Proteins that contain all nine essential amino acids are called complete proteins. Animal-based foods like meat, fish, eggs, and dairy are the most well-known complete proteins, but several plant foods qualify too, including soy, quinoa, amaranth, buckwheat, and hemp seeds. The nine essential amino acids your body cannot make on its own are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
What Makes a Protein “Complete”
Your body uses amino acids as building blocks for everything from muscle tissue to neurotransmitters and hormones. Of the 20 amino acids your body needs, it can manufacture 11 on its own. The remaining nine must come from food. A protein source is considered complete when it provides adequate amounts of all nine in a single food or meal.
The distinction matters because not all protein sources deliver these nine in equal proportion. Every plant food technically contains all 20 amino acids, but many have one or two essential amino acids in such low quantities that they fall below the threshold for “complete.” Grains, for instance, tend to be low in lysine, while legumes tend to be low in methionine. When a food has too little of one amino acid, that amino acid is called the “limiting” amino acid because it limits how effectively your body can use the protein overall.
Animal-Based Complete Proteins
Nearly all animal-derived foods are complete proteins. This includes beef, poultry, pork, fish, shellfish, eggs, milk, cheese, yogurt, and whey. These sources also tend to have a higher concentration of essential amino acids overall. Whey protein, for example, is about 43% essential amino acids by weight, milk protein is 39%, egg is 32%, and muscle protein is 38%. By comparison, most plant protein isolates land between 21% and 22%.
Animal proteins also score higher in two amino acids that plant foods commonly lack: lysine and methionine. Animal-based proteins average about 7.0% lysine and 2.5% methionine, while plant-based proteins average roughly 3.6% lysine and 1.0% methionine. This is one reason animal proteins are often considered higher quality from a purely nutritional standpoint, though “quality” depends on more than amino acid profile alone.
Digestibility plays a role too. Scientists measure protein quality using a score called DIAAS (Digestible Indispensable Amino Acid Score), which accounts for how well your body actually absorbs each amino acid. Milk protein concentrate scores the highest at 1.18, while some grain-based proteins score as low as 0.01. That said, many plant proteins perform surprisingly well. Soy protein isolate, pea protein, and even lupine flour score between 89% and 94% for digestibility, which is comparable to eggs (91%) and meat (90–94%).
Plant-Based Complete Proteins
If you eat a plant-based diet, several individual foods deliver all nine essential amino acids in meaningful amounts.
- Soy foods: Tofu, tempeh, and edamame are the most established plant-based complete proteins. Soy protein isolate has digestibility on par with animal proteins.
- Quinoa: A pseudocereal (technically a seed, not a grain) that provides a balanced amino acid profile.
- Amaranth: Another pseudocereal and complete protein, often used in porridge or baked goods.
- Buckwheat: Despite the name, buckwheat is not related to wheat and is a complete protein source.
- Hemp seeds: These contain all nine essential amino acids, though lysine is the limiting amino acid with a score of 0.5 to 0.62. Hemp’s overall amino acid profile has been compared to egg white protein and casein.
- Chia seeds: A complete protein in a small package, often added to smoothies or oatmeal.
- Spirulina: This blue-green algae is 55–70% protein by dry weight and contains all essential amino acids.
- Nutritional yeast: A deactivated yeast grown specifically as a food product, providing complete protein along with B vitamins.
One important nuance: being “complete” doesn’t mean these plant foods are identical to animal proteins in amino acid concentration. Hemp seeds, for instance, qualify as complete but are still relatively low in lysine. Spirulina is impressively protein-dense but is consumed in small enough quantities that it works better as a supplement than a primary protein source. Context matters.
Combining Foods for Complete Protein
You don’t need every food to be complete on its own. Pairing two incomplete proteins that complement each other’s weaknesses creates a complete amino acid profile. This is the principle behind some of the most common food pairings across cultures.
Rice and beans are the classic example. Rice is low in lysine but high in methionine, while beans are high in lysine but low in methionine. Together, they cover all nine. Pita bread with hummus works the same way: wheat is low in lysine, but chickpeas are rich in it. A peanut butter sandwich on whole grain bread follows the same logic, with peanuts compensating for wheat’s lysine shortfall.
Sprouted grain products like Ezekiel bread take a different approach. By combining sprouted whole grains (barley, wheat, millet, spelt) with legumes (soybeans, lentils), the bread itself becomes a complete protein. Sprouting grains and legumes also increases their amino acid content, particularly lysine.
Older nutrition advice suggested you needed to eat complementary proteins at the same meal. That’s no longer considered necessary. As long as you eat a variety of protein sources throughout the day, your body can pool the amino acids it needs.
Why Essential Amino Acids Matter
Your body needs these nine amino acids for more than just building muscle. Leucine, isoleucine, and valine (the branched-chain amino acids) are particularly important for muscle growth and repair. Tryptophan is a precursor to serotonin, a neurotransmitter that regulates mood and sleep. Phenylalanine is converted into tyrosine, which your body then uses to produce thyroid hormones, adrenaline, and the skin pigment melanin. Methionine plays a critical role in cellular processes by donating chemical groups that help regulate gene expression and detoxification. It’s also a primary source of sulfur in your diet, along with the amino acid cysteine.
Daily requirements vary by amino acid and body weight. Leucine has one of the highest requirements at roughly 40 mg per kilogram of body weight per day, while threonine requires about 15 mg per kilogram. For a 70 kg (154 lb) person, that translates to about 2,800 mg of leucine and 1,050 mg of threonine daily. Most people eating adequate total protein from varied sources meet these thresholds without tracking individual amino acids. The people most likely to fall short are those eating very limited diets, whether due to food insecurity, extreme restriction, or reliance on a single staple grain without complementary protein sources.