L-Tyrosine: What It Is and How It Works

L-tyrosine is an amino acid found in protein foods and at the same time sold as a supplement for concentration and "stress resistance." Students take it before exams, athletes before grueling workouts, and also people who work night shifts. To understand what can and cannot be expected from tyrosine, our editorial team has examined what this substance is, how it works in the body and why its effect depends on the situation.

What L-tyrosine is

Tyrosine is one of the twenty proteinogenic amino acids, that is, those that make up the body's proteins. Formally it is classified as a non-essential amino acid: a healthy person's body is able to synthesize tyrosine from another amino acid - phenylalanine - with the help of the enzyme phenylalanine hydroxylase, which works mainly in the liver.

At the same time, tyrosine is called conditionally essential. In people with phenylketonuria, phenylalanine hydroxylase does not work, so tyrosine becomes a mandatory dietary component and is part of special therapeutic formulas. And if a person gets little phenylalanine from food, their own tyrosine synthesis is also limited.

The letter "L" in the name denotes the spatial form of the molecule. It is the L-form that human proteins and enzymes use, so sports and health supplements use L-tyrosine. Alongside it, the market also offers N-acetyl-L-tyrosine (NALT) - a modified form with better water solubility but, according to pharmacokinetic observations, a poorer ability to raise blood tyrosine levels, since a significant part of NALT is excreted unchanged in the urine.

The US Institute of Medicine (IOM, 2005) considers phenylalanine and tyrosine together, setting a shared requirement for them. An ordinary diet with a sufficient amount of protein usually fully covers this requirement, so there is no reason to speak of a "deficiency" of tyrosine in a healthy person with normal nutrition.

The role of tyrosine in the body

Tyrosine is interesting not only as a "building block" of proteins. It is a precursor of several key signaling molecules that affect mood, attention, the cardiovascular system and metabolism.

• Catecholamines:dopamine, noradrenaline and adrenaline - neurotransmitters and hormones responsible for motivation, attention and the "fight or flight" response.
• Thyroid hormones:thyroxine (T4) and triiodothyronine (T3) are formed from the tyrosine residues of the protein thyroglobulin and from iodine.
• Melanin:the pigment of skin, hair and eyes.
• Body proteins:tyrosine is part of structural and enzymatic proteins, and its phosphorylation is one of the main mechanisms of signal transmission in the cell.

It is precisely the link with catecholamines that explains tyrosine's popularity as a supplement. Dopamine and noradrenaline are critically important for working memory, cognitive flexibility and sustaining attention under stress. The logic is simple: give the brain more "raw material" for these neurotransmitters.

Regarding the thyroid gland it is important to understand: although the hormones are formed from tyrosine, their synthesis is limited by iodine and regulated by the pituitary gland through TSH. Additional tyrosine does not "rev up" the thyroid gland in a healthy person, but people with thyroid diseases should discuss the supplement with a doctor.

Likewise melanin: taking tyrosine does not make a tan more intense, despite the fact that similar promises sometimes appear in advertising for tanning products.

How tyrosine is converted into dopamine and noradrenaline

The path from tyrosine to catecholamines consists of several enzymatic stages. The first and key one is the conversion of tyrosine into L-DOPA by the enzyme tyrosine hydroxylase. This stage is considered rate-limiting, that is, it is precisely this that determines the speed of the whole process.

Next, L-DOPA is converted into dopamine, dopamine in the corresponding neurons into noradrenaline, and in the adrenal medulla into adrenaline. Each stage is catalyzed by a separate enzyme, whose work requires cofactors, in particular tetrahydrobiopterin, iron, vitamin B6, copper and vitamin C.

In a resting state, tyrosine hydroxylase is normally largely saturated with substrate, so additional tyrosine has little effect on catecholamine synthesis. The situation changes when neurons work intensively - for example, during acute stress, cold, sleep deprivation or grueling exertion. Under such conditions the enzyme is activated, neurotransmitter reserves are used up faster, and the availability of tyrosine can become significant (Fernstrom, Fernstrom, 2007).

It is precisely this feature that explains why in studies tyrosine usually does not improve performance in rested people but can help maintain cognitive productivity in difficult conditions. This idea is often described as "replenishing" the neurotransmitters depleted by stress (Jongkees et al., 2015).

So tyrosine is not a stimulant in the classic sense. It does not raise dopamine levels arbitrarily but only creates the conditions for systems that are already working intensively not to exhaust their resource so quickly.

How tyrosine enters the brain

To affect neurotransmitters, tyrosine must cross the blood-brain barrier. It does this with the help of the large neutral amino acid transporter (LAT1). This same transporter is also used by tryptophan, phenylalanine, leucine, isoleucine, valine and methionine.

Since all these amino acids compete for one "entrance," the delivery of tyrosine to the brain depends not only on its blood level but also on its ratio to competitors. After a protein meal, the level of all amino acids in the blood rises simultaneously, and the share of tyrosine changes little. In contrast, isolated intake of tyrosine markedly raises its share specifically.

This mechanism also explains important practical nuances. For example, the drug levodopa, used in Parkinson's disease, competes with the same amino acids for transport, so combining it with tyrosine requires consultation with a neurologist.

In studies, plasma tyrosine levels after intake rose during the first to second hour, which determined the typical timing of intake before cognitive tests or exertion (Jongkees et al., 2015). We write in more detail about the practical side in a separate article on taking tyrosine.

Why tyrosine is taken as a supplement

The main area of application of tyrosine as a supplement is supporting cognitive function under stress. Early studies, in particular military ones, showed that tyrosine can reduce the deterioration of mood and cognitive measures in cold and hypoxia (Banderet, Lieberman, 1989). In cadets during an intensive combat training course, taking tyrosine was associated with better working memory (Deijen et al., 1999).

In sport, interest in tyrosine is linked to the central fatigue hypothesis: it is assumed that during prolonged exertion, especially in the heat, changes in neurotransmitter balance in the brain contribute to fatigue. The results of studies here are contradictory - one study showed improved endurance in the heat (Tumilty et al., 2011), another did not confirm this (Watson et al., 2012).

People also take tyrosine for mood, motivation, appetite control or "thyroid support." For these uses quality evidence is either lacking or absent altogether.

It is important to understand the limits: tyrosine is not a remedy for depression, attention deficit disorder or hypothyroidism. These conditions require diagnosis and treatment prescribed by a doctor.

We present a detailed review of studies on athletes in the article on tyrosine's evidence base.

Editorial conclusions

L-tyrosine is an amino acid precursor of dopamine, noradrenaline, adrenaline, thyroid hormones and melanin. A healthy person gets it from protein food and synthesizes it from phenylalanine.

As a supplement, tyrosine works not as a stimulant but as a "reserve" for neurotransmitter systems that work intensively under stress. That is why its effect is most noticeable during sleep deprivation, cold and multitasking, and in a rested person it may be imperceptible.

The delivery of tyrosine to the brain depends on competition with other amino acids, which is important to consider when choosing the timing of intake.

The articles "The Benefits of L-Tyrosine for Athletes: The Evidence Base," "How to Take L-Tyrosine: Dosage, Timing, Duration" and "Side Effects of L-Tyrosine" will help continue the topic.

References

1. Fernstrom JD, Fernstrom MH. Tyrosine, phenylalanine, and catecholamine synthesis and function in the brain. J Nutr. 2007;137(6 Suppl 1):1539S–1547S.
2. Jongkees BJ, Hommel B, Kühn S, Colzato LS. Effect of tyrosine supplementation on clinical and healthy populations under stress or cognitive demands — a review. J Psychiatr Res. 2015;70:50–57.
3. Banderet LE, Lieberman HR. Treatment with tyrosine, a neurotransmitter precursor, reduces environmental stress in humans. Brain Res Bull. 1989;22(4):759–762.
4. Deijen JB, Wientjes CJ, Vullinghs HF, et al. Tyrosine improves cognitive performance and reduces blood pressure in cadets after one week of a combat training course. Brain Res Bull. 1999;48(2):203–209.
5. Tumilty L, Davison G, Beckmann M, Thatcher R. Oral tyrosine supplementation improves exercise capacity in the heat. Eur J Appl Physiol. 2011;111(12):2941–2950.
6. Watson P, Enever S, Page A, et al. Tyrosine supplementation does not influence the capacity to perform prolonged exercise in a warm environment. Int J Sport Nutr Exerc Metab. 2012;22(5):363–373.
7. Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: National Academies Press; 2005.