 975 mg |
| Phenylalanine + tyrosine | 25 | 1 625 mg |
| Threonine | 15 | 975 mg |
| Tryptophan | 4 | 260 mg |
| Valine | 26 | 1 690 mg |
Don't let the table scare you. In food, amino acids come built into protein, so EFSA doubts that separate targets for each one are useful and considers the numbers themselves still imprecise. If you eat enough protein from different foods, the nine add up on their own. For how much protein is enough, see our article on how much protein you need a day.
Your body has no protein store
The body stockpiles fat in its fat tissue. There's no such store for protein: almost all the protein in the body is built into something that's working.
Half of this protein comes from just four kinds: the main protein of skin, tendons and bones, the two muscle proteins myosin and actin, and hemoglobin in the blood. The free amino acids floating around the body add up to only about 100 g.
And everything keeps moving. Every day, the body of a 70 kg man breaks down and rebuilds 250 to 300 g of protein, two to three times more than we usually eat. Some of the amino acids are lost for good, mostly by being burned. Protein from food covers these losses. And when we eat more protein, the body simply burns more.
That's why protein is an everyday matter, and it's even easier when you spread it across your meals. How to do that is in our article on spreading your protein through the day. And the claim authorized by the EU reads: protein contributes to the maintenance of muscle mass.
Where to get all nine
All fresh foods, plant and animal, contain essential amino acids. The difference is in the amount. Plant foods usually have less per gram of dry matter, especially of lysine, methionine, threonine and tryptophan. A good mix of animal and plant foods covers your needs.
Grains are low in lysine, and legumes are low in methionine and cysteine. That's why the two groups complement each other. Bulgarian cooking does this out of habit: bob chorba (bean soup) with a slice of bread, lentils with rice.
Myth
“Beans and bread have to be in the same bite, or the protein isn't complete.” A 2013 review in the Medical Journal of Australia says strict combining isn't necessary if you eat enough varied plant foods every day. The body also keeps a small reserve of free amino acids that it uses to top up the protein from food.
Milk and meat are rich sources of essential amino acids. The FAO notes that a protein scoring above 100 can make up for a weaker one. So a little cheese on your beans or an egg next to your slice of bread raises the quality of the whole plate. If you want to add complete protein to pasta, soup or beans without cooking another dish, MEALUP is one option: egg white and milk protein concentrate with all 9 essential amino acids. You sprinkle it over the finished food.
Frequently asked questions
Do I need to count amino acids?
If you eat enough protein from different foods, you get all nine. Amino acids come built into protein, and EFSA doubts that separate targets for each one are useful.
Do vegetarians get all nine?
Yes, if they eat enough and a good variety. The review in the Medical Journal of Australia shows that vegetarians in Australia meet the protein recommendations and that varied plant foods complement each other. If you're pregnant or on a special diet, ask your doctor.
Who needs to be careful with phenylalanine?
People with phenylketonuria. It's a rare inherited condition in which the body can't turn phenylalanine into tyrosine, so it builds up. They follow a special diet under medical supervision.
What is a “conditionally essential” amino acid?
One the body usually makes itself but, in some situations, for example in premature babies, can't make enough of. These are arginine, cysteine, glutamine, glycine, proline and tyrosine.
Sources
- EFSA Panel on Dietetic Products, Nutrition and Allergies, “Scientific Opinion on Dietary Reference Values for protein”, draft for public consultation 2011, final text EFSA Journal, 2012
- Hou Y, Wu G, “Nutritionally Essential Amino Acids”, Advances in Nutrition, 2018
- FAO, “Dietary protein quality evaluation in human nutrition”, FAO Food and Nutrition Paper 92, 2013
- Holeček M, “Histidine in Health and Disease…”, Nutrients, 2020
- Holeček M, “Branched-chain amino acids in health and disease…”, Nutr Metab (Lond), 2018
- Lauinger L, Kaiser P, “Sensing and Signaling of Methionine…”, Metabolites, 2021
- Richard DM et al., “L-Tryptophan: Basic Metabolic Functions, Behavioral Research and Therapeutic Indications”, International Journal of Tryptophan Research, 2009
- MedlinePlus Genetics, U.S. National Library of Medicine, “PAH gene”
- Marsh KA, Munn EA, Baines SK, “Protein and vegetarian diets”, Medical Journal of Australia, 2013
