The 9 Essential Amino Acids and Why Your Body Needs Them

Protein wears many hats: It's crucial for everything from building your muscles to protecting you from a pesky virus. But the macronutrient is only able to carry out these body-benefitting perks due to the amino acids it's made up of.

"Amino acids are the building blocks of proteins. Our muscle tissue is made of proteins and our bodies need 20 different amino acids to grow and function properly," Jim White, RD, CPT, tells us.

While protein is comprised of 20 amino acids, only nine amino acids are classified as "essential," according to the Harvard T.H. Chan School of Public Health. These amino acids are considered essential because our bodies can't produce them on their own so we have to obtain them via our diets.

To nourish your body with the highest quality protein it deserves, you'll want to make sure you're getting all nine essential amino acids (EAAs), which make for a complete protein. These EAAs are absolutely necessary for your body to carry out day-to-day functions, helping maintain muscle mass, a healthy immune system and strong hair and nails, Eudene Harry, MD, tells LIVESTRONG.com.

The nine essential amino acids are:

1. Histidine: Your body uses histidine to grow and repair tissues, according to the National Institutes of Health (NIH). Histidine also helps your body produce histamines, a neurotransmitter that plays a role in your immune health, gastric secretion and sexual function.
2. Leucine: Leucine helps your body regulate blood sugar, process protein and repair your muscles and bones. It is one of the three branched-chain amino acids (BCAAs).
3. Isoleucine: Isoleucine (isolated leucine) plays a big part in your body's muscle metabolism, energy and stress production. Isoleucine also stimulates your immune system. It's one of the three BCAAs.
4. Lysine: Lysine helps boost your body process and use calcium, promoting collagen development.
5. Methionine: Required for growth and tissue repair, methionine helps protect your cells from pollutants as well as slows cell aging and helps your body absorb selenium and zinc.
6. Phenylalanine: Your body converts phenylalanine to tyrosine, a non-essential amino acid that produces neurotransmitters dopamine and norepinephrine.
7. Threonine: Threonine is an important amino acid for your nervous system and helps prevent fat buildup in the liver.
8. Tryptophan: Tryptophan is converted into serotonin, a neurotransmitter that helps regulate appetite, sleep, mood and pain.
9. Valine: Valine helps your body maintain good cognitive function and muscle coordination. It's one of the three BCAAs.

Read more: Breaking Down Amino Acid Supplements: What to Know About EAAs and BCAAs

All animal-based proteins are complete proteins (foods that contain all nine essential amino acids), according to the FDA. However, there are some plant-based foods that are complete proteins, too. So whether you're vegan, plant-based or a frequent meat-eater, there are plenty of ways to incorporate complete proteins into your daily diet.

Animal-Derived Complete Proteins:

• Fish
• Poultry
• Eggs
• Dairy
• Pork
• Beef

Plant-Based Complete Proteins:

• Tofu
• Edamame
• Tempeh
• Quinoa
• Miso
• Hemp
• Buckwheat
• Chia seeds

Generally, there's no need to worry about getting enough essential amino acids in your diet, according to the Cleveland Clinic. If you eat a variety of foods each day and meet your recommended daily protein count — which is about 0.8 grams of protein per kilogram (or 2.2 pounds) of body weight, according to Harvard Health Publishing — you're probably getting all the amino acids your body needs.

If you don't eat animal products, you'll want to make sure you're eating a variety of plant-based foods to ensure you're getting all of the amino acids you need, Dr. Harry says. To get all the EAAs in one sitting, combine plant-based foods like rice and beans, whole-grain pita and hummus or peanut butter and bread. These are known as complementary proteins.

But eating one plant-based protein (such as peanut butter) for lunch and a different vegetarian protein source (such as lentils) for dinner will also ensure you're getting a complete amino acid profile. The key is to eat a variety of plant-based proteins throughout the day.

It is possible to be deficient in EAAs, however, it's not too common, according to Dr. Harry.

"Since amino acids are responsible for building protein, any process in the body that depends on this can be affected," Dr. Harry says. "Signs and symptoms are numerous and can range from loss of muscle mass, fatigue, hair loss, weak and brittle nails, premature aging, weakened immune system, poor skin quality to depression or other mood disorders."

In some cases, amino acid deficiency can occur if your body has trouble breaking down or storing amino acids for future use. This is also known as an amino acid metabolism disorder and can lead to a buildup of harmful toxins in the body over time, according to the U.S. National Library of Medicine.

These conditions are genetic, so standard newborn screening tests look for potential amino acid metabolism disorders, according to the U.S. National Library of Medicine.

Read more: 7 Popular Protein Myths Totally Busted By Science