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Phenylalanine
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What is Phenylalanine?
Phenylalanine is an essential amino acid: Your body cannot produce it on its own, so you must obtain it from protein-rich foods. It is a building block of proteins and also a precursor to the amino acid tyrosine.¹ The name itself reveals a lot about its structure. “Phenyl” refers to a ring-shaped structure made of carbon atoms, while “alanine” refers to the simpler amino acid it is derived from. Because of this ring, phenylalanine—along with tyrosine and tryptophan—is classified as an aromatic amino acid. “Aromatic” describes its chemical structure and has nothing to do with its smell. In foods, phenylalanine occurs as L-phenylalanine.
This is the form that your body incorporates into proteins. There is also D-phenylalanine, essentially its mirror image, and DL-phenylalanine, a mixture of both forms in equal parts, which is primarily used in dietary supplements.
Phenylalanine and Tyrosine: A Well-Coordinated Team
Under normal conditions, your liver converts about three-quarters of the phenylalanine from your diet into tyrosine.¹ This process is carried out by the enzyme phenylalanine hydroxylase, which requires tetrahydrobiopterin as a cofactor. As long as you consume enough phenylalanine, your body can produce tyrosine on its own. That is why the recommended intake for both amino acids is usually listed together.²
The congenital metabolic disorder phenylketonuria (PKU) illustrates just how important this metabolic pathway is. In PKU, phenylalanine hydroxylase does not function properly, causing phenylalanine to accumulate in the blood. ³ The body then switches to an alternative pathway: it converts phenylalanine into phenylpyruvate, a so-called phenylketone that is excreted in the urine. This is where the name phenylketonuria comes from.³ You can read more about what phenylketonuria means for nutrition in the section on overdose below.
What are the functions of Phenylalanine?
Phenylalanine is a proteinogenic amino acid, which means that it is used to build proteins. It is also glucogenic and ketogenic, which is why it can be introduced into the citrate cycle to generate energy, but can also be used to synthesize ketone bodies. Together with tyrosine, phenylalanine is also the amino acid from which catecholamines are produced¹. These are adrenaline, noradrenaline and dopamine. Phenylalanine is also involved in melanin synthesis - melanin is the dark pigment of skin and hair.²
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What makes Phenylalanine unique?
Phenylalanine is important for the production of stress and performance hormones. Dopamine is one of the most important neurotransmitters of the central nervous system. Among other things, it is part of the reward system, especially the anticipatory reward system, and is also jointly responsible for performance, memory and concentration.³
How much Phenylalanine do you need per day?
The German Nutrition Society (DGE) has not yet issued a general recommendation for the daily intake of phenylalanine. The requirement for adults is probably around 14 mg phenylalanine per kilogram of body weight per day. The requirement is likely to be higher in children and adolescents who are growing, as well as in pregnant and breastfeeding women.⁶
When do you need Phenylalanine most?
During times of stress or high mental strain, your brain’s need for the neurotransmitters dopamine and norepinephrine may increase, temporarily reducing their availability.⁷ Phenylalanine is a precursor to these neurotransmitters via tyrosine.⁴ Researchers are therefore investigating whether supplementing with these precursors plays a role in such situations.
A review article analyzed studies on tyrosine: In brief, stressful situations—such as exposure to cold or when participants had to juggle multiple tasks at once—those who took supplemental tyrosine were better able to maintain their mental performance.⁷ This effect was most pronounced when neurotransmitter levels were low.
The studies were conducted using tyrosine. Because phenylalanine is a precursor to tyrosine, this area of research is also relevant to phenylalanine, although it has hardly been studied directly in this context.
How does an Phenylalanine deficiency develop and how does it manifest itself?
Serious health problems such as renal insufficiency, liver disease or cancer can lead to the body not being able to absorb or utilize phenylalanine properly. In addition, people who suffer from stress should keep an eye on their nutrient supply, as protein breakdown can be increased in them. Eating disorders, coeliac disease and Crohn's disease can also promote a deficiency. An undersupply can lead to hormonal imbalances, which can manifest themselves in fatigue and pigmentation disorders, among other things.⁸
What happens if there is an overdose of Phenylalanine?
Anyone who consumes significantly too much phenylalanine over a longer period of time can damage their kidneys⁸. Intake is particularly problematic for people who suffer from the congenital metabolic disorder phenylketonuria: They are unable to metabolize this amino acid due to a missing enzyme and must therefore follow a phenylalanine-restricted diet.⁹
Which foods are particularly high in Phenylalanine?
Phenylalanine is found in many protein-rich foods. These include poultry, dairy products, eggs, grains, legumes, nuts, and soy. The table shows how much phenylalanine is in each food.⁹ The values are based on 100 g of the food in its listed form; the serving sizes are examples.
| Food | Phenylalanine per 100 g | Example serving |
|---|---|---|
| Soybeans (dried) | 2,120 mg | 50 g: approx. 1,060 mg |
| Parmesan | 1,920 mg | 30 g: approx. 580 mg |
| Pumpkin seeds (dried) | 1,730 mg | 30 g: approx. 520 mg |
| Emmental cheese | 1,660 mg | 30 g: approx. 500 mg |
| Peanuts | 1,380 mg | 30 g: approx. 410 mg |
| Chicken breast (skinless, roasted) | 1,230 mg | 150 g: approx. 1,850 mg |
| Lentils (dried) | 1,220 mg | 60 g: approx. 730 mg |
| Tuna (canned in oil, drained) | 1,140 mg | 80 g: approx. 910 mg |
| Almonds | 1,130 mg | 30 g: approx. 340 mg |
All information is based on the studies listed in the references.
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Further blog articles on the topic
This dictionary entry is based on carefully researched sources:
Bibliography & Sources
- Flydal MI, Martinez A. Phenylalanine hydroxylase: function, structure, and regulation. IUBMB Life. 2013;65(4):341-349.
- World Health Organization. Protein and amino acid requirements in human nutrition. Report of a joint FAO/WHO/UNU expert consultation. WHO Technical Report Series 935. Genf: WHO; 2007.
- van Spronsen FJ, Blau N, Harding C, Burlina A, Longo N, Bosch AM. Phenylketonuria. Nat Rev Dis Primers. 2021;7(1):36.
- Daubner SC, Le T, Wang S. Tyrosine hydroxylase and regulation of dopamine synthesis. Arch Biochem Biophys. 2011;508(1):1-12.
- Slominski A, Tobin DJ, Shibahara S, Wortsman J. Melanin pigmentation in mammalian skin and its hormonal regulation. Physiol Rev. 2004;84(4):1155-1228.
- Berridge KC, Robinson TE. What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience? Brain Res Brain Res Rev. 1998;28(3):309-369.
Cools R, D'Esposito M. Inverted-U-shaped dopamine actions on human working memory and cognitive control. Biol Psychiatry. 2011;69(12):e113-e125. - 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.
- https://www.netdoktor.de/medikamente/phenylalanin/
- U.S. Department of Agriculture, Agricultural Research Service. FoodData Central, SR Legacy.
