How Many Essential Amino Acids Are There? Functions, RDA, Risks & Food Sources

Amino acids are the building blocks of proteins. They are characterized by a basic amino group (-NH2), an acidic carboxyl group (-COOH), and an organic R group. Each amino acid has its unique R group which makes the structure of one amino acid different from another.

There are 20 amino acids present in the human body and they are of two types: essential and nonessential amino acids. Essential amino acids are those amino acids that are not naturally present in the body. These amino acids must be obtained from the diet.

However, this does not mean that the body can function with the essential amino acids alone. It still requires the nonessential amino acids, amino acids the body can produce, to carry out specific biological functions.

In this article, I will be discussing the essential amino acids in detail – their unique R group, dietary sources, and dietary requirements.

How many essential amino acids are there?

There are 9 essential amino acids. These 9 of the 20 amino acids cannot be synthesized by the human body. They must be ingested through meals to carry out normal body functions and keep the body healthy.

Different protein sources contain different amino acids. Some proteins contain all nine essential amino acids and are known as complete proteins. Others contain a combination of amino acids and are also very healthy.

List of essential amino acids

1. Histidine

histidine essential amino acid - gezro

Histidine (His or H) is a basic amino acid and a precursor for histamine. It contains a protonated amino group (-NH3+), a deprotonated carboxyl group (-COO), and a partially deprotonated imidazole side chain. Histidine has a chemical formula of C6H9N3O2 and a molar mass of 155.157 g/mol.


  • Histidine can help improve feeding behavior and energy metabolism
  • It also helps reduce inflammation and oxidative stress
  • Eating histamine-rich foods can help regulate iron, copper, molybdenum, zinc, and manganese levels
  • Histamine, a product of histidine, helps to improve immune responses and digestion
  • Histidine regulates the production of red blood cells, maintenance of hemoglobin levels, and regulation of blood pH
  • It helps to increase alertness and cognition
  • Histidine stimulates the release of carnosine, which reduces plaque buildup in the arteries and lower the risk for some chronic diseases
  • Histidine plays a vital role in tissue repair and healing of wounds
  • In addition, histidine is used in the treatment of high blood pressure, anemia, rheumatoid arthritis, stomach ulcers, heart diseases, and poor growth and development in infants

RDA and risks

The recommended daily allowance (RDA) for every 19 years and older is 14 mg/kg/day. Going above this limit up to 32 mg/kg/day could lead to drowsiness, fatigue, muscle weakness, loss of appetite, nausea, depression, and loss of memory.

Food sources

Histidine-rich foods are common protein sources. Examples are eggs, soy products, legumes, beans, poultry, grass-fed beef and meats, fish, seafood, potatoes, corn, quinoa, oats, rice, and buckwheat.

2. Isoleucine

isoleucine essential amino acid - gezro

Isoleucine (Ile or I) is one of the branched-chain essential amino acids. It is a non-polar, uncharged, α-amino acid with a protonated α-amino group, a deprotonated α-carboxylic group, and a branched hydrocarbon side chain.

It has a chemical formula C6H13NO2 and a molar mass of 131.175 g/mol.


  • Isoleucine helps to control blood sugar
  • As an amino acid present in the muscles, it helps to boost energy levels and resilience
  • It also helps to improve muscle development and the buildup of lean body mass
  • As a key function of proteins, foods rich in isoleucine can help aid healing and recovery in injured muscles

RDA and risks

The recommended daily allowance for isoleucine is 19 mg/kg/day. Exceeding this may lead to diarrhea, vomiting, stomach bloating, high blood pressure, skin whitening, and loss of coordination. Furthermore, the inability of the body to digest isoleucine can cause maple syrup urine disease (MSUD).

Additionally, isoleucine deficiency is not so common. But, when it happens, some of the symptoms are weakening and wasting of muscle and blood sugar level fluctuations.

Food sources

Foods rich in isoleucine include eggs, poultry, grass-fed beef, watercress, black beans, pinto beans, lentils, cottage cheese, lamb, chard, sunflower seeds, and sesame seeds.

3. Leucine

leucine essential amino acid - gezro

Leucine (Leu or L) is an α-amino acid with a protonated amino group, a deprotonated carboxyl group, and an isobutyl side chain. It is a non-polar aliphatic amino acid with a molecular formula of C6H13NO2 and a molar mass of 131.175 g/mol.

It makes one of the three branched-chain essential amino acids and is the most essential ketogenic amino acid present in the human body. Leucine is present in muscle fiber and highly concentrated in hemoglobin.


  • Leucine is the key regulator of muscle protein synthesis
  • Leucine is regarded as the athlete’s amino acid because of its ability to improve muscle mass and athletic performance
  • It also slows done the rate of muscle deterioration
  • Leucine plays a vital role in blood sugar regulation
  • Leucine is good for weight regulation in overweight persons, elderlies and people predisposed to obesity
  • Eating food rich in leucine post-workout can help reduce soreness and speed up muscle recovery

RDA and risks

The recommended daily allowance for leucine is 42 mg/kg/day.

There are no risks associated with overconsumption of leucine-rich foods, but an excess intake of leucine supplements can cause chronic fatigue, impaired coordination, reduction in the release of serotonin, and aggravated pellagra.

In addition, very high doses of leucine can raise blood ammonia levels.


Leucine-rich foods include eggs, oats, chicken, turkey breast, cottage cheese, spirulina, oats, tuna fish, lentils, venison, peanuts, grass-fed beef, sesame seeds, hemp seeds, almonds, edamame, cooked chickpeas, and wheat germ.

4. Lysine

lysine essential amino acid - gezro

Lysine (Lys or K) is an α-amino acid that is vital to the synthesis of many proteins. It contains an α-amino group, an α-carboxyl group, and a side chain lysyl ((CH2)4NH2) group. Lysine is a basic, aliphatic amino acid with the chemical formula C6H14N2O2 and a molar mass of 146.190 g/mol.


  • Lysine is an essential amino acid necessary for the growth and repair of tissues
  • It also plays a significant role in the production of certain hormones, proteins, and enzymes
  • It is useful for treating health conditions, such as cold sores, herpes, high blood pressure, and diabetes
  • Lysine aids the absorption of calcium, iron, and zinc
  • Lysine-rich foods promote collagen growth and wound healing
  • Lysine can also help boost the immune system and promote healthy skin and hair growth
  • It is also a muscle-building amino acid and can aid muscle recovery in athletes
  • Eating lysine-rich foods helps to prevent mobility issues, bone loss, and osteoporosis

RDA and risks

The recommended daily allowance (RDA) of lysine is 38 mg/kg/day. The risks of lysine come with lysine supplements. Excess intake of lysine causes stomach pain, diarrhea, kidney and liver impairment, and increased viral load in HIV patients.


Some lysine-rich foods are red meat, chicken, beans, lentils, eggs, cod, sardines, lamb, white beans, pork, shrimp, parmesan cheese, tuna, pumpkin seeds, wheat germ, cottage cheese, nuts, soybeans, spirulina, and brewer’s yeast.

5. Methionine

methionine - gezro

Methionine (Met or M) is an α-amino acid with a protonated amino group, deprotonated carboxyl group, and an S-methyl thioether side chain. It is a non-polar, aliphatic amino acid with the chemical formula C5H11NO2S and a molar mass of 149.21 g/mol.

It is a soluble crystalline white solid in powder form that decomposes at a melting point of 538°F (281°C). Methionine, through certain reactions, can produce important sulfur-containing molecules. It is also a precursor to cysteine and several important proteins.  


  • Methionine is important for DNA methylation
  • As mentioned before, it is used to produce other important molecules such as cysteine, glutathione, DNA, and proteins, among many others
  • It may help reduce the risk of developing colorectal cancer
  • It may also soothe the tremors that come with Parkinson’s
  • Foods rich in methionine can help improve bone strength
  • They can also aid weight loss and support liver functions
  • Methionine is useful in drug withdrawal therapy

RDA and risks

The recommended daily allowance (RDA) for methionine is 19 mg/kg/day in adults and 22-27 mg/kg/day for children between the ages of 2 to 12. Consuming too much methionine can predispose a person to heart disease and tumor growth.


Methionine-rich foods include oats, sunflower butter, Brazil nuts, sesame seeds, egg whites, whitefish, halibut, cod, pike, tuna, free-range chicken, free-range elk, turkey, and seaweed.

6. Phenylalanine

phenylalanine - gezro

Phenylalanine (Phe or F) is an α-amino acid with a protonated α-amino group and a deprotonated α-carboxyl group. It has a chemical structure similar to alanine, except that the methyl group in alanine is replaced with a benzyl group.

It is a neutral, non-polar amino acid because of the inert, hydrophobic benzyl side chain. Methionine has a chemical formula C9H11NO2 and a molar mass of 165.192 g/mol. L-Phenylalanine is a precursor for the synthesis of L-DOPA, dopamine, norepinephrine, and epinephrine.

In plants, phenylalanine begins the synthesis of flavonoids, such as lignan.


  • It is a precursor for several compounds
  • Phenylalanine can help relieve depression and improve your mood
  • It has analgesic properties, which makes it a good pain reliever for chronic pain
  • Eating phenylalanine-rich foods can aid weight loss
  • Taking phenylalanine supplements can stimulate the production of melanin to improve the appearance of depigmentation in people with vitiligo

RDA and risks

The recommended daily allowance (RDA) for phenylalanine is 33 mg/kg/day in adults. If you are taking phenylalanine supplements with a prescription, you can take up to 45 mg/kg/day. Above this dosage may lead to heartburn, fatigue, constipation, anxiety, and nausea.


Some phenylalanine-rich foods are beef, pork, venison, fish, poultry, nuts, soy, seafood, nut butter, legumes, dairy, whole grains, diet soda, sugar-free ice cream, gum, and yogurt made with aspartame.

7. Threonine

threonine - gezro

Threonine (Thr or T) is an α-amino acid with a protonated amino group, a deprotonated carboxyl group, and a side chain with a hydroxyl group. It is a polar, uncharged amino acid with the chemical formula C4H9NO3 and a molar mass of 119.120 g/mol.

Threonine is a precursor to serine and glycine. It is highly beneficial in people whose bodies cannot synthesize glycine due to a health condition.


  • Threonine produces a mucus gel layer which it uses to protect the digestive tract from digestive enzymes that can damage the digestive tract
  • It boosts the immune system by producing T-cells that help to fight off infections
  • Threonine directly increases the production of glycine. Increased levels of glycine help to improve muscle contractions
  • Eating foods that contain threonine supports muscle and bone strength
  • It helps to prevent the accumulation of fat in the liver by regulating fat metabolism. Foods that are rich in threonine prevent threonine deficiency, thereby preventing fatty liver and liver failure
  • Threonine can work with phenylalanine to tackle the early stages of depression
  • It also increases the production of collagen, thereby speeding up the healing of wounds

RDA and risks

The recommended daily allowance (RDA) of threonine is 20 mg/kg/day. Threonine supplements are safe to use but excessive consumption can cause nausea, skin rash, stomach upset, and headache.


Threonine-rich foods are high-quality protein foods such as wild-caught fish like salmon, eggs, cottage cheese, dairy, poultry, carrots, banana, lentils, sesame seeds, pumpkin seeds, edamame, and kidney beans.

8. Tryptophan

tryptophan - gezro

Tryptophan (Trp or W) is an α-amino acid that contains an α-amino group, an α-carboxyl group, and an indole side chain. It is a polar molecule that is soluble in water, hot alcohol, and alkali hydroxides. It has a chemical formula C11H12N2O2 and a molar mass of 204.229 g/mol.

This essential amino acid is a precursor to serotonin, melatonin, niacin, and auxins (phytohormones).


  • It is a precursor for serotonin, the hormone responsible for making you feel good and calm, thereby reducing the symptoms of depression
  • Taking tryptophan supplements can aid weight loss. It works through a mechanism that controls the appetite and keeps your weight in check
  • Tryptophan helps improve the quality of your sleep
  • It also helps to reduce headaches and migraines
  • Research shows that increased tryptophan levels can help treat attention-deficit/hyperactivity disorder (ADHD) symptoms

RDA and risks

The recommended daily allowance for tryptophan is 5 mg/kg/day. When using tryptophan supplements, it is good to start with small doses. You can increase your dosage if you do not experience any side effects.

Some side effects of taking too much tryptophan are headache, dry mouth, diarrhea, nausea, drowsiness, and light-headedness. Pregnant women, lactating mothers, and people with kidney or liver disease should stay off tryptophan supplements.


Some tryptophan-rich foods are chicken breast, roast lamb, skirt steak, ground turkey, roast turkey breast, tuna fillet, salmon, tofu, edamame, egg whites, cheese, peanuts, chocolate, dried dates, canned sardines, oatmeal, hamburger patty, red kidney beans, spinach, and pumpkin seeds.

9. Valine

valine - gezro

Valine (Val or V) is the third branched-chain essential amino acid. It is an α-amino acid that contains an α-amino group, an α-carboxyl group, and an isopropyl group as the side chain. Valine is a non-polar aliphatic amino acid with the chemical formula C5H11NO2 and a molar mass of 117.148 g/mol.


  • Valine is one of the athlete’s amino acids. It helps to improve muscle health by supporting muscle metabolism, growth, and repair
  • It also provides the muscle with extra glucose during workout sessions to prevent muscle breakdown
  • Valine boosts immunity by enhancing the production of antibodies that help to wade off pathogens that cause diseases
  • Valine boosts the proper functioning of the nervous system
  • Valine also supports weight loss
  • It enhances the production and maintenance of proteins

RDA and risks

The recommended daily allowance (RDA) of valine is 24 mg/kg/day. Going beyond this limit will result in high concentrations of valine in the body. High amounts of valine can cause hallucinations, high concentrations of toxic ammonia, impaired liver and kidney function.

Furthermore, excessive consumption of valine supplements can cause nausea, bloating, fatigue, and loss of coordination.


Examples of foods high in valine include eggs, halibut, soybeans, veal roast, lamb roast, beef chuck roast, buffalo sirloin steak, pork, hard cheese, tofu, podded peas, lean chicken breast, tuna, low-fat yogurt, navy beans, and oatmeal.

Mushrooms, gluten-free grains, sesame seeds, pumpkin seeds, squash seeds, chia seeds, flaxseeds, almonds, pistachios, cashews, and fermented soy products, such as natto and tempeh are also rich sources of valine.


Are essential amino acids the same as branched-chain amino acids?

Branched-chain amino acids (BCAAs) are three of the essential amino acids. They are leucine, isoleucine and valine. The difference between these amino acids and the rest of the essential amino acids is their branched structure and the lack of the enzyme necessary for their catabolism in the liver.

As a result, they are broken down in the muscles instead of the liver. They are then channeled into energy production and muscle synthesis during exercise.

Which food has the highest amino acids?

Animal proteins, such as meat, poultry, eggs, fish, and dairy have the highest concentration of amino acids. They are known as complete proteins because they contain all nine essential amino acids.

Quinoa and buckwheat are some examples of plant-based sources of complete proteins.

Can you have a deficiency in essential amino acids?

Yes, it is possible. Essential amino acid deficiency isn’t so common because rich sources of these biomolecules are everyday meals, such as meat, eggs, pumpkin, black beans, and soy.

It’s important to note that a deficiency in amino acids may not necessarily be caused by a lack of protein in your diet. It could also be due to incomplete breakdown of protein, stress, or genetics.

Signs of amino acid deficiency are fatigue, muscle loss, slow recovery, diarrhea, skin depigmentation, pressure sores, and digestive inconveniences.


9 out of the 20 amino acids the human body needs are essential. The body cannot produce them, but they are important for healthy living. Ensure that you meet up your required daily allowance for these amino acids.

Amino acids are the building blocks of life. The different proteins formed are responsible for various functions. These functions encompass body tissue repair, growth, and aiding digestion.

Besides glucose, amino acids are another class of biomolecules that can be used as an energy source.

You can also learn about the chemical formula for cellular respiration and the process involved.

Thanks for reading.