L-Phenylalanine: A Comprehensive Reference
1. Identity: Chemical Names, Structural Properties, and Common Forms
Phenylalanine (symbol Phe or F) is an α-amino acid with the molecular formula C9H11NO2. It is one of the four aromatic amino acids and the 21 proteinogenic amino acids common to all life forms, and it is also one of the nine essential amino acids. Its three-letter code is Phe, its one-letter code is F, its codons are UUU and UUC, and its systematic IUPAC-IUB name is 2-amino-3-phenylpropanoic acid.
Phenylalanine has two enantiomers: naturally occurring L-phenylalanine and synthetic D-phenylalanine. L-phenylalanine is one of the 20 standard amino acids used to synthesize proteins in the body and is classified as a non-polar amino acid due to its hydrophobic benzyl side chain. This hydrophobicity contributes to the stability and structural integrity of protein molecules. It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. It is classified as neutral and nonpolar because of the inert and hydrophobic nature of the benzyl side chain.
Forms and Preparations
This amino acid exists in three forms: L-phenylalanine, the natural form found in food and used in protein synthesis; D-phenylalanine, a synthetic form not commonly used by the human body; and DL-phenylalanine, a combination of both forms. The L-form becomes incorporated into proteins, whereas the D-form acts as a pain reliever. L-phenylalanine occurs naturally in certain foods, whereas D-phenylalanine is a synthetic variant of L-phenylalanine.
- L-Phenylalanine: The only form of phenylalanine found in proteins; it is the biologically essential dietary form.
- D-Phenylalanine: A synthetic version of, and mirror image of, L-phenylalanine created in the laboratory. It has a different mechanism of action in the body than the L-form. D-phenylalanine slows the action of the enzymes carboxypeptidase A, endorphinase, and enkephalinase — enzymes that degrade endorphins — and slowing down these enzymes may help reduce pain.
- DL-Phenylalanine (DLPA): A 50/50 combination of L-phenylalanine and D-phenylalanine.
Supplements may contain various forms, including D-phenylalanine, L-phenylalanine, and D,L-phenylalanine. L-phenylalanine is the form naturally found in proteins and foods. D-phenylalanine and D,L-phenylalanine are made in a laboratory. Each of these forms may have slightly different effects in the body.
Commercially, L-phenylalanine is produced for medical, feed, and nutritional applications, such as aspartame, in large quantities by utilizing the bacterium Escherichia coli, which naturally produces aromatic amino acids. The quantity of L-phenylalanine produced commercially has been increased by genetically engineering E. coli, such as by altering regulatory promoters or amplifying the number of genes controlling enzymes responsible for its synthesis.
2. Natural Sources
As an essential amino acid, phenylalanine is not synthesized by animals, which must obtain it from dietary sources such as meat, dairy, eggs, and legumes. Major dietary sources of L-phenylalanine include meat, fish, eggs, cheese, and milk. Human hemoglobin, the oxygen-carrying pigment of red blood cells, is one of the richest sources of phenylalanine, yielding 9.6 percent by weight.
Bacteria, archaea, fungi, algae, some protozoans, and plants biosynthesize phenylalanine via the shikimate pathway. First isolated in 1881 from lupine seedlings, phenylalanine is one of several essential amino acids for fowls and mammals. Microorganisms synthesize it from glucose and pyruvic acid, products of the breakdown of carbohydrates.
Another common source of phenylalanine is anything sweetened with the artificial sweetener aspartame, such as diet drinks, diet foods, and medications; the metabolism of aspartame produces phenylalanine as one of the compound's metabolites.
3. History and Discovery
The first description of phenylalanine was made in 1879, when Schulze and Barbieri identified a compound with the empirical formula C9H11NO2 in yellow lupine (Lupinus luteus) seedlings. It was isolated from lupins in 1881 by the chemist Ernst Schulze (1840–1912). Isolated from lupine seedlings, phenylalanine was named for its phenyl ring — a benzene ring attached to its side chain.
In 1882, Erlenmeyer and Lipp first synthesized phenylalanine from phenylacetaldehyde, hydrogen cyanide, and ammonia. In 1901, German chemist Emil Fischer developed a method for the synthesis of phenylalanine, which helped elucidate its chemical structure.
The genetic codon for phenylalanine (UUU and UUC) was the first to be discovered. Marshall W. Nirenberg discovered that when he inserted mRNA made up of multiple uracil repeats into E. coli, the bacterium produced a new protein made up solely of repeated phenylalanine amino acids. This discovery helped to establish the nature of the coding relationship that links information stored in genomic nucleic acid with protein expression in the living cell.
Traditional and Historical Use
Unlike many herbal supplements, L-phenylalanine does not have a documented history of isolated traditional use as a phytochemical extract or botanical preparation. It is an amino acid constituent of all protein-containing foods and has thus been a universal component of human diets across all cultures and time periods. Its identification as a discrete chemical entity is entirely a product of 19th-century Western analytical chemistry. The formal study of its physiological effects and its development as a dietary supplement belong to the 20th century, emerging from biochemistry and nutritional science rather than from ethnobotanical or traditional medicine traditions.
The development of phenylalanine as a therapeutic supplement gained momentum in the 1970s and 1980s, when researchers investigated DL-phenylalanine (DLPA) for depression and pain. It is used in the manufacture of food and drink products and sold as a nutritional supplement, as it is a direct precursor to the neuromodulator phenethylamine.
4. Key Constituents, Biochemistry, and Mechanisms of Action
4.1 Primary Biochemical Roles
The daily requirement for the essential amino acid L-phenylalanine for protein synthesis is about 10 mg/kg body weight. Phenylalanine is also the precursor of the amino acid L-tyrosine, which itself is needed for the synthesis of proteins, compounds acting as neurotransmitters and hormones (adrenaline, noradrenaline, and dopamine), and melanin.
L-phenylalanine is a proteinogenic amino acid and is a component of almost all proteins, with a share of 4–5%. Around 75% of the phenylalanine introduced via food is generally required for the formation of L-tyrosine. L-tyrosine itself contributes to the normal synthesis of catecholamines and helps with the normal synthesis of dopamine.
4.2 Phenylalanine Hydroxylase and Catecholamine Pathway
The conversion of phenylalanine to tyrosine removes excess phenylalanine and supplements tyrosine supplies. The enzyme phenylalanine hydroxylase (PAH), which is responsible for this conversion, uses tetrahydrobiopterin (BH4) as a cofactor. Human phenylalanine hydroxylase (hPAH) catalyzes the hydroxylation of L-phenylalanine into L-tyrosine. The reaction is the first step in the catabolic pathway of L-Phe/L-Tyr and proceeds to feed neurotransmitter biosynthetic pathways. In non-pathological conditions, degradation of excessive L-Phe by hPAH sustains physiological plasma levels of L-Phe below 120 µM.
L-phenylalanine is biologically converted into L-tyrosine, another DNA-encoded amino acid. L-tyrosine in turn is converted into L-DOPA, which is further converted into dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline). These three are known as the catecholamines.
4.3 Blood-Brain Barrier Transport
Phenylalanine is a large, neutral amino acid (LNAA) that competes with other LNAAs for transport across the blood-brain barrier (BBB) via the large neutral amino acid transporter (LNAAT). Excessive phenylalanine in the blood saturates the transporter, effectively decreasing the levels in the brain of other LNAAs whose entry into the brain is impeded. Since these amino acids are required for protein and neurotransmitter synthesis, phenylalanine accumulation disrupts brain development in children.
4.4 Phenylethylamine Production
L-phenylalanine is a direct precursor to the neuromodulator phenethylamine. Research has concluded that the antidepressive action of L-phenylalanine combined with the MAO inhibitor L-deprenyl is based on the accumulation of phenylethylamine in the brain.
4.5 NMDA Receptor Interaction
At the glycine binding site of the NMDA receptor, L-phenylalanine has an apparent equilibrium dissociation constant (KB) of 573 µM. L-phenylalanine also inhibits neurotransmitter release at glutamatergic synapses in the hippocampus and cortex with an IC50 of 980 µM, a brain concentration seen in classical phenylketonuria.
4.6 Gut Hormone and Satiety Signaling
L-Phe is an essential amino acid abundant in dietary protein that activates the calcium-sensing receptor (CaSR), a promiscuous amino-acid-sensing receptor expressed in the gastrointestinal tract. Oral administration of L-Phe acutely reduced food intake in rats and mice, and chronically reduced food intake and body weight in diet-induced obese mice. Ileal L-Phe also reduced food intake in rats. These effects are discussed further under the appetite section below.
4.7 Melanin and Skin Pigmentation
The human epidermis has the full machinery for autocrine L-phenylalanine turnover to L-tyrosine in keratinocytes and melanocytes. Phenylalanine hydroxylase (PAH) activities increase linearly with inherited skin colour, yielding eightfold more activities in black skin compared to white skin. L-phenylalanine uptake and turnover in the pigment-forming melanocytes is vital for initiation of melanogenesis.
5. Scientific Evidence by Area of Use
5.1 Vitiligo (Skin Depigmentation)
The most extensively studied clinical application of supplemental L-phenylalanine is in the treatment of vitiligo, a depigmenting skin condition. The rationale rests on its role in melanogenesis: in vitro L-phenylalanine uptake/turnover studies on primary epidermal melanocytes from vitiligo patients demonstrated a significantly decreased calcium-dependent L-phenylalanine uptake and turnover compared to healthy control cells.
Key clinical studies: To evaluate the efficacy of L-phenylalanine (L-Phe) in combination with UVA therapy for vitiligo, an open trial (149 patients, 18 months) and a small double-blind trial (32 patients, 6 months) were conducted. Oral L-Phe loading resulted in peak plasma levels of L-Phe after 30–60 minutes. Response to L-Phe plus UVA irradiation was positive, and various grades of repigmentation not exceeding 77% in the open and 60% in the blind trial were observed. An increased L-Phe dose resulted in increased L-Phe plasma levels but not in improved clinical results. The optimal L-Phe dose appears to be lower than 50 mg/kg/day. Although it is difficult to draw firm conclusions from the present investigation, the authors suggest that L-Phe may have a place in the treatment of vitiligo and its role merits further investigation.
The administration of phenylalanine combined with UVA exposure was found to be effective in vitiligo. When orally administered in a dose of 50 mg/kg body weight, it results in an elevated plasma level. Since peak concentrations in the blood are reached between 30 and 45 minutes after ingestion, UVA exposure was administered at this time. After 4 months (32 treatments), reasonable repigmentation preferentially occurred in the skin area of subcutaneous fat.
In another study involving topical plus oral therapy, to demonstrate the effectiveness of topical and oral L-phenylalanine in combination with light plus 0.025% clobetasol propionate, an open trial was performed on a group of 70 patients with evolutive vitiligo. Participants were treated with oral (100 mg/kg/day) and topical (gel at 10%) L-phenylalanine, exposed to sunlight (spring-summer) or UVA lamps (autumn-winter), and given 0.025% clobetasol propionate at night. Overall, 90.9% of participants showed improvement, with 68.5% of patients achieving an improvement of 75% or more. This 75% improvement rate was reached 87.9% of the time on the face, 60.4% on the trunk, and 54.6% on the limbs.
Systematic review evidence: L-phenylalanine monotherapy was assessed in one trial, and as an adjuvant to phototherapy in three trials. All reported beneficial effects. However, it was not possible to pool the data from any studies for meta-analytic purposes due to the wide difference in outcome measures and poor quality of reporting. Reports investigating the efficacy of natural health products for vitiligo exist but are of poor methodological quality and contain significant reporting flaws. L-phenylalanine used with phototherapy shows promise and warrants further investigation.
Evidence strength: Moderate/preliminary. Available trials are small, methodologically limited, and largely unblinded. The combination with UVA/sunlight consistently shows benefit, but controlled data are insufficient for firm conclusions.
5.2 Depression and Mood Disorders
The mechanism of DL-phenylalanine's supposed antidepressant activity may be accounted for in part by the precursor role of L-phenylalanine in the synthesis of the neurotransmitters norepinephrine and dopamine, though clinical trials have not found an antidepressant effect from L-phenylalanine alone. Elevated brain levels of norepinephrine and dopamine are thought to have an antidepressant effect.
Key clinical studies: In an open study, DL-phenylalanine in doses from 75–200 mg/day was administered to 20 depressed patients for 20 days. At the end of the trial, 12 patients (8 with complete, 4 with good response) could be discharged without any further treatment. Four patients with partially atypical depressions experienced mild to moderate responses, whereas 4 patients did not respond at all. Depressive core symptoms such as depressed mood, retardation, and/or agitation were preferentially affected.
In one clinical trial with 155 depressed patients, a combination of L-phenylalanine (250 mg daily) and standard treatment was beneficial in 80–90% of the cases. Specifically, the antidepressive efficacy of L-deprenyl (5–10 mg daily) plus L-phenylalanine (250 mg/day) was evaluated in 155 unipolar depressed patients. Both oral and intravenous administration showed beneficial effects in 90% of outpatients and 80.5% of inpatients. It was concluded that this combined treatment has a potent antidepressive action based on the accumulation of phenylethylamine in the brain.
DLPA (150–200 mg/day) had the same effect as an antidepressant, imipramine, in 40 depressed patients. However, the authors pointed to study design flaws that may have skewed the results.
Evidence strength: Weak to preliminary. The strongest evidence supports its use for mood, though the research is limited and dated. Available studies are small, methodologically flawed, largely underpowered, and mostly conducted in the late 1970s and 1980s. No large-scale randomized controlled trials exist, and regulatory bodies have not approved phenylalanine for treating depression.
5.3 Appetite Regulation and Glycaemic Control
L-Phe is an essential amino acid abundant in dietary protein that activates the CaSR, a promiscuous amino-acid-sensing receptor expressed in the gastrointestinal tract. Research has examined whether L-phenylalanine can mediate the satiety effects associated with high-protein diets.
Preclinical evidence: Oral administration of L-Phe acutely reduced food intake in rats and mice, and chronically reduced food intake and body weight in diet-induced obese mice. Ileal L-Phe also reduced food intake in rats.
Human clinical trial: Previous studies used relatively large doses of phenylalanine (10 g), and a subsequent trial evaluated the effects of a lower dose (5 g) on gastrointestinal functions, energy intake, and postprandial blood glucose. The study evaluated the effects of intragastric administration of L-phenylalanine, at doses of 5 g (Phe-5 g) or 10 g (Phe-10 g), or control, on energy intake, plasma CCK and PYY concentrations and appetite perceptions, as well as plasma glucose, insulin, glucagon, and GLP-1 responses and gastric emptying of a mixed-nutrient drink. Both Phe-10 g and Phe-5 g stimulated insulin and glucagon. Phe-10 g, but not Phe-5 g, reduced overall plasma glucose and peak plasma glucose in response to the mixed-nutrient drink. Phenylalanine had no effect on gastric emptying of the drink. The authors concluded that the energy intake-suppressant effect of phenylalanine is related to the stimulation of CCK and PYY, while the glucoregulatory effect may be independent of stimulation of plasma GLP-1 or slowing of gastric emptying.
Evidence strength: Preliminary. Animal data are promising, and one human trial provides mechanistic insight, but the doses tested in humans (5–10 g) are far larger than typical supplement doses. This area requires further adequately powered human trials.
5.4 Pain (via DL-Phenylalanine)
The pain-related research has been conducted primarily on the D-form or the racemic DL mixture, based on the putative mechanism of endorphin preservation. D-phenylalanine is primarily studied for its potential therapeutic applications, particularly in treating chronic pain. Some research indicates that D-phenylalanine may help manage chronic pain by inhibiting an enzyme that breaks down natural pain-relieving substances in the body.
The enkephalinase inhibition theory behind D-phenylalanine is scientifically plausible but clinically unproven. Despite promising findings, substantial evidence supporting the efficacy of phenylalanine supplements remains limited, particularly for chronic pain relief.
Evidence strength: Very weak/insufficient. The available controlled trial data have not demonstrated significant analgesic effects over placebo.
5.5 Parkinson's Disease (Historical Interest)
Substantial evidence supporting the efficacy of phenylalanine supplements remains limited for conditions like Parkinson's disease. Safety concerns arise, especially for individuals with phenylketonuria and those on certain medications, as phenylalanine can interact with drugs like levodopa. No contemporary clinical trials establish benefit in Parkinson's disease, and this area is not supported by regulatory bodies.
6. Body Systems and Health Areas Associated with L-Phenylalanine
- Central Nervous System / Neurotransmitter Synthesis: Phenylalanine is needed to produce chemical messengers (neurotransmitters) in the brain, including dopamine, epinephrine, and norepinephrine, as well as other amino acids.
- Endocrine / Catecholamine System: Phenylalanine is transformed into tyrosine in the body, which is then utilized to make L-DOPA. L-DOPA is subsequently transformed into dopamine, epinephrine, and norepinephrine. These neurotransmitters have been linked to mood regulation.
- Skin / Integumentary System: L-phenylalanine uptake and turnover in the pigment-forming melanocytes is vital for initiation of melanogenesis.
- Gastrointestinal / Metabolic System: L-Phe activates the calcium-sensing receptor (CaSR), an amino-acid-sensing receptor expressed in the gastrointestinal tract, stimulating release of satiety hormones.
- Protein Synthesis / Structural Roles: L-phenylalanine is a proteinogenic amino acid and is a component of almost all proteins, with a share of 4–5%.
- Metabolic Flexibility: Like L-tyrosine, L-phenylalanine can also be converted into fats or glucose when required, and is therefore one of the glucoplastic and ketoplastic amino acids.
7. Dosage Forms and Doses Reported in Studies
L-phenylalanine is available as a free-form amino acid supplement in capsule, tablet, and powder forms. The following dosages derive strictly from reported clinical and research contexts:
- Vitiligo (oral, combined with UVA): An increased L-Phe dose resulted in increased plasma levels but not in improved clinical results. The optimal L-Phe dose appears to be lower than 50 mg/kg/day. When orally administered in a dose of 50 mg/kg body weight, it results in an elevated plasma level used in conjunction with UVA exposure.
- Vitiligo (oral + topical): Oral (100 mg/kg/day) and topical (gel at 10%) L-phenylalanine was used in combination with light and clobetasol propionate in one open trial.
- Depression (DL-phenylalanine, open study): Doses from 75–200 mg/day were administered to 20 depressed patients for 20 days.
- Depression (L-phenylalanine + L-deprenyl): L-phenylalanine at 250 mg/day in combination with L-deprenyl (5–10 mg daily) was evaluated in 155 depressed patients.
- Depression (DLPA vs. imipramine): DLPA at 150–200 mg/day was compared to the antidepressant imipramine in 40 depressed patients.
- Appetite/Glycaemia (human intragastric): Doses of 5 g (Phe-5 g) or 10 g (Phe-10 g) were administered intragastrically in a human trial evaluating energy intake, gut hormone release, and blood glucose.
- Food supplement doses (EFSA assessment): In children (10 to under 14 years), specified doses of 100, 250, 500, 750, and 1000 mg/day L-phenylalanine in food supplements are considered unlikely to cause adverse health effects.
- Estimated dietary intake (EU population): The mean dietary intake of phenylalanine in the EU population ranges from 0.4–4.1 g/day, corresponding to approximately 79.0 mg/kg body weight per day for adolescents (10–17 years) and 58.7 mg/kg body weight per day for adults.
8. Safety Considerations and Drug Interactions
8.1 Phenylketonuria (PKU) — Absolute Contraindication
The long-term safety of phenylalanine in any of its forms is not known. Both L- and D-phenylalanine must be avoided by those with the rare metabolic disease phenylketonuria (PKU). For individuals with phenylketonuria (PKU), managing phenylalanine intake is critical to prevent serious health complications. PKU is a genetic disorder where the enzyme phenylalanine hydroxylase is deficient or absent, leading to an inability to metabolize phenylalanine into tyrosine. Dangerously high blood concentrations of phenylalanine can cause severe brain damage, delayed growth, intellectual disability, and problems with the transport of other amino acids to the brain.
8.2 Maternal PKU and Pregnancy
Excessive supplementation of phenylalanine is strictly contraindicated in pregnant people with PKU. Serum levels of phenylalanine greater than 360 mmol/L are associated with an increased risk of birth defects. Those who are pregnant or breastfeeding should avoid taking phenylalanine supplements, as there is limited research on their safety and potential long-term effects.
8.3 Tolerable Upper Intake and Absence of a Formal UL
No data on adverse health effects after chronic ingestion of supplemental phenylalanine in apparently healthy subjects are available, and thus no tolerable upper intake level (UL) can be established. None of these conclusions are applicable for patients with phenylketonuria. No conclusion can be made regarding DL-phenylalanine.
8.4 Drug Interactions
- Levodopa (L-DOPA): Phenylalanine can interact with drugs like levodopa. Individuals taking medications for Parkinson's disease such as levodopa should avoid supplementing with phenylalanine. Because both phenylalanine and levodopa compete for the same large neutral amino acid transporter at the intestinal wall and blood-brain barrier, supplemental phenylalanine may reduce the absorption and efficacy of levodopa.
- MAO Inhibitors (MAOIs): L-phenylalanine is absolutely contraindicated with MAOI medications. Since phenylalanine is metabolized in part to phenylethylamine, and MAOIs inhibit the enzyme that breaks down phenylethylamine, concurrent use could result in dangerous accumulation of biogenic amines.
- Tardive Dyskinesia / Antipsychotics: L-phenylalanine may worsen tardive dyskinesia in people with schizophrenia. Special caution is needed in this case.
8.5 General Tolerability
Supplements containing phenylalanine are used for various reasons, but their benefits are not well defined. Common side effects include headache and upset stomach. People with inherited metabolic disorders should avoid phenylalanine supplements because of the toxic accumulation of this amino acid, which can lead to developmental delay, high blood pressure, and an increased risk of stroke.
8.6 Regulatory Status
The FDA has not reviewed phenylalanine for safety and effectiveness. L-phenylalanine is sold as a dietary supplement in the United States and is regulated as a food ingredient. L-phenylalanine is an important amino acid used for the synthesis of L-aspartame, the artificial sweetener. Its worldwide production has been cited at approximately 15,000 tons per year.
References
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