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What Is Phenylalanine?

Phenylalanine is an essential amino acid that belongs to a larger group of amino acids collectively referred to as large neutral amino acids (LNAA)1. Other amino acids that make up this group include tyrosine, leucine, valine, isoleucine and tryptophan. The LNAA designation refers to a similarity in the structure and charge of their side chains. This group of amino acids compete for the same carrier to diffuse through the blood-brain barrier. For this reason, phenylalanine should be taken on its own, if trying to raise levels in brain.

Phenylalanine serves as the precursor to tyrosine, which in turn serves as the precursor to the catecholamine’s (i.e. dopamine, norepinephrine and epinephrine), melanin and thyroid hormones1. The catecholamine’s are involved in basic nervous system activities such as movement, memory, attention, problem solving, desire, motivation, “fight or flight” response, heart rate and stroke volume, anaphylaxis and energy production via glycolysis or lipolysis3.

Where Does Phenylalanine Come From?

As phenylalanine is an essential amino acid, it is found in a large range of foods. However, foods with the highest content of phenylalanine include meat, fish, eggs, milk, cheese, nuts and pulses. In terms of phenylalanine protein content; soy, casein and whey are the best respective sources.

Phenylalanine Benefits

Because phenylalanine serves as a precursor to tyrosine, which is a precursor to thyroid hormones, it is sometimes included in supplements to stimulate thyroid activity. Thyroid hormones play a crucial role in regulating metabolism and therefore weight management. For this reason, phenylalanine is sometimes included in fat metaboliser supplements; however, tyrosine is used in the place of phenylalanine in most instances. Because phenylalanine also serves as a precursor to the catecholamines, dopamine, epinephrine and norepinephrine, it sometimes features in pre-workout supplements for its purported mood-enhancing effects; however, as with fat metabolisers, tyrosine is more commonly used in its place.

Negative Side-Effects of Phenylalanine

There are no known specific adverse side-effects from phenylalanine consumption or supplementation, other than its obvious contraindication for individuals with phenylketonuria (PKU); a genetic disease that restricts the metabolism of phenylalanine to tyrosine. In an interesting aside, dairy food scientists have recently developed a new low phenylalanine whey based protein substitute for PKU, called glycomacroprotein (GMP). It is derived from cheese whey (naturally low in phenylalanine) and it is supplemented with phenylalanine-free essential amino acids. In PKU, recent short term studies in mice and humans demonstrate improved protein retention with GMP compared with phenylalanine-free amino acids4, 5.

Phenylalanine Recommended Dosages and Timing

There are very few clinical trials (if any) where oral phenylalanine has been used to treat a condition or symptom. For this reason it is hard to provide proven recommendations on phenylalanine supplementation. Having said that, most supplements contain added phenylalanine in amounts below 1g. Whey protein typically provides between 2 and 3g of phenylalanine per 100g.

Phenylalanine Supplements

Nowadays, phenylalanine does not feature very prominently in supplements. It most commonly occurs naturally as part of protein powders.

1. Fernstrom JD & Fernstrom MH. Tyrosine, phenylalanine, and catecholamine synthesis and function in the brain. The Journal of Nutrition. 2007;15:1539S-1547S.
2. Start K. Treating phenylketonuria by a phenylalanine-free diet. Prof Care Mother Child. 1998;8(4):109-110.
3. Lord RS & Bralley JA. (2008). Laboratory Evaluations for Integrative and Functional Medicine. Metametrix Institute, Duluth, Georgia Atlanta.
4. D.M. Ney, S.T. Gleason, S.C. van Calcar, E.L. MacLeod, K.L. Nelson, M.R. Etzel, G.M. Rice, J.A. Wolff, Nutritional management of PKU with glycomacropeptide from cheese whey. J Inherit Metab Dis. 2009;32:32–39.
5. E.L. MacLeod, M.K. Clayton, S.C. van Calcar, D.M. Ney, Breakfast with glycomacropeptide compared with amino acids suppresses plasma ghrelin levels in individuals with phenylketonuria. Mol Genet Metab. 2010;100:303–308.

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