Essential amino acids are the nine amino acids your body cannot make on its own. You have to get them from food. Your body uses 20 amino acids total to build proteins, but it can manufacture 11 of them internally. The remaining nine must come from your diet, which is why they’re called “essential.”
The Nine Essential Amino Acids
The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Each plays distinct roles in the body, from building muscle to producing brain chemicals.
A few stand out for the breadth of what they do. Leucine, isoleucine, and valine are the three branched-chain amino acids (BCAAs), all heavily involved in muscle growth and energy production. Phenylalanine is a building block for dopamine and other chemical messengers in the brain. Tryptophan is a precursor to serotonin, which regulates mood and sleep. Histidine helps produce histamine (the compound behind allergic reactions, but also a neurotransmitter) and plays a role in wound healing and blood sugar regulation. Lysine supports hormone production, calcium absorption, and immune function. Methionine aids tissue growth, detoxification, and the absorption of minerals like zinc and selenium. Threonine contributes to the structural proteins in your skin and connective tissue.
How Much You Need Each Day
The World Health Organization sets recommended intakes for each essential amino acid in milligrams per kilogram of body weight per day. For an adult weighing about 70 kg (154 pounds), the daily targets look like this:
- Leucine: 39 mg/kg (about 2.7 g total)
- Lysine: 30 mg/kg (about 2.1 g)
- Valine: 26 mg/kg (about 1.8 g)
- Phenylalanine + tyrosine: 25 mg/kg (about 1.75 g)
- Isoleucine: 20 mg/kg (about 1.4 g)
- Threonine: 15 mg/kg (about 1.05 g)
- Methionine + cysteine: 15 mg/kg (about 1.05 g)
- Histidine: 10 mg/kg (about 700 mg)
- Tryptophan: 4 mg/kg (about 280 mg)
Leucine has the highest requirement of any single essential amino acid. That’s partly because it plays a unique role in triggering muscle repair. Research suggests that roughly 2 to 3 grams of leucine per meal is needed to maximally stimulate muscle protein building, with older adults likely needing the higher end of that range.
Complete vs. Incomplete Proteins
A food counts as a “complete protein” when it contains adequate amounts of all nine essential amino acids. Most animal-based foods clear this bar easily: fish, poultry, eggs, beef, pork, and dairy all qualify. Whole soy foods like tofu, edamame, tempeh, and miso are the major plant-based exception, also providing all nine.
Incomplete proteins contain all or most of the essential amino acids, but in lower amounts for at least one. This category includes legumes (beans, peas, lentils), nuts, seeds, whole grains, and vegetables. The term “incomplete” sounds alarming, but it doesn’t mean these foods are nutritionally poor. It just means they’re low in one or two specific amino acids.
How Plant-Based Eaters Get All Nine
Different plant foods tend to be low in different amino acids, which makes combining them straightforward. Beans are low in methionine but rich in lysine. Grains are the opposite: low in lysine but good sources of methionine. Eating both over the course of a day fills the gaps. Classic pairings like rice and beans, hummus and pita, or peanut butter on whole wheat bread reflect this principle naturally.
Here are the common pairings:
- Beans (low in methionine): pair with grains, nuts, or seeds
- Grains (low in lysine and threonine): pair with legumes
- Nuts and seeds (low in lysine): pair with legumes
- Corn (low in tryptophan and lysine): pair with legumes
You don’t need to combine these foods at the same meal. Eating beans at lunch and almonds as an afternoon snack still gives your body the full range of amino acids it needs. Your body maintains a pool of free amino acids that it draws from throughout the day.
What Happens When You Don’t Get Enough
When your diet consistently falls short on one or more essential amino acids, blood levels of that amino acid drop, and so do levels in the brain. This can shift appetite and suppress hunger signals, which creates a cycle: the deficiency makes you eat less, which deepens the shortfall. Over time, the effects resemble protein malnutrition more broadly, with muscle wasting, weakened immunity, fatigue, poor wound healing, and changes in skin and hair.
In developed countries, outright deficiency is uncommon for people eating a varied diet with enough total calories. The people most at risk are those on very restrictive diets, those with digestive conditions that impair absorption, and older adults who tend to eat less protein overall. Vegans who rely heavily on a single food group without much variety can also run low on specific amino acids, particularly lysine.
How Protein Quality Is Measured
Not all protein sources deliver amino acids equally well. Scientists measure this using scoring systems that account for both the amino acid profile of a food and how well your body actually absorbs those amino acids.
The older method, called PDCAAS, estimates digestibility based on what passes through your entire digestive tract. Its replacement, called DIAAS, is considered more accurate because it measures absorption at the small intestine, where amino acids actually enter your bloodstream. DIAAS also specifically accounts for lysine availability, which PDCAAS ignores. Another key difference: PDCAAS caps scores at 100%, meaning a food like eggs and a food like soy might get the same perfect score even if eggs deliver more usable amino acids. DIAAS allows scores above 100%, giving a more honest picture of high-quality protein sources.
For most people, these scoring systems matter less than the practical takeaway: eat a variety of protein sources throughout the day. If you eat animal products, you’re almost certainly covered. If you eat only plants, a mix of legumes, grains, nuts, and seeds across your meals will reliably provide all nine essential amino acids in the amounts your body needs.