Phenylalanine: All you Need to Know

Phenylalanine: All you Need to Know

In this article, we are going to talk about all you need to know about the essential amino acid L-Phenylalanine.

What is L-phenylalanine

It is an amino acid, that is, an element that makes up proteins.

It is an essential nutrient at a functional level, an indispensable amino acid since our body cannot synthesize it endogenously.

That is why we have to take it from the diet or any other form of  external administration to avoid suffering cachexia.


Figure I. Graphic representation of cachexia.

Phenylalanine deficiency

Suffering cachexia exclusively due to an L-phenylalanine deficiency is practically impossible. In fact, it has never been reported in the scientific literature, except for patients with a parenteral nutrition when there is a lack of this amino acid.

Cachexia and protein malnourishment in general occur due to a poor protein intake and not just because of one amino acid.

However, phenylalanine is not just an essential amino acid for the protein synthesis. Moreover, it also performs an important endocrine function in the catecholamine synthesis. The synthesis of its chemical structure results in a very interesting property when compared to other amino acids.

Do you want to learn more about L-phenylalanine? Then, keep reading!

Where can I buy phenylalanine?

L-phenylalanine is practically available in any dietary source of protein:

  • Meat (beef, poultry, pork).
  • Fish.
  • Milk and dairy products.
  • Eggs.
  • Soy products (protein, flour and tofu).
  • Some nuts have small concentrations too.

Aspartame and phenylalanine

Something interesting is that aspartame is a dietary source of L-phenylalanine.

Evidently, it is significantly available in developed and industrial countries (specially the United States, where the aspartame intake is very high).

Aspartame Breakdown

Figure II. Breakdown of the chemical structure of aspartame, a dipeptide methyl ester.

Still, an adult will take 5g of L-phenylalanine a day on average. Although the recommended daily amount for an isoenergetic diet for an adult is 8g a day (HDMB, s.f.)

Despite the availability of phenylalanine in common products, it seems that our intake is not enough according the the established amino acid proportions.

Amino acid score

Figure III. Recommended amino acid intake (g/kg/day) according to age.

We have to bear in mind that the recommended amount of L-phenylalanine are listed as AAA (aromatic amino acids) or L-phenylalanine + L-tyrosine. The latter amino acid can actually partially replace the dietary requirements of hydroxy-L-phenylalanine.

Phenylalanine conversion process

Figure IV. Graphic representation of the conversion process (hydroxylation) of L-phenylalanine to L-tyrosine.

What is L-phenylalanine used for

Muscle protein synthesis

Any amino acid that belongs to the proteinogenic amino acids that encode our genome can and will be used to synthesize proteins in the  tissues, including the muscle tissue.

Twenty common amino acids

Figure V. Graphic representation of 20 out of the 22 proteinogenic amino acids.

From these, the essential amino acids are specially sensitive for this process.

While the rest can be synthesized from these ones, but essential amino acids cannot be created “de novo”.

So we have to take them from an external source or we will suffer a catabolic process in order to obtain them from our own proteins.

Non-essential and essential amino acids

Figure VI. Graphic representation of the functional categorization of amino acids into essential/non-essential.

That is why it is crucial to make sure we take all the essential amino acids. You can look at the previous chart with the recommended daily amount of these amino acids and see if your diet is meeting your requirements.

If there is barely any variety of foods and a bad nutrient quality, we will have to take more essential amino acids.

Catecholamine synthesis

L-phenylalanine is amino acid that sets the catecholamine synthesis into motion.

If we are being strictly academic, the amino acid that starts this process is L-tyrosine. But since phenylalanine becomes tyrosine, we can say that this it really the “first functional step”.

This process takes place in the liver, through the effect of phenylalanine hydroxylase.

Catecholamine synthesis

Figure VII. Catecholamine synthesis.

From then on…

Tyrosine goes to the dopaminergic and noradrenergic neurons of the central nervous system. As well as to the sympathetic nerves, the pheochromocytes from the adrenal medulla and the paraganglia in the periphery to synthesize L-dopa. In turn, the latter continues to produce dopamine and this one will produce epinephrine and norepinephrine.

All this process has important neurometabolic implications that are specific to each hormone that has been synthesized. In other words, dopamine plays a role in the limbic system and other cortical or extracortical areas that regulate the emotions.

Dopamine and serotonin production

Figure VIII. Simplified graphic representation of the dopamine and serotonin production mechanism and its main effects.

While the functions of adrenaline and noradrenaline focus on stimulating the sympathetic nervous system, mediated by adrenergic receptors that regulate the contraction/relaxation of the smooth muscle, increasing the lipolysis, activating the RAAs system, etc.


Figure IX. Main effects of the agonism to alpha and beta-adrenergic receptors in different tissues.

That is the reason why L-phenylalanine plays a role on processes like the lipolysis (first step before “burning fat”). Part of it is used to synthesized the hormones in charge of this effect.

The evidence is a little bit problematic, mainly because tyrosine hydroxylase is the hormone that limits the catecholamin synthesis. Moreover, its activity will not be affected after taking a precursor.

Still, interventions like the ones by Ueda et al. (2017) show a sharp increase in the glycerol plasma concentrations. This is proof of the intracellular triglyceride hydrolysis from the adipocyte lipid droplet.

We need more studies, but phenylalanine can be a useful pre-workout supplement, specially during cutting phase and hypophagia.

Satiety regulator

Perhaps the most curious and interesting property of this amino acid:

Phenylalanine is a highly satiating nutrient.

This is due to its effect on the cholecystokinin secretion (CKK), a peptide hormone that is produced in the duodenum and which regulates certain aspects of the digestion. For example, it regulates the gastric emptying.

Due to this effect, cholecystokinin has been regarded as an effective appetite suppressant (Johnson, 2013). It is the hormone that blocks the effect of ghrelin more than leptin.

The production of cholecystokinin is what produces a feeling of satiety, even if we eat just lettuce. It actually shares mechanism with the PYY.

In addition, the cholecystokinin release is extremely sensitive to the fatty acid and amino acid concentrations present in the chyme.

Aromatic amino acids (like phenylalanine, tyrosine or tryptophan) are more effective when it comes to increasing the CKK release (Liddle, 1994). From all of them, phenylalanine seems to be the most effective even if we still do not fully understand its mechanisms.

Still, it has proven that it can regulate the intracellular calcium concentrations in the enterocytes in animals. More importantly, it has an anorexygen effect in humans.

Taking 10g of L-phenylalanine 20 minutes before a meal ad libitum reduced the intake in a 32% in humans.


Effects of L-phenylalanine on calorie intake

Figure X. Effects on the calorie intake after taking L-phenylalanine (L-PA), D-phenylalanine (D-PA) and Placebo in an ad libitum intervention.

The group that took L-phenylalanine before a meal was satiated much sooner than those who took D-phenylalanine or placebo. Therefore, their calorie intake was way lower.

This effect took place thanks to the changes in the CKK release in the L-phenylalanine group. However, against all logic, the group that took D-phenylalanine did not experience the same changes.

CKK release

Figure XI. CKK release curve after taking L-phenylalanine (crosses) and D-phenylalanine (diamonds).

As we know, the chirality and stereo-isometric configuration of amino acids can affect their action mechanism, pharmacokinetics and pharmacodynamics.

L-phenylalanine can be a useful anorexigenic agent for those who want to control their calorie intake. Specially when we talk about people with obesity and/or diabetes with central resistance to leptin.

L-Phenylalanine by HSN

In HSN we have done a screening of the scientific literature about the use of L-phenylalanine. We have concluded that it is a nutrient that has been studied in depth and it has a strong evidence in humans.

That is why we have decided to come up with a food supplement made from L-phenylalanine.

Following our traditional HSNraw philosophy: a highly pure product without additives that may alter the content of the food supplement.

100% Vegan

In the ingredient list of our phenylalanine you will find:

L-phenylalanine (produced through a vegetable fermentation process). And that’s it!

It has been “produced through a vegetable fermentation process” because we strongly believe that supplements should be available for everyone, regardless of their condition or beliefs.

That is why you will find the following section in our products:

Remarkable features

Figure XII. “Remarkable Features” in HSN, lack of allergens and suitable for vegetarians and vegans.

Which means:

Lack of allergens

We always work to select high quality raw ingredients to make products that will not have this type of compounds.

Suitable for vegetarians and vegans

For a while now, we have decided to reject amino acids that come from animal sources. We only used raw ingredients from plant sources.

The highest purity from plants! What else do you want?


  1. Ballinger, A. B., & Clark, M. L. (1994). l-Phenylalanine releases cholecystokinin (CCK) and is associated with reduced food intake in humans: Evidence for a physiological role of CCK in control of eating. Metabolism, 43(6), 735–738.
  2. EFSA Panel on Dietetic Products, N. and A. (NDA). (2010). Scientific Opinion on the substantiation of health claims related to L phenylalanine and increased alertness (ID 708, 1629), enhancement of mood (ID 657), pain relief (ID 657) and improvement of memory (ID 658) pursuant to Article 13(1) of Regulation (EC) No 1924/2006. EFSA Journal, 8(10), 1748.
  3. Konturek, S. J., Radecki, T., Thor, P., & Dembinski, A. (1973). Release of Cholecystokinin by Amino Acids. Proceedings of the Society for Experimental Biology and Medicine, 143(2), 305–309.
  4. Matthews, D. E. (2007). An Overview of Phenylalanine and Tyrosine Kinetics in Humans. The Journal of Nutrition, 137(6), 1549S-1555S.
  5. National Center for Biotechnology Information. PubChem Database. Phenylalanine, CID=6140.
  6. National Center for Biotechnology Information. PubChem Database. Cholecystokinin, CID=16129670.
  7. Pacak, K., Timmers, H. J. L. M., & Eisenhofer, G. (2015). Pheochromocytoma. In J. L. Jameson, L. J. De Groot, D. M. de Kretser, L. C. Giudice, A. B. Grossman, S. Melmed, … G. C. B. T.-E. A. and P. (Seventh E. Weir (Eds.), Endocrinology: Adult and Pediatric (Vol. 2–2, pp. 1902-1930.e6).
  8. Pohle-Krauza, R. J., Navia, J. L., Madore, E. Y. M., Nyrop, J. E., & Pelkman, C. L. (2008). Effects of l-phenylalanine on energy intake in overweight and obese women: Interactions with dietary restraint status. Appetite, 51(1), 111–119.
  9. Wang, Y., Chandra, R., Samsa, L. A., Gooch, B., Fee, B. E., Michael Cook, J., … Liddle, R. A. (2011). Amino acids stimulate cholecystokinin release through the Ca2+-sensing receptor. American Journal of Physiology – Gastrointestinal and Liver Physiology, 300(4), G528-37.

Related Entries

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About Alfredo Valdés
Alfredo Valdés
A specialist in Pathophysiology and biomolecular effects on nutrition and sportive activity who will show you the elaborate world of sports nutrition in his articles, employing a simple and critical writing.
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