What Are Amino Acids?
Amino acids (AAs) are often called the “building blocks of protein.” They are organic compounds made of oxygen, nitrogen, carbon, and hydrogen.
Amino acids are necessary to form proteins; create enzymes, hormones, and neurotransmitters; provide energy for the body; break down food; absorb nutrients; build muscle; boost your immune system; and repair body tissues.
In this article, we will review the classification of amino acids, their functions and benefits, deficiency symptoms, food sources, and amino acid requirements.
Amino acids vs peptides vs proteins
Peptides are chains of multiple amino acids, usually 2 to 50.
Proteins are longer peptide chains, containing at least 50 amino acids. Proteins are also called polypeptides.
Both peptides and proteins are held together by peptide bonds (1).
Amino acid classification
There are about 20 amino acids required in the human body, also referred to as proteinogenic amino acids, and they each have unique properties that serve different functions. There are three primary classifications of amino acids in the diet, including essential, nonessential, and conditionally essential which are reviewed below.
Essential amino acids
Essential amino acids (EAAs), also called indispensable amino acids, cannot be made by the body and must be consumed from food sources. There are nine EAAs and their names, functions, and food sources are listed next.
Names, functions, and food sources of essential amino acids
| Amino Acid | Functions | Food Sources |
|---|---|---|
| Histidine | - Makes histamine (a neurotransmitter) - Improves immune function, digestive function, sleep regulation, and sexual -function - Maintains the myelin sheath which is a protective barrier surrounding nerve cells (2) | - Meat, poultry, & fish - Eggs - Legumes - Nuts and seeds - Grains and pseudo-grains - Cauliflower and potatoes |
| Isoleucine | - Makes hemoglobin (a protein in red blood cells that carries oxygen to tissues) - Promotes muscle function, immune function, and energy regulation (3) | - Meat, poultry, and fish - Cheese and cottage cheese - Eggs - Legumes - Sunflower seeds and sesame seeds - Spirulina, watercress, and chard |
| Leucine | - Makes protein and growth hormones - Promotes muscle growth and repair, wound healing, and blood sugar regulation (4) | - Meat, poultry, and fish - Cottage cheese - Eggs - Legumes - Hemp seeds and sesame seeds - Oats and wheat germ - Spirulina |
| Lysine | - Makes hormones, enzymes, collagen, and elastin - Regulates energy production, immune function, and calcium absorption (5) | - Meat, fish, and poultry - Cheese - Eggs - Legumes - Pumpkin seeds |
| Methionine | - Regulates tissue growth, metabolism, detoxification, and mineral absorption (zinc and selenium) (6) | - Meat, poultry, and fish - Dairy - Eggs - Soybeans - Quinoa - Nuts and seeds - Seaweed and spirulina |
| Phenylalanine | - Makes tyrosine, dopamine, epinephrine, and norepinephrine (neurotransmitters) - Helps produce other amino acids (7) | - Meat, poultry, and fish - Dairy - Eggs - Soybeans - Nuts and seeds |
| Threonine | - Makes collagen and elastin which are part of your skin and connective tissue - Promotes fat metabolism and immune function (8) | - Meat, poultry, and fish - Dairy - Eggs - Legumes - Quinoa and oats - Nuts and seeds - Carrots and bananas |
| Tryptophan | - Makes serotonin (neurotransmitter) - Improves mood, appetite, and sleep regulation (9) | - Meat, poultry, and fish - Dairy - Eggs - Soybeans and kidney beans - Oats - Pumpkin seeds - Spinach |
| Valine | - Promotes uscle growth, tissue regeneration, and energy production (10) | - Meat, poultry, and fish - Dairy - Eggs - Legumes - Nuts and seeds - Quinoa and rice - Mushrooms |
Complete vs incomplete proteins
Since essential amino acids cannot be produced by your body, it is important to eat foods that contain them on a daily basis. When you eat foods that contain protein, your body breaks the protein down into amino acids which are then used for many functions in the body.
Some foods contain all nine EAAs in optimal amounts (i.e. ratio to total protein). These are called complete proteins. Animal foods, like meat, poultry, fish, eggs, and dairy, and a few plant-based foods, mainly soy products, are complete proteins (11).
Foods that are missing or low in at least one EAA are called incomplete proteins. The amino acid that is lacking in these proteins is called the limiting amino acid.
Nearly all plant-based foods are incomplete proteins, so if you eat a vegetarian or vegan diet, you will need to eat a wide variety of foods with protein to ensure you consume all of the EAAs.
Branched-chain amino acids
Branched-chain amino acids (BCAAs) are a group of three essential amino acids: isoleucine, leucine, and valine. BCAAs are unique compared to the other EAAs because of their role in muscle protein synthesis (MPS).
Muscle protein synthesis is the process of building new muscle tissue, and it occurs when protein (or amino acid) consumption exceeds muscle protein breakdown (12).
BCAAs help stimulate MPS and prevent muscle protein breakdown. Of the three BCAAs, leucine has the most involvement in MPS and helps to increase skeletal muscle fiber size and mass (13).
Mnemonic for essential amino acids
A frequently used tool to remember the essential amino acids is a mnemonic, like PVT TIM HaLL (recited as “private Tim Hall”). The mnemonic includes the first letter of each EAA. Here you can find even more mnemonics for essential amino acids.
Nonessential amino acids
Nonessential amino acids (NEAAs), also called dispensable amino acids, are the amino acids that the body makes on its own. While you usually do not need to get these from your diet, they are still present in many foods and serve important functions.
Names and functions of nonessential amino acids
| Amino Acid | Functions |
|---|---|
| Alanine | - Regulates immune function and glucose metabolism - Provides energy for muscle tissue, brain, and the central nervous system (14) |
| Arginine | - Promotes wound healing, blood flow and circulation, immune function, and cardiovascular function (15, 16) - Promotes fetal growth during pregnancy |
| Asparagine | - Aids removal of ammonia from the body - Acts as a diuretic - Aids cell functions in nerve and brain tissue (17) |
| Aspartic acid | - Makes nucleotides which are part of DNA and RNA - Acts as a neurotransmitter - Helps produce other amino acids (18) |
| Cysteine | - Involved in keratin and collagen production - Component of glutathione (antioxidant), many tissues, and hormones - Aids metabolism of coenzyme A, heparin, and biotin (19) |
| Glutamic acid | - Involved in cognitive functions like learning and memory (20) |
| Glutamine | - Promotes wound healing, immune function, and digestive tract function (16) |
| Glycine | - Promotes fetal growth during pregnancy - Aids production of hemoglobin (21) |
| Proline | - Component of collagen and necessary for proper joint and tendon function - Maintains and strengthens heart muscles (22) |
| Serine | - Aids metabolism of fats, fatty acids, and cell membranes - Aids muscle growth - Promotes immune function - Helps produce other amino acids (23) |
| Tyrosine | - Makes dopamine, epinephrine, and norepinephrine (neurotransmitters) - Helps produce other amino acids (24) |
Conditionally-essential amino acids
Conditionally-essential amino acids (CEAAs), also called semi-essential amino acids, are a group of nonessential amino acids that have higher requirements during times of increased growth, illness, or injury. Your body has a limited capacity to make NEAAs, so if your requirements increase, you will need to obtain an additional amount from your diet.
Names and food sources of conditionally-essential amino acids
| Amino Acid | Food Sources |
|---|---|
| Arginine | - Meat, poultry, and fish - Liver and organ meats - Dairy - Eggs - Legumes - Grains - Nuts and seeds - Coconut meat - Spirulina, seaweed, and sea vegetables |
| Cysteine | - Meat, poultry, and fish - Dairy - Eggs - Legumes - Oat bran - Sunflower seeds |
| Glutamine | - Meat, poultry, and fish - Dairy - Eggs - Gelatin and bone broth - Soybeans - Corn and rice - Spirulina, Chinese cabbage, asparagus, and broccoli rabe |
| Glycine | - Meat, poultry, and fish - Dairy - Eggs - Gelatin and bone broth - Legumes - Spinach, kale, cauliflower, cabbage, pumpkin, bananas, and kiwis |
| Proline | - Meat, poultry, and fish - Dairy - Eggs - Gelatin and bone broth |
| Serine | - Meat, poultry, and fish - Eggs - Legumes - Nuts & seeds - Sweet potatoes - Seaweed |
| Tyrosine | - Meat, poultry, and fish - Dairy - Eggs - Legumes - Grains - Nuts and seeds |
Conditions that increase your need for conditionally-essential amino acids
- Physiological periods of growth, like childhood, adolescence, and pregnancy
- Recovery from illness, surgery, or trauma
- Healing of severe wounds or burns
- Frequent high-intensity strength or endurance training
Amino acid structure
Amino acids can be grouped by the structure of their side chain, which may determine how they function in the body. The side chain can be electrically charged, polar uncharged, hydrophobic, or have another structure. Below, we’ll review the amino acids in each group, and you can learn more about the side-chain differences here.
Electrically charged
Side chains that possess a charge at neutral pH can help with solubility in water and aid enzymatic reactions. Three amino acids have a positively charged side chain and two have a negatively charged side chain.
Positive (or basic)
- Arginine
- Histidine
- Lysine
Negative (or acidic)
- Aspartic acid
- Glutamic acid
Polar uncharged
Side chains that are polar uncharged bind with water and other amino acids via hydrogen. There are four amino acids with polar uncharged side chains.
- Asparagine
- Glutamine
- Serine
- Threonine
Hydrophobic
Side chains that are hydrophobic do not dissolve in water. There are eight amino acids with hydrophobic side chains.
- Alanine
- Isoleucine
- Leucine
- Methionine
- Phenylalanine
- Tryptophan
- Tyrosine
- Valine
Other structures
Some amino acids have unique properties that don’t clearly fit into the charged, polar uncharged, or hydrophobic categories.
- Cysteine can form hydrogen bonds and disulfide bonds which are important for protein folding and stability
- Glycine has only a hydrogen atom as its side chain and functions in several environments
- Proline is hydrophobic and contributes to the structure and shape of proteins
General functions & benefits of amino acids
- Boosting your immune system
- Building muscle tissue
- Digesting and absorbing food
- Growing & repairing body tissues
- Maintaining healthy hair, skin, and nails
- Regulating mood, sleep, & cognitive function
- Producing enzymes, hormones, & neurotransmitters
- Promoting a healthy digestive system
- Protein synthesis
- Providing energy to the body
Deficiency symptoms & consequences
While amino acid deficiency occurs most frequently in poorer parts of the world or amongst older adults eating a limited diet, people who don’t prioritize protein in their diet or people who eat a vegan diet without proper consideration of protein sources can also experience deficiency symptoms.
Protein deficiency and amino acid imbalances increase your risk for dysfunction of your nervous, reproductive, immune, and digestive systems.
Signs of essential amino acid deficiencies
- Anemia
- Anxiety
- Decreased immune function
- Depression
- Digestive issues
- Fatigue
- Growth stunting in children
- Hair loss
- Insomnia
- Muscle wasting
- Poor mental function
- Skin rashes and other conditions
- Weakness or loss of muscle strength
Potential health consequences of amino acid deficiencies
In the most extreme, severe malnutrition with inadequate protein intake can lead to the clinical disorders, kwashiorkor or marasmus.
Kwashiorkor occurs in the setting of adequate calorie intake with insufficient protein intake. Some symptoms include edema of the extremities, dry peeling skin, hyperkeratosis and hyperpigmentation, ascites, liver failure, immune dysfunction, anemia, and poor muscle tone.
Marasmus occurs in the setting of combined inadequate calorie and protein intake. The main symptoms are generalized muscle wasting, low body fat, improper growth in children, bradycardia, and hypotension (11).
Dietary sources of amino acids
Here is a review of complete and incomplete protein sources.
Complete protein sources
Foods that contain all nine essential amino acids are called complete proteins. Here we’ve listed the top 2-4 EAAs in each of these food sources.
| Food Source | Top-Ranked Amino Acids |
|---|---|
| Meat | Isoleucine, Histidine, Lysine, Phenylalanine |
| Poultry | Isoleucine, Histidine, Phenylalanine, Tryptophan |
| Fish | Isoleucine, Histidine, Phenylalanine |
| Eggs | Isoleucine, Lysine, Methionine |
| Milk | Leucine, Phenylalanine |
| Cheese | Isoleucine, Leucine, Phenylalanine, Valine |
| Cottage cheese | Leucine, Phenylalanine, Threonine, Tryptophan |
| Soybeans | Leucine, Lysine, Phenylalanine, Valine |
Almost complete protein sources
Here is a list of incomplete protein sources that are only missing or low in 1-2 essential amino acids, also called the limiting amino acids.
| Food Source | Limiting Amino Acids |
|---|---|
| Collagen & gelatin | Tryptophan |
| Corn | Lysine, Tryptophan |
| Grains | Isoleucine, Lysine, Threonine |
| Legumes | Methionine, Tryptophan |
| Nuts & seeds | Isoleucine, Lysine |
Complementary proteins
Complementary proteins are combinations of incomplete protein sources that together provide adequate amounts of essential amino acids. When combined, they can be considered complete proteins.
While it’s most important to focus on your daily consumption of amino acids (or protein) rather than your individual meals or snacks, if you are prioritizing muscle growth, you’ll want to ensure you are getting complete proteins at each main meal for optimal muscle protein synthesis.
Some common vegan food combinations include the following:
- Corn or wheat tortilla + beans
- Hummus (made with chickpeas + tahini)
- Hummus + pita bread
- Muesli (made with nuts + grains)
- Rice + beans
- Rice + tofu
- Wheat bread + peanut butter
- Wheat pasta + peas
Alternatively, if you include animal protein (especially meat, poultry, fish, eggs, or dairy) in your diet, you likely don’t need to worry about pairing your plant-based proteins together as your diet likely already contains adequate EAAs.
Amino acid supplements
Increased exercise, especially frequent strength training, can increase amino acid needs. Some people choose to take amino acid supplements, like BCAAs or EAAs, before and/or after their exercise session to deliver a quick supply of amino acids to their muscles and fuel their workout.
When taken according to the dosage listed on the product, BCAA & EAA supplements will be safe for most people.
Individual amino acids should only be taken if it’s likely you are not meeting your needs from food sources or if prescribed by your qualified healthcare provider. Taking individual amino acid supplements may increase your risk for amino acid imbalances, so they usually shouldn’t be taken for an extended period of time.
| Type of Supplement | What They Contain | Benefits |
|---|---|---|
| Essential amino acids | All 9 EAAs | - Reduce delayed onset muscle soreness (25) |
| Branched-chain amino acids | Leucine, isoleucine, and valine in the ideal ratio of 2:1:1, respectively | - Reduce fatigue, improve athletic performance, stimulate muscle recovery after exercise (26) - Reduce soreness and muscle damage after resistance training (27) |
| Individual amino acids | Any single essential amino acid | - Can target potential deficiencies |
Amino acid requirements
Factors influencing amino acid requirements
Most people will meet their essential amino acid needs by eating enough total protein. Your protein and amino acid intake may be higher if you exercise or strength train frequently, are older than 50 years of age, have a goal of fat loss, or are recovering from an injury, surgery, or illness.
Recommended daily amino acid intake
The recommended daily allowance (RDA) for each of the essential amino acids is listed below. The amounts shown are reflected in milligrams (mg) per kilogram (kg) of body weight. You can convert pounds to kilograms by dividing your weight in pounds by 2.2 (28, 29).
- Histidine: 10-14 mg/kg
- Isoleucine: 19-20 mg/kg
- Leucine: 39-42 mg/kg
- Lysine: 30-38 mg/kg
- Methionine: 10-19 mg/kg
- Phenylalanine: 25-33 mg/kg
- Threonine: 15-20 mg/kg
- Tryptophan: 4-5 mg/kg
- Valine: 24-26 mg/kg
For further recommendations on protein needs, view our article on recommended daily protein intake.
More resources
- BCAAs vs EAAs
- When to Take Amino Acids: EAAs & BCAAs
- Mnemonic for Essential Amino Acids
- What is Protein?
- Recommended Protein Intake
Frequently asked questions
There are 20 standard proteinogenic amino acids.
There are 9 essential amino acids.
Amino acids are responsible for forming proteins; creating enzymes, hormones, and neurotransmitters; providing energy to the body; breaking down food; absorbing nutrients; building muscle; boosting your immune system; and repairing body tissues.
Amino acids from food sources are not dangerous unless your total protein intake is extremely high, usually > 3.4 g/kg body weight. Some amino acid supplements, mostly individual amino acids, may be dangerous if the amount you take causes amino acid imbalances in your body. Consult a healthcare professional before starting any supplement.
Amino acids can be converted into enzymes, hormones, neurotransmitters, and proteins.
Amino acids are made of oxygen, nitrogen, carbon, and hydrogen.
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