Did you know that the Practical Handbook of Biochemistry and Molecular Biology lists over 300 different amino acids? That’s a lot of amino acids. Yet, out of all of these, the human body uses only 20 to synthesize the proteins it requires to carry out the business of living. In fact, protein is so necessary to life that it’s required for almost every single biological process, from energy production to muscle growth and the maintenance of proper immune system and brain function. Moreover, 11 of these amino acids can be produced inside the body—and are thus known as nonessential amino acids—but the 9 essential amino acids must be obtained through diet. In this article, we’re going to discuss these essential amino acids and uncover what they do and why these building blocks of protein are so necessary for our overall health and well-being.
Nonessential vs. Essential Amino Acids
Before we get into the 9 essential amino acids, it may be helpful to first make note of the 11 nonessential amino acids. These are:
- Aspartic acid
- Glutamic acid
Contrast these with the nine essential amino acids, which are:
Don’t be confused if you’ve read there were just eight essential amino acids, as experts used to think infants were the only humans who couldn’t make their own histidine. But modern research has shown that adults also rely on dietary sources of histidine, which is why it’s now considered the ninth essential amino acid.
While we certainly don’t want to add to the confusion, it’s also important to note that some sources include arginine as the tenth essential amino acid. However, most experts prefer to classify arginine as a conditionally essential amino acid, which means that it may become essential only under certain conditions, like prolonged stress, injury, or illness. Similarly, premature infants need dietary sources of arginine because their bodies aren’t yet able to produce it on their own.
What Do the 9 Essential Amino Acids Do?
The body needs an appropriate supply of all 20 amino acids—essential and nonessential—to ensure the proper operation of the vast majority of cellular processes. And this means that both essential and nonessential amino acids have a lot of responsibilities.
But since this article is devoted to the nine essential amino acids, let’s take this opportunity to delve more deeply into the many important functions performed by this particular group of amino acids.
If the name histidine seems familiar, it’s probably because this amino acid is a precursor of histamine. Although histamine is widely recognized as a critical ingredient in allergic reactions, its role in the inflammatory process is an important component of the immune response and necessary for proper immune function. Interestingly, histamine also has a part to play in sexual arousal, which explains why antihistamines can decrease sexual desire.
But creating histamine isn’t all histidine does. On the contrary, histidine is also involved in tissue repair and maintenance of the body’s pH, as well as synthesis of the protein in red blood cells that transports oxygen—otherwise known as hemoglobin. In addition, studies have shown that people with rheumatoid arthritis have lower baseline serum levels of histidine, which may indicate a role for histidine in the treatment of this autoimmune disease.
Moreover, histidine helps maintain the myelin sheaths that protect nerve cells in the central nervous system, and it protects tissues from damage by both radiation and heavy metals.
Good dietary protein sources of histidine include:
- Whole grains
- Dairy products
Isolecuine is one of the three branched-chain amino acids, or BCAAs, so-called because this group of amino acids consists of branched side chains—the shorter chains of atoms attached to the main chain, or backbone, of the molecule.
Like histidine, isoleucine is a vital component of hemoglobin. However, isoleucine also assists in blood clotting and wound healing. In addition, like the other BCAAs, isoleucine is found in its highest concentrations in muscle tissue, where it helps regulate both energy and blood sugar levels, aids in muscle repair, and assists the body in recovering from strenuous exercise.
Good food sources of isoleucine include:
- Dairy products
The second of the three BCAAs, leucine is the most abundant essential amino acid in muscle, where it promotes muscle growth and repair by assisting in protein synthesis. Like isoleucine, leucine contributes to both muscle recovery and energy production within muscle tissue, where it actually increases the number of muscle mitochondria—the energy centers of the cells.
Moreover, leucine promotes the growth and repair of bone tissue, stimulates growth hormone production and insulin secretion, and speeds wound healing.
Good dietary protein sources of leucine include:
- Dairy products
- Pumpkin seeds
Like all the other essential amino acids, lysine is required for proper protein synthesis—but it’s also needed to ensure appropriate intestinal absorption of calcium. In addition, lysine assists the immune system in producing antibodies and helps defend the body against viruses.
Lysine is also required for tissue repair and the building of collagen—a structural protein necessary for the creation of a number of different types of connective tissue, including joint cartilage, skin, and tendons. Due to its role in collagen production, lysine also helps maintain the strength of artery walls, which is crucial for the prevention of atherosclerosis.
Moreover, lysine has been shown to reduce stress-induced anxiety and diarrhea and may even protect the gut by exerting an anti-inflammatory effect on the intestinal lining.
While lysine is abundant in many animal proteins, it’s typically the limiting amino acid—the amino acid in shortest supply during protein synthesis—in plant proteins, especially those found in cereal grains. For this reason, vegetarians, and especially vegans, must be diligent when choosing dietary protein sources or opt for supplements to ensure adequate lysine intake.
Good food sources of lysine include:
- Parmesan cheese
Methionine is a sulfur-containing amino acid that occupies a unique position among the essential amino acids because, without it, the synthesis of protein never gets started. And like lysine, methionine also assists in the formation of cartilage by encouraging cells to create more cartilage tissue.
In addition, methionine helps strengthen joints by forming sulfurous chains that link together, and its anti-inflammatory, antioxidant, and analgesic properties have been shown to help ease arthritis symptoms.
Methionine is also essential for the absorption and bioavailability of the minerals selenium and zinc. Moreover, it aids in the detoxification and excretion of chemicals such as lead and mercury and helps the liver metabolize fats.
Good food sources of methionine include:
- Dairy products
- Brazil nuts
- Sesame seeds
While phenylalanine is a crucial component of the structure and function of many proteins and enzymes, its most important role is as the precursor of the amino acid tyrosine—which itself is then converted into thyroid hormones as well as a number of brain chemicals that affect mood, focus, and other facets of cognitive function. Some of these important chemicals include dopamine, epinephrine, and norepinephrine.
Phenylalanine is also found in three different forms—D-phenylalanine, L-phenylalanine, and DL-phenylalanine (DLPA). The L-form is the type used by the body for protein synthesis, while the D-form is the synthetic mirror image of the L-form and has been shown in at least some studies (results have been conflicting) to act as a painkiller. And, as you might guess, the DL-form is a mixture of the L- and D-forms.
Due mainly to its association with neurotransmitter production, phenylalanine has been suggested in some studies to play a beneficial role in depression, skin conditions like vitiligo, multiple sclerosis (MS), Parkinson's disease, attention deficit-hyperactivity disorder (ADHD), and rheumatoid arthritis. However, results continue to be mixed, though many of the purported benefits of this amino acid appear to depend on the form used.
Interestingly, about 1 out of every 10,000 to 15,000 children is born with a condition called phenylketonuria, or PKU. People with this inherited disorder are born without the gene required to produce the enzyme phenylalanine hydroxylase. Without this enzyme, levels of phenylalanine may become toxic and lead to serious health issues, including brain damage, seizures, and behavioral and developmental problems.
For individuals who weren’t born with PKU, good dietary protein sources of phenylalanine are:
- Parmesan cheese
- Roasted soybeans
- Pumpkin seeds
Threonine helps support the cardiovascular, immune, and central nervous systems and is a precursor of both serine and glycine, which are required for the creation of muscle tissue, collagen, and elastin—a protein found in elastic connective tissues like skin, blood vessels, and tendons.
Threonine also helps build robust bones and tooth enamel, assists in the metabolism of fatty acids, promotes wound healing, and aids in the prevention of fat accumulation in the liver. Moreover, it’s necessary for maintaining the integrity of gastrointestinal mucosa, which is a factor in leaky gut syndrome.
In addition, studies have found that threonine may be useful for the treatment of depression and multiple sclerosis.
Good dietary protein sources of threonine are:
- Pumpkin seeds
Tryptophan is necessary for normal growth in infants and for maintaining a balance between protein synthesis and breakdown in adults. Tryptophan is also a precursor of niacin as well as the inhibitory neurotransmitter serotonin.
Serotonin is often referred to as the happy chemical due to its role in regulating mood and promoting self-esteem. However, the majority of serotonin (about 90%) is actually produced in the digestive tract, where low levels have been linked to irritable bowel syndrome (IBS).
In addition, serotonin plays a part in cardiovascular disease, osteoporosis, and many physiological and cognitive functions, including vasoconstriction, vasodilation, memory, and learning. Low levels of serotonin are also thought to play a role in conditions as diverse as fibromyalgia, insomnia, and eating disorders.
Good sources of tryptophan in the diet are:
Like isoleucine and leucine, the third and final BCAA, valine, is necessary for tissue repair, muscle metabolism, and blood sugar control. It also assists in supporting the immune and central nervous systems as well as normal cognitive function. Moreover, valine helps regulate the body’s nitrogen balance, which is an important indicator of overall health.
Some of the best dietary protein sources of valine are:
- Whole grains
So there you have it! The nine essential amino acids and their functions. And since essential amino acids are our favorite topic, stick around our blog and explore more hot topics about these building blocks of life.
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