The 9 Essential Amino Acids and Their Importance to Your Body

Which Amino Acids Are Essential?

There are nine "essential" amino acids—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 as follows:

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

Are There Really Only Nine?

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 adult humans as well.

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

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 for 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 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?

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.

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The 22 Amino Acids

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 psychological issues or mood disorders. Not enough can lead to rheumatoid arthritis, deafness from nerve damage, chronic kidney failure, or schizophrenia.

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

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.

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 are found in the liver, adipose, and muscle tissues.

Leucine is used as a dietary supplement for body-building to enhance physical performance because it may decrease the net rate of protein degradation and improve mental and physical performance.

Leucine is also an ideal dietary supplement for people recovering from major surgical procedures, serious trauma, or extreme muscle pain due to its apparent ability to promote muscle recovery.

It also has the ability to help reduce 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 helps to protect from fatigue and regulates the body's glucose levels.

High doses 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

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 may help prevent mood swings and 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 that 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 an amino acid and natural sedative 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 thought 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.

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

High levels of methionine intake 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 helps to prevent the bone loss that leads to osteoporosis, though there's no evidence that lysine prevents osteoporosis.

It may also help prevent the occurrence of herpes simplex infections or cold sores, but again, further research 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 or kidney problems, and those under medications must consult with a physician or health practitioner before taking lysine.

Lysine can 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 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

This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

© 2011 Zee Mercado

Comments

Zee Mercado (author) on May 23, 2019:

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