Amino acids are the smallest building blocks of protein. They are individual compounds that all have a similar chemical structure. When we eat protein, it is broken into these individual amino acids, which are then used in our body both as individual substances, and to build other proteins that our body needs to survive. There are around 20 amino acids found in protein, of which nine are classed as ‘essential’. This means that the body can’t make them itself from other amino acids, and has to get them from the protein in our diet.
Why do we need protein/amino acids?
Amino acids and protein form the basis of all human tissue – not only muscle, but also our skin, organs, bones, blood vessels, tendons and ligaments. They are vital for all growth and repair.
Protein and amino acids are also essential for:
- Making antibodies and immune chemicals, to help fight infection.
- Making enzymes, which catalyse billions of chemical reactions that happen every day in our body.
- Making haemoglobin, the substance that contains iron and transports oxygen around the body in our red blood cells.
- Making some hormones, such as thyroid hormone that governs our metabolism.
- Making brain chemicals and neurotransmitters, such as serotonin and dopamine, which contribute to mood and a feeling of happiness and ‘reward’. Some individual amino acids are used directly by the brain as neurotransmitters.
- Transporting substances across cell membranes. In the membrane of every cell sit protein molecules that allow nutrients to pass into the cell and waste substances to pass out.
- Our liver needs various amino acids to properly detoxify and cleanse our blood.
Essential, non-essential and non-protein amino acids
The nine essential amino acids (that our body cannot make itself) are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Protein foods that contain all nine essential amino acids in the right proportions for human health are said to provide ‘complete protein’, and those that are low in one or more are said to be ‘incomplete’.
Six other amino acids found in protein are said to be ‘conditionally essential’ – meaning the body needs to get them from food under certain circumstances. These are arginine, cysteine, glycine, glutamine, proline and tyrosine. The truly ‘non-essential’ amino acids found in protein are alanine, asparagine, aspartic acid, glutamic acid and serine.
In addition, the body also uses amino acids that are not found in protein. These include carnitine and taurine.
In supplements we can find all of these different categories of amino acids.
L, D and DL-forms of amino acids
You might notice that sometimes the names of amino acids are shown with the ‘L’ in front of their name and sometimes without it. The ‘L’ and ‘D’ refer to different isomers of the amino acid – which means they have the same chemical formula and number /type of atoms, but the atoms are arranged differently. In supplements, the L form is usually used, but the ‘L’ doesn’t always appear in the name; hence, ‘L-Lysine’ means the same as ‘Lysine’, and ‘L-Glutamine’ means the same as ‘Glutamine’ on a supplement label – in other words, don’t think you’re getting something different because the ‘L’ is not in the name! The only ‘D’ form commonly found in supplements is ‘DL-Phenylalanine’ which is a mixture of the D and L types. More on this below.
Supplements: amino acid blends vs. individual amino acids
Some supplements contain a broad range of amino acids – from the nine essential amino acids up to all 20. Supplements of this type tend to be used primarily as a ‘protein replacement’ – perhaps for individuals who can’t get enough protein in their diet, or find it hard to digest protein. Because the amino acids in these supplements are ‘free form’ – i.e. already broken down into individual amino acids rather than inside a complex protein structure – the body may absorb and utilise them more easily. However, they shouldn’t be used as a substitute for eating enough protein foods for a healthy person.
In contrast, single amino acid supplements are usually taken for specific reasons, relating to the roles they have in the body as individual molecules rather than as part of a protein. They can be better taken individually for these purposes because
In the next section, we have a look at some of these in more detail.
Individual amino acids
5-HTP / L-Tryptophan
Tryptophan is one of the essential amino acids. Protein foods that are a good source of tryptophan include mozzarella, Swiss and parmesan cheese, pumpkin and sesame seeds, and meats – chicken, pork, beef and lamb.
Tryptophan is used by the body to make the brain chemical serotonin. As mentioned above, serotonin contributes to mood and a feeling of happiness; it also has roles in sleep, appetite, behaviour and even tolerance to pain. To make serotonin, tryptophan is first converted to 5-HTP (5-hydroxytryptophan), and then serotonin. Hence, 5-HTP is an ‘intermediate’ substance between the two and can be more easily converted to serotonin. 5-HTP is more commonly found in supplement form than tryptophan.
Through their role in making serotonin, 5-HTP and tryptophan in supplement form may be helpful for supporting mood , anxiety , sleep , and supporting normal appetite .
Arginine is a ‘conditionally essential’ amino acid. The body can make it from other amino acids, but it may not be able to make enough during periods of growth, when arginine may become essential. Foods that are a good source of arginine include pumpkin and sesame seeds, most nuts, beef and pork, and beans and lentils.
Apart from building protein, one of the important roles of arginine is that it’s converted to a substance called nitric oxide in the body. Nitric oxide causes blood vessels to relax and dilate, supporting normal blood circulation. Because of this, arginine may help to keep blood pressure down  and generally support heart function . By supporting circulation, arginine may be helpful for male sexual function too .
Arginine may also stimulate growth hormone production , which makes it a popular choice for supporting muscle building, strength and tone .
L-Carnitine / Acetyl-L-Carnitine
Carnitine is made in the body from lysine and methionine, two of the essential amino acids. In foods, carnitine is found primarily in red meat, dairy products and fish. However, it is not a constituent of the actual protein itself, and so it is not considered among the 20 protein amino acids as described above.
Acetyl-L-carnitine is derivative of L-carnitine – in biochemical terms, it’s an ‘acetylated’ form. The different carnitines can be converted between each other in the body; however, taking the different forms may be helpful for specific purposes.
As one of its primary roles in the body, carnitine is part of an enzyme that helps to transport fats into the mitochondria (the energy-producing factories) of our cells to be converted to energy. It is highly concentrated in the muscles and the heart, which of course need a lot of energy. For this reason, taking L-carnitine may be beneficial for heart health , and it’s a popular supplement for supporting exercise performance .
Carnitine is also found in high amounts in sperm and seminal fluid , and may be helpful for supporting sperm motility .
Acetyl-L-carnitine may be more specifically helpful for supporting brain function. This may be because it is structurally similar to acetylcholine, a brain chemical strongly associated with memory and cognitive function, and/or because it provides ‘acetyl groups’ for acetylcholine production . It may also work better as an antioxidant, helping to reduce free radical damage in the brain better than standard L-carnitine .
L-Cysteine / N-Acetyl Cysteine (NAC)
Cysteine is a ‘conditionally essential’ amino acid. It can be made in the body from methionine, through several steps. It is also found in most of the same foods as methionine (see L-Methionine, below).
The best-known role of cysteine is in making a substance called glutathione in the body. Glutathione has a vital role in detoxification, in maintaining the health of our cells, and in DNA synthesis and repair. It is also used to make powerful antioxidant enzymes called glutathione peroxidase and glutathione-S-transferase, which are critical to protect our cells from free radical damage. Glutathione is made from three amino acids: cysteine, glycine and glutamine (glutamic acid). However, cysteine is often the ‘limiting’ amino acid in glutathione production, meaning it’s the one most likely to ‘run out’; for this reason, cysteine may be the most helpful of all three for supporting glutathione levels. N-acetyl cysteine, which is a derivative of cysteine, may be better than cysteine itself, and is more often used for this purpose. [36,37]
L-Glutamine is considered a conditionally essential amino acid, as it is thought to become essential at times of severe stress or trauma. Glutamine exists primarily as a free amino acid in the body, i.e. not as part of a protein. Glutamine (or glutamic acid, which is related to glutamine) is found in dairy foods, sesame seeds, nuts and red meat.
Glutamine is actually the most abundant amino acid in muscle tissue, and has a primary role in muscle tissue formation and growth . For this reason (amongst others) it can be a popular supplement with those looking to support muscle growth and recovery after exercise.
Glutamine is also thought to have the following roles [16,17]:
- Acts as a ‘fuel’ for cells lining the digestive tract. In fact, the digestive tract is said to use more glutamine than any other part of the body. For this reason, glutamine may be helpful for supporting the integrity of the gut lining.
- Plays a role in the immune response.
- Making neurotransmitters (brain chemicals). Glutamic acid, which is related to and produced from glutamine, is itself an important neurotransmitter in itself, and it can also be converted into GABA (gamma-aminobutyric acid).
- For glutathione production. As mentioned under L-Cysteine, glutathione is an important antioxidant and detoxification-supporting compound. Glutamine is one of the amino acids needed to make it, in conjunction with cysteine and glycine.
- ‘Mops up’ excess ammonia in the body – a substance that becomes toxic if allowed to build up.
Glycine is a conditionally essential amino acid. It can be produced in the body from the amino acids threonine and serine.
Glycine has a direct role as a neurotransmitter in the brain – a substance that helps to transmit messages between nerve cells. It’s actually an inhibitory neurotransmitter, which means it slows down the transmission of nerve signals, and so may help with calming . As part of this action, glycine may support sleep quality too .
Like cysteine and glutamine, glycine is used to make glutathione – an important antioxidant and detoxification-supporting compound.
Glycine has a direct role in detoxification too. The liver attaches toxins to glycine (and other amino acids) to help remove them from the body .
Lastly, glycine may help to support muscle repair. It is a primary component of collagen, which is needed for muscle and all other connective tissue in the body; and it also may enhance the release of growth hormone .
Lysine is another of the essential amino acids! Good food sources of lysine include most meats, fish and cheeses. In plant foods, the best source is soya beans, followed by other beans and pulses. Grains are particularly low in lysine, and this makes the protein in most grains an ‘incomplete’ protein.
As well as being a constituent of protein in general, lysine is important for collagen production; however, it is not clear whether supplementing lysine can directly improve collagen production.
Perhaps the most common reason for supplementing lysine is its potential to suppress the Herpes simplex virus type 1, which can cause cold sores; several human studies have found it to be successful in reducing recurrence of cold sores and reducing the severity of symptoms. It is said to be less successful against Herpes simplex type 2, which causes genital herpes [21,22,23].
Methionine is an essential amino acid too. Like many other amino acids, it’s found in high amounts in cheeses and meats. Some nuts and seeds also contain a good amount – especially sesame and pumpkin seeds and Brazil nuts. Beans and pulses are often low in methionine compared to the other amino acids, which again makes many of them an ‘incomplete’ protein: it’s important for those who get a lot of their protein from beans and pulses (e.g. vegans) to make sure they are getting enough methionine elsewhere, for example in seeds, nuts and higher-protein grains such as quinoa.
Methionine is a vital part of a process called methylation in the body. This process occurs billions of times a day in all our cells. It is necessary to help repair DNA, for production of brain chemicals such as serotonin that regulate our mood, for regulating inflammation and for detoxification of waste substances (methylation is one of the liver’s ‘phase 2’ detoxification pathways). However, methionine is not usually taken as a supplement to support methylation directly, as there are other nutrients that are more often lacking.
Methionine can help to reduce histamine levels in the body through methylation. For this reason, methionine is sometimes recommended as a supplement to help those with allergies or high histamine levels [24,25].
L-Phenylalanine / DL-Phenylalanine (DLPA)
Phenylalanine is an essential amino acid. Like most other protein amino acids, it is found in meat, fish, dairy, eggs and also in beans. L-Phenylalanine is the type found in proteins.
Phenylalanine is a precursor to the amino acid tyrosine, and so is eventually used by the body for many of the same purposes (see L-Tyrosine, below). Phenylalanine is said to be better absorbed than tyrosine , and so supplementing phenylalanine may be more useful for some people.
D-Phenylalanine, or DL-Phenylalanine (which is a synthetic mixture of the L and D types) may be helpful for reducing sensitivity to pain. It is thought to work by blocking the breakdown of endorphins and enkephalins, substances that make us feel good and reduce pain sensitivity [34,35].
Taurine is an amino acid not found in protein. It is not usually ‘essential’ in adults, as it can be made in the body from cysteine and methionine, with the help of vitamin B6. However, taurine may be essential in some circumstances, such as during prolonged stress or illness, when its production in the body may be decreased . Taurine is found primarily in meat and seafood, and is not present in plant foods.
Like glycine, taurine acts directly as an inhibitory neurotransmitter in the brain, meaning it slows down the transmission of nerve signals and so may have a sedative or calming effect .
Taurine also has essential roles in the following, amongst others: [31,32,33]
- Making bile salts, which help us to absorb fats and fat-soluble vitamins
- Protecting cells from damage (i.e. antioxidant activity)
- Aids the movement of electrolyte minerals in and out of the cells, helping to generate nerve impulses
- Helps to regulate cell volume (acts as an ‘osmolyte’ – a substance that affects osmosis)
- Eye function – it is found in high concentrations in the retina, where it is said to be critical for development of the rods and cones that perceive light.
- Kidney function.
Theanine is an amino acid found in tea leaves. Like carnitine and taurine, it’s not a protein amino acid. It is not considered essential for human health.
Theanine is said to be responsible for the relaxing effects of drinking tea. It has been found to increase alpha brainwave activity, which is associated with deep relaxation and mental alertness similar to that achieved with meditation. Theanine may also increase levels of GABA (gamma-amino butyric acid), an inhibitory or ‘relaxing’ neurotransmitter. For this reason, theanine may be used as a supplement to support calming and relaxation, but also to support alertness and mental focus too [26,27].
Tyrosine is a non-essential amino acid for most people, as the body can make it from phenylalanine (as described above). It is only known to be essential for people with a condition called phenylketonuria, who can’t make tyrosine from phenylalanine. Like most other protein amino acids, it is found in meat, fish, dairy, eggs and also in beans.
As well as being a constituent of protein, tyrosine is used to make thyroid hormones, as well as the brain chemicals/hormones dopamine, adrenaline and noradrenaline. Thyroid hormones drive our metabolism, including how quickly we burn food as fuel, our rate of digestion, and the rate and strength of our heartbeat, and have an influence on other hormone-producing glands throughout the body. Adrenaline and noradrenaline are involved in the body’s fight or flight stress response – they give us ‘get up and go’ and help us to feel motivated and energised. Dopamine contributes feelings of pleasure and enjoyment and a sense of reward, and is also involved with motivation.
Because of these roles of tyrosine, supplementation may be supportive for mood [28,30], reducing the negative effects of stress on the body [29,30], for supporting those with low thyroid function, and for supporting energy.
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