The 9 Essential Amino Acids and Their Importance to Your Body

Updated on June 5, 2018

Essential Amino Acids

There are 9 "essential" amino acids and they are called "essential" because our bodies can't produce them, so it's essential that we include them in our daily diet.

The nine essential amino acids are histidine, valine, isoleucine, leucine, phenylalanine, threonine, tryptophan, methionine, and lysine.

Why are amino acids important? Amino acids are the building blocks of protein, which is a very important element for nearly all physiological functions. Most skeletal tissues, cells, organs, and muscles are made of amino acids. They help make the proteins that allow our bodies to grow, repair tissue, break down food, and perform many other essential biological processes.

Branched-chain amino acids (BCAA) have a branched molecular structure. There are three of them: leucine, isoleucine, and valine. Like other essential amino acids, they are building blocks of proteins and muscles. They also help regulate blood sugar levels. BCAAs also help reduce fatigue during exercise.

The 9 Essential Amino Acids

 
1. Histidine
2. Isoleucine
3. Leucine
4. Lysine
5. Methionine
6. Phenylalanine
7. Threonine
8. Tryptophan
9. Valine

The 9 Essential Amino Acids

Some sources list eight essential amino acids and others list nine. This is because histidine, which used to be considered essential only for infants was later reclassified as an essential amino acid when it was found to be indispensable for humans

Other lists include histidine as well as arginine, which is only essential (must be added to the diet) for premature infants, who can't make it on their own.

Protein shakes made with whey powder are an excellent complete plant-based protein source for vegetarians.
Protein shakes made with whey powder are an excellent complete plant-based protein source for vegetarians.

Vegetarian Food Sources of Essential Amino Acids

Essential amino acids are the building blocks of protein. These foods can be plant or animal proteins, but it can be more challenging to people on plant-based or vegan diets to make sure they get enough essential amino acids.

When we eat protein, it is broken down in our gastrointestinal tracts into individual amino acids and then put back together again as new proteins. This complex biological process is called protein biosynthesis.

Plant proteins rich in essential amino acids include combinations of whole grains, soybeans, lentils, chickpeas, black beans, pinto beans, almonds, walnuts, and sunflower seeds. Of these, only soybeans are considered a complete protein. Other plant proteins are considered "incomplete" because they contain only some essential amino acids.

Protein powders, especially whey isolate powders, are an excellent source of complete protein and contain all of the essential amino acids. Pea protein contains all nine essential amino acids except for methionine.

Animal proteins that are rich in essential amino acids include beef, pork, turkey, chicken, eggs, milk, yogurt, cheese, and seafood. Animal proteins are considered "complete proteins" because they contain all of the essential amino acids.

What's the Recommended Daily Amount of Essential Amino Acids?

Amino acids are present in protein foods, so recommended levels of amino acids are made in terms of daily recommended protein intake.

Protein needs vary widely from one individual to another. Pregnant women, for example, need more than other women. People who exercise vigorously also need more protein than sedentary people.

A way to calculate the minimum amount of protein you should eat every day is to multiply your body weight in kilograms by 0.8, or your weight in pounds by 0.36.

This amounts to:

  • 56 grams per day for the average sedentary man.
  • 46 grams per day for the average sedentary woman.

The 22 Amino Acids

 
 
 
Alanine
Cysteine *
Aspartic Acid
Glutemic Acid
Phenylalanine
Glycine *
Histidine
Isoleucine
Lysine
Leucine
Methionine
Asparagine
Pyrrolysine
Proline *
Glutamine *
Arginine *
Serine *
Threonine
Selenocysteine
Valine
Tryptophan
Tyrosine *
 
 
Conditionally, or semi-essential, amino acids are marked with an *. Essential amino acids are in bold.
Beans are an excellent plant-based protein source of the essential amino acid histidine.
Beans are an excellent plant-based protein source of the essential amino acid histidine.

1. Histidine

Histidine is unique because it is both an essential and nonessential amino acid. The body needs histidine to develop and maintain healthy tissues, especially myelin sheath that coats nervous cells to ensure the transmission of messages from your brain to organs throughout your body.

Too much histidine is associated with physiological disorders like anxiety and schizophrenia. Not enough can to lead to rheumatoid arthritis and deafness from nerve damage.

Adults can typically produce enough histidine from other amino acids in the liver to support the body’s daily needs. But children must get histidine from food. This is especially true during infancy when adequate histidine levels are essential for proper growth and development.

Foods that are high in protein generally contain high histidine levels as well. These foods include:

  • meat, poultry, and fish
  • dairy
  • rice, wheat, and rye
  • seafood
  • beans
  • eggs
  • buckwheat
  • corn, cauliflower, mushrooms, potatoes, bamboo shoots, bananas, cantaloupe, and citrus fruits

Cottage cheese is an excellent source of the essential amino acids valine and threonine.
Cottage cheese is an excellent source of the essential amino acids valine and threonine.

2. Valine

Valine, apart from being an essential amino acid, is one of the three branched-chain amino acids. The other two are leucine and isoleucine.

Also together with leucine and isoleucine, valine belongs to the group of proteinogenic amino acids, which are the building blocks of proteins produced by cells that are recorded in the genetic code of each living thing.

Valine is an important source of nitrogen, an important component in alanine and glutamine synthesis in the muscles.

Foods rich in valine include:

  • cottage cheese
  • fish and poultry
  • sesame seeds
  • lentils
  • tofu
  • egg whites
  • peanuts
  • beef and lamb
  • gelatin

3. Isoleucine

Isoleucine is another branched-chain amino acid. It cannot be produced in the body and must be obtained from the food we eat.

Isoleucine is essential for proper blood-clotting and muscle repair. While isoleucine deficiency is uncommon, symptoms include dizziness, fatigue, headaches, confusion, irritability, and depression.

Isoleucine has several key roles in healthy body functions.

  • It regulates blood sugar and boosts the body's energy levels.
  • It plays a key role in the transport of oxygen from the lungs to the various parts of the body and the production of hemoglobin, which contains iron.
  • It is important for the efficient metabolism of glucose, as manifested by the increase in the absorption of sugar.

When given orally, isoleucine reduces the level of sugar in the blood by 20 percent and increases sugar absorption in the muscles by 71 percent without necessarily increasing the level of insulin in the blood.

Good sources of isoleucine are:

  • eggs
  • chicken
  • fish
  • cheese
  • soybeans
  • seaweed
  • turkey

4. Leucine

Leucine is the third branched-chain amino acid. It is not produced naturally by the body and must be absorbed through the food we eat.

Leucine is called a "buffer" protein because it has the ability to protect the body when it lacks iron. It is used to produce sterols, which resemble fats (a good example is cholesterol) and which are found in the liver, adipose, and muscle tissues.

Leucine is used as dietary supplement for body-building to enhancing physical performance because it boosts stamina and endurance while delaying the deterioration of muscle tissues through the significant increase in the production of muscle proteins.

Leucine is also an ideal dietary supplement for people recovering from major surgical procedures, serious trauma, or extreme muscle pain. It also has the ability to dissolve visceral fat, the kind of fat found in the deepest layer of the skin that does not respond to the usual weight loss exercises or non-surgical procedures.

Leucine is a great energy source especially during intense athletic performance and other extreme physical activities. It protects from fatigue and regulates the body's glucose levels.

An overdose of leucine can lead to Maple Syrup Urine Disease (MSUD), a disorder characterized by a deficiency in keto hydrogenase complex that can cause the accumulation of leucine, isoleucine, and valine in the blood and urine. The name comes from the smell of an infant's urine when suffering from MSUD. The disorder can cause delirium, neurologic disorders, and death.

Good food sources of leucine include:

  • soybeans
  • beef
  • chicken
  • parmesan cheese
  • pork
  • seeds and nuts
  • white beans

Fish, including canned tuna, is an excellent source of several essential amino acids, including isoleucine, phenylalanine, and threonine.
Fish, including canned tuna, is an excellent source of several essential amino acids, including isoleucine, phenylalanine, and threonine.

5. Phenylalanine

Phenylalanine is a forerunner of tyrosine, the anti-depressant dopamine, norepinephrine, epinephrine and the skin pigment, melanin. It is also a precursor of phenylethylamine, a popular anti-depressant dietary supplement. It is naturally present in mammalian breastmilk.

Eating foods rich in phenylalanine will help prevent mood swings, help you out of lethargy, sluggishness, feelings of low morale, and anxiety.

There are three forms of phenylalanine: L-phenylalanine, D-phenylalanine, and DL-phenylalanine. L-phenylalanine is converted to L-tyrosine, then to L-DOPA and to dopamine. D-phenylalanine produces endorphins which are released by the pituitary glands during exercise, excitement, orgasm, when experiencing pain, and after eating spicy food.

Compared to L-phenylalanine, D-phenylalanine cannot efficiently cross the blood-brain barrier. D-phenylalanine is excreted in the urine without entering the central nervous system.

DL-phenylalanine is sold as a nutritional supplement and is used for its analgesic and antidepressant characteristics. Its pain-relieving property can be attributed to its ability to block the enzyme carboxypeptidase, which causes enkephalin damage.

Good sources of phenylalanine are:

  • beef, poultry, pork, and fish
  • milk, yogurt, eggs, and cheese
  • soy products (including soy protein isolate, soybean flour, and tofu)
  • certain nuts and seeds

6. Threonine

Threonine is important in the formation of bones, cartilage, hair, teeth, and nails. It is also responsible for the growth and development of liver muscles, skeletal muscles, and the small intestines. Threonine also aids in the production of antibodies to strengthen the immune system. It is mostly found in the central nervous system and can help combat depression.

Threonine helps speed the healing of wounds and recovery from injury by helping build strong bones.

Good food sources of threonine are:

  • cottage cheese
  • milk
  • eggs
  • sesame seeds
  • beans
  • poultry
  • fish
  • meat
  • lentils
  • corn
  • various grains

7. Tryptophan

Tryptophan is a sleep-inducing amino acid important in the production of serotonin, vitamin B3 (niacin), and auxin (a plant hormone). Serotonin is the "feel-good" hormone that helps boost mood, prevents oversleeping, and helps fight depression.

Tryptophan is used to treat premenstrual syndrome and seasonal affective disorder (SAD), or the winter blues caused by the absence of sunlight during the gloomy winter months.

Research studies show that excessive tryptophan, combined with a vitamin B6 deficiency, may lead to the onset and development of bladder cancer. The metabolites kynurenic acid, acetyl-L-kynurenine, L-kynurenine, 3-hydroxy-L-kynurenine, and 3-hydroxyanthranilic acid were found in large quantity in the urine of urinary bladder cancer patients.

Tryptophan is found in these foods:

  • milk
  • chocolate
  • oats
  • bananas
  • dried dates
  • cottage cheese
  • turkey
  • peanuts

8. Methionine

Together with cysteine, methionine is one of two sulfur-containing proteinogenic amino acids. Methionine also serves as an effective antioxidant and helps in body metabolism at the cellular level. It is a perfect scavenging agent against oxidative stress, due to its ability to be converted to methionine sulfoxide. It is important because it can provide the body with the sulfur and methyl elements essential for human growth.

Methionine is used to treat diseases of the liver, especially those caused by carbon tetrachloride and arsenic. It is known to minimize the spread of the flu by inhibiting the virus' further proliferation in the body.

However, a study conducted in the 1970s showed a correlation between methionine and the growth of cancer cells. Some cancer cells, the study says, need methionine to survive.

Since animal proteins are the highest sources of this amino acid (chicken and fish especially), those interested in pursuing an anti-cancer diet may want to get their protein from plant sources.

An overdose of methionine increases the acidity of urine and causes the elimination of calcium from the body. This is the reason why this amino acid is given to dogs as a dietary supplement to protect plants from damage by animal urine.

Calcium supplementation is recommended to compensate for the lost amount of calcium in the body.

This amino acid is also used by plants for synthesis of ethylene. The process is known as the Yang, or the methionine, cycle.

Methionine can contribute to the development of atherosclerosis (fatty deposits in the arteries). Taking methionine beyond its allowable levels can increase the amount of fat in the blood and contribute to the accumulation of plaque in the arterial walls which is the main cause of atherosclerosis.

Excessive methionine intake can also cause injury and damage to the endothelial cells.

Sources of methionine include:

  • chicken
  • fish
  • milk
  • red meat
  • eggs
  • nuts
  • grains
  • beans

9. Lysine

Lysine is a key component in the production of hormones and enzymes and plays an important role in the production of collagen, a substance that is critical in bone, muscle, cartilage, and skin formation.

Lysine can be obtained by eating protein-rich foods and can also be taken as a supplement, in the form of tablets, powder, or injection.

Lysine plays a key role in calcium absorption by reducing the amount of calcium being excreted in the urine. It promotes the growth of hair, nails, teeth, and bones. It also prevents bone loss that leads to osteoporosis, though there's no evidence that lysine prevents osteoporosis. It also prevents the occurrence of herpes simplex infections or cold sores, but again, further study has to be conducted to prove this claim.

Lysine is considered safe except for a few cases of abdominal cramps and diarrhea when taken in high doses. Patients suffering from cardiovascular diseases and those under medications must consult with a physician or health practitioner before taking lysine.

Lysine should be taken by athletes who engage in strenuous physical activities for stamina and endurance. Vegetarians need a bigger amount of lysine intake, because vegetables, except for legumes, contain a very minimal amount of lysine.

Continuous research is being conducted on the potential of lysine as an important component in muscle-building, reducing cholesterol level and speeding up recovery after surgery.

Foods that are high in lysine are:

  • meat
  • fish
  • eggs
  • soybeans
  • poultry products
  • nuts
  • dairy products

Essential Amino Acids on a Gluten-Free Diet

People who eat a gluten-free diet may already be getting enough animal-based proteins to ensure an adequate supply of essential amino acids. There are ways, however, to boost proteins using plant-based, gluten-free sources.

Amaranth, quinoa, and buckwheat contain high amounts of lysine. In general, amaranth is unusually high in many essential amino acids. Also, adding legume (bean or chickpea) flour can boost protein content to make baked goods more nutritious.

Sources Cited

  1. Amino acids. (n.d.). Retrieved November 27, 2017, from https://medlineplus.gov/ency/article/002222.htm
  2. L-isoleucine. (n.d.). Retrieved November 27, 2017, from http://aminoacidstudies.org/l-isoleucine/
  3. Amino acids. (n.d.). Retrieved November 27, 2017, from https://medlineplus.gov/ency/article/002222.htm
  4. Halpern, B. C., Clark, B. R., Hardy, D. N., Halpern, R. M., & Smith, R. A. (1974, April). The effect of replacement of methionine by homocystine on survival of malignant and normal adult mammalian cells in culture. Retrieved November 30, 2017, from https://www.ncbi.nlm.nih.gov/pubmed/4524624
  5. PHENYLALANINE: Uses, Side Effects, Interactions and Warnings. (n.d.). Retrieved November 30, 2017, from https://www.webmd.com/vitamins-supplements/ingredientmono-653-phenylalanine.aspx?activeingredientid=653&activeingredientname=phenylalanine

What Are Essential Amino Acids?

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