Phenylalanine

Phenylalanine is an essential amino acid, one of the building blocks of protein that the body cannot make, so food has to supply it. The 2007 report of the WHO (World Health Organization), FAO and UN University calls it nutritionally indispensable. The US National Academies (NASEM, 2005) list it as an indispensable amino acid too.

Much of it is not built into protein but converted to tyrosine. The National Academies call tyrosine conditionally indispensable, because the liver makes it from phenylalanine. Tyrosine in turn is a precursor of the catecholamine neurotransmitters (brain messengers such as dopamine), hormones and melanin skin pigments.

Almost everyone who eats enough protein gets enough. In US survey data from 1988 to 1994, the mean intake of phenylalanine from food and supplements was 3.4 g a day. The WHO sets the need for phenylalanine plus tyrosine at 25 mg per kg of body weight a day. That is about 1,750 mg for a 70 kg adult (our arithmetic).

Soybeans and chicken lead our USDA ranking per serving, and pumpkin seeds, hemp seeds and peanuts lead per 100 g.

Here is what most readers do not know. Phenylalanine sits behind one of the best-known food warnings: “contains phenylalanine” on products sweetened with aspartame. Aspartame is about half phenylalanine by weight, according to the National Academies.

The warning is for people with phenylketonuria (PKU), a rare inherited condition in which the body cannot break phenylalanine down. Newborns are screened for it with a blood phenylalanine test, using a simple method published by Robert Guthrie and Ada Susi in 1963. Treatment is a low-phenylalanine diet for life, with prescription drugs for some people.

For everyone else, we found no evidence that phenylalanine from food or from aspartame at the amounts studied does harm.

Health benefits of phenylalanine

• ★★PKU drug sapropterin lowered blood phenylalanineIn 89 people with PKU (phenylketonuria, an inherited condition), 6 weeks of sapropterin cut blood phenylalanine far more than placebo (by 236 µmol/L). In responding children it let them eat 17.7 mg/kg a day more phenylalanine than placebo.
• ★★PKU drug sepiapterin cut blood phenylalanine 63%In 98 people who responded to it, 6 weeks of sepiapterin cut blood phenylalanine by 63%, against a 1% change on placebo. Side effects were mostly mild stomach and bowel events (59% against 33% on placebo).
• ★★Stopping pegvaliase brought blood phenylalanine back upIn adults with PKU who had cut blood phenylalanine on pegvaliase, switching to placebo for 8 weeks raised it by 664.8 to 949.8 µmol/L. Those who kept taking the drug saw a rise of only 26.5.
• ★★Aspartame did not affect thinking in healthy adultsIn 48 healthy young adults, aspartame at 15 or 45 mg/kg a day for 20 days did not change thinking tests, brain waves or behavior. In 48 carriers of one PKU variant (without PKU), thinking matched placebo, and only the 45 mg/kg dose raised blood phenylalanine, within the normal range.
• ★No proven link to alertness, mood or memoryIn 2010 EFSA, the EU food safety agency, found no cause-and-effect link for L-phenylalanine claims about alertness, mood, pain relief and memory. For alertness, the 29 references given held no usable data.

Foods high in phenylalanine

Beans and poultry lead our USDA ranking per serving. One cup of soybeans (172 g) holds 1.49 g of phenylalanine. A 4 oz chicken breast follows with 1.03 g, then a chicken drumstick with skin (0.94 g) and adzuki beans (0.92 g per cup).

Per 100 g, seeds and nuts take over: pumpkin seeds have 1.73 g, hemp seeds 1.45 g and peanuts 1.38 g. Per 100 kcal, cod (0.85 g), tuna (0.83 g), shrimp (0.83 g) and chicken breast (0.76 g) lead, followed by basil and spinach. The ranking below shows the full lists.

Phenylalanine is in every protein, so animal and plant foods both supply it. The WHO says the content of each indispensable amino acid in its example proteins is considerably greater than its requirement pattern, with two exceptions. Neither exception is phenylalanine.

The WHO table lists total aromatic amino acids (phenylalanine plus tyrosine) at 243% to 314% of the pattern. That covers egg, beef, milk, soy, potato, rice, maize and wheat protein. Lysine, by comparison, is only 57% in wheat and 58% in maize. So lysine is the amino acid to plan around in grain-based diets, not phenylalanine.

In the National Academies’ table, phenylalanine plus tyrosine is 98 mg per g of protein in eggs and 96 in milk. It is 81 in whole wheat flour and 77 in navy beans. The WHO adult pattern is 38 (see the chart in the next section).

By our arithmetic, one chicken breast (1.03 g) gives 59% of the 1,750 mg WHO figure for phenylalanine plus tyrosine. One cup of soybeans (1.49 g) gives 85%. That is from phenylalanine alone, before counting any tyrosine in the same food.

For people with PKU the same list is a list of foods to limit, because protein-rich foods are the main source. A UK consensus of metabolic dietitians covers foods such as nuts, seeds, meat, fish, eggs and dried legumes. It says they are added only for people who tolerate at least 20 g a day of natural protein. The richer foods need a higher tolerance.

Products made with aspartame carry a phenylalanine warning for the same reason.

How phenylalanine works

Most of the biochemistry below comes from reviews, drug labels and tracer studies. The human findings that matter most are trials in people with PKU.

Four ideas cover it. Most phenylalanine is turned into tyrosine, and phenylalanine shares a carrier into the brain with other amino acids. The conversion fails in PKU and the treatments work around that, and almost every food protein carries more than enough of it. Here is each one, and what has been seen in people.

Most of it is turned into tyrosine

An enzyme called phenylalanine hydroxylase (PAH) does the rate-limiting step of phenylalanine breakdown, the slowest step that sets the pace. According to a 2013 review, it consumes about 75% of the phenylalanine that comes from the diet and from protein breakdown. PAH, helped by a molecule called tetrahydrobiopterin (BH4), converts phenylalanine into tyrosine.

The WHO says tyrosine depends on there being enough phenylalanine. It adds that at present it is not possible to set a specific value for how much tyrosine spares phenylalanine. That is why requirements are given for the pair.

It shares a carrier into the brain, and tyrosine is the preferred fuel

Phenylalanine is one of the large neutral amino acids, which cross into the brain on shared carriers. A review of the LAT1 carrier says it takes up essential amino acids at the blood-brain barrier and the placenta.

The best-known human effect is in Parkinson’s disease. Oral phenylalanine, leucine or isoleucine at 100 mg per kg reversed the effect of infused levodopa, as a high-protein meal did. Levodopa’s blood level did not fall, and glycine and lysine had no effect.

Phenylalanine can also feed the pathway to dopamine, but the enzyme that starts it prefers tyrosine. So unless tyrosine is abnormally low, changes in phenylalanine do not change catecholamine production, according to a 2007 review. Separately, 10 g of L-phenylalanine released the gut hormone cholecystokinin in 6 people and cut how much they ate at the next meal.

In PKU the conversion fails, and treatments work around it

PKU is caused by PAH deficiency, and most PAH gene variants lead to a misfolded, unstable enzyme. Phenylalanine then builds up in blood and brain. Left untreated, it causes severe intellectual disability, epilepsy and behavioral problems, according to the European guidelines.

Treatment tries to lower it in three ways. A phenylalanine-restricted diet reduces what goes in. Sapropterin is a synthetic form of BH4. Sepiapterin is a precursor that cells convert into BH4, so both help PAH work in people who respond.

Pegvaliase is an injected, PEGylated form of an enzyme (coated with a polymer so it lasts longer), phenylalanine ammonia lyase from Anabaena variabilis. It converts phenylalanine to trans-cinnamic acid and ammonia without PAH.

How low is low enough? The American College of Medical Genetics and Genomics (ACMG) issued a guideline in 2023. It strongly recommends keeping blood phenylalanine at 360 µmol/L or lower for life. The PRISM authors call 120 µmol/L the upper limit of normal in people without PKU.

The US label for pegvaliase is for people whose levels stay above 600 µmol/L despite existing management. Adults entering the PRISM trials averaged 1,232.7 µmol/L.

Almost every food protein carries plenty

The WHO adult pattern is 38 mg of phenylalanine plus tyrosine per g of protein. That is the 25 mg per kg requirement divided by the 0.66 g per kg protein requirement (our arithmetic). Egg, milk, whole wheat and navy bean protein all sit well above it in the National Academies’ table. This is the opposite of lysine in grains or methionine in legumes, and it is why phenylalanine is rarely the amino acid to plan around.

What has been seen in people

The firmest human findings are in PKU. Randomized trials show that sapropterin, sepiapterin and pegvaliase lower blood phenylalanine. But they measured blood phenylalanine and diet tolerance, not IQ, and the makers sponsored most of them.

Open-label results, where everyone knows the treatment, are larger. In the PRISM program, 261 adults on pegvaliase went from a mean of 1,232.7 µmol/L to 311.4 µmol/L at 24 months. Of them, 60.7% reached 360 µmol/L or lower, and 12 people had sudden whole-body allergic reactions consistent with anaphylaxis criteria.

In the AMPLIPHY crossover trial, sepiapterin lowered blood phenylalanine by 437.0 µmol/L, against 256.6 for sapropterin at its top dose. But that trial was open-label. A registry comparison found a median of 167 µmol/L after 3 years on pegvaliase. That compares with 660 on sapropterin plus diet and 1,157 on diet alone.

Why the target matters comes from observational studies. Higher blood phenylalanine goes with lower IQ in early-treated children, losing diet control early costs IQ, and high maternal levels harm the baby.

The few trials of phenylalanine pills are small. With UVA light, oral L-phenylalanine repigmented vitiligo in small trials. A 2021 review lists it among the most promising nutrition-based add-ons.

In 6 people, 10 g cut calories eaten. In a later trial, 10 g (not 5 g) lowered energy intake in 16 lean men. In 32 overweight women it had no overall effect.

We found no trial showing a benefit of phenylalanine pills in healthy people. All of these are among the study cards above, or in the sources below.

How much phenylalanine do you need?

Adults need about 25 mg of phenylalanine plus tyrosine per kg of body weight a day, according to the WHO/FAO/UNU report of 2007. That is about 1,750 mg for a 70 kg adult. The US recommended dietary allowance (the amount that covers nearly everyone) for the pair is 33 mg per kg (about 2,310 mg). EFSA has not set a value for phenylalanine.

Source	Requirement (mg per kg a day)	For a 70 kg adult	Notes
WHO/FAO/UNU 2007, adults	25	1,750 mg	Phenylalanine plus tyrosine, 38 mg per g of protein. A best estimate near the midpoint of a range of estimates that all carry considerable uncertainty.
NASEM 2005, EAR, adults 19 and older	27	1,890 mg	Phenylalanine plus tyrosine. The average of estimates ranging from 15.1 to 39 mg per kg.
NASEM 2005, RDA, adults 19 and older	33	2,310 mg	Phenylalanine plus tyrosine. Set at 1.24 times the EAR.
NASEM 2005, pregnancy	36 (EAR), 44 (RDA)	Not calculated	Phenylalanine plus tyrosine. Scaled from the nonpregnant EAR by 1.33, as there are essentially no direct data.
NASEM 2005, lactation	41 (EAR)	Not calculated	Phenylalanine plus tyrosine. Adds the amount in human milk to the nonlactating EAR.

For children, the WHO values for phenylalanine plus tyrosine fall with age. They are 59 mg per kg a day at 6 months and 40 at 1 to 2 years. They are 30 at 3 to 10 and at 11 to 14 years, 28 at 15 to 18 and 25 from age 18.

EFSA reproduced these values in its 2012 opinion on protein but did not derive reference values for individual amino acids. It said amino acids are not provided as individual nutrients but in the form of protein, and that more data are needed.

Actual intakes are well above the requirement. The mean from US survey data was 3.4 g of phenylalanine a day, about 1.9 times the WHO figure for the pair (our arithmetic). At the 99th percentile in men 31 to 50 it was 7.7 g.

Phenylalanine deficiency: signs and who is at risk

We found no report of a phenylalanine deficiency syndrome in healthy people who eat enough protein. Shortfalls would come from a very low protein intake overall. The WHO lists aromatic amino acids at well over the requirement pattern in staple proteins. The reverse problem, too much phenylalanine, is PKU, covered in the safety section.

Too little can happen during PKU treatment. The US labels for sapropterin, pegvaliase and sepiapterin all warn of low blood phenylalanine (hypophenylalaninemia). The sapropterin label says children under 7 years given 20 mg per kg a day are at increased risk. That is why blood phenylalanine is checked regularly in treatment.

Forms of phenylalanine: absorption and effects

Phenylalanine reaches you in several ways. It is bound in the protein of every food and sold as free L-phenylalanine in supplements. The D- and DL- forms are sold for mood and pain. Phenylalanine also makes up half of the sweetener aspartame. People with PKU use phenylalanine-free amino acid mixtures and a low-phenylalanine milk protein called glycomacropeptide.

The honest bottom line: no trial compares forms for a health benefit. The comparisons below measure how the body handles each form, and several are small.

Form	What it is	What studies show	Source
Phenylalanine in food protein	Bound in protein, the form in all foods	Mean intake from food and supplements was 3.4 g a day in US survey data. The National Academies found no data to set an upper limit.	1
Free L-phenylalanine	The single amino acid in its natural L-form, as capsules or powder	In vitiligo trials with UVA light, oral doses of 50 mg per kg and 100 mg per kg were used, and in one trial a higher dose raised blood levels but not repigmentation. 10 g before a meal cut calories eaten in 6 people. The National Academies report no adverse effects from single doses of 10 g in 13 men, and no data on long-term use.	1, 2, 3, 13, 14, 15
D-phenylalanine	The mirror-image form of the amino acid	10 g did not release the gut hormone CCK or cut food intake, and 250 mg four times a day did not relieve chronic pain better than placebo. EFSA found no cause and effect link with pain relief.	2, 4, 6
DL-phenylalanine	An equal mix of the D- and L-forms	At 150 to 200 mg a day it scored the same as imipramine in a trial with no placebo group. EFSA found only uncontrolled studies in depression.	5, 6
Aspartame	The methyl ester of the dipeptide aspartyl-phenylalanine, about 50% phenylalanine by weight	Blood phenylalanine rises with dose. In 48 carriers of one PKU variant, 45 mg per kg a day stayed within the normal range, and in 48 healthy adults it had no effect on thinking. US and EU rules require a phenylalanine statement on the label.	1, 7, 8, 11, 12
Phenylalanine-free amino acid mixtures and glycomacropeptide (GMP)	Protein substitutes for PKU. GMP is a milk protein with only a small amount of phenylalanine	In 30 people with PKU, blood phenylalanine control was similar over 3 weeks, and GMP was rated more acceptable. In healthy adults, amino acids from GMP and free amino acids were absorbed faster than from casein.	9, 10

The trials in the benefits section used free L-phenylalanine by mouth (vitiligo at 50 to 100 mg per kg a day, appetite at 10 g). Others used D-phenylalanine (pain), DL-phenylalanine (depression) and, in PKU, drugs rather than the amino acid itself. Here are the form-specific studies, strongest evidence first, 4 in all.

Is phenylalanine safe? Supplements, dose and interactions

Phenylalanine from food is not a concern for almost everyone. The National Academies set no tolerable upper intake level (the most you can take each day without likely harm). They said data on chronic supplemental phenylalanine in apparently healthy adults were not available.

EFSA derived no reference values for individual amino acids and found its data insufficient for an upper level for protein. We found no EFSA upper level for phenylalanine.

The National Academies list no adverse effects from single doses of L-phenylalanine as high as 10 g in 13 men. They also list dose-related rises in blood phenylalanine when adults were given aspartame. The doses were 4 to 200 mg per kg of aspartame (2 to 100 mg per kg of phenylalanine). Single doses up to 60 mg per kg of aspartame had no effect on behavior or thinking.

Phenylketonuria: the condition where phenylalanine is not safe

PKU, also called PAH deficiency, is rare. A 2026 meta-analysis puts classic PKU at 0.29 per 10,000 births worldwide, 0.58 in North America and 0.67 in Europe. The National Academies give about 1 in 15,000 US infants.

Newborns are screened with a blood phenylalanine test, and an out-of-range result is confirmed with a plasma amino acid test and genetic testing. The National Academies say irreversible brain damage can follow if phenylalanine is not restricted within 1 month of birth. Restriction is continued at least through childhood and adolescence. The ACMG guideline now recommends lifelong treatment for anyone whose untreated level is above 360 µmol/L, and genetic testing at birth.

The diet limits natural protein and adds phenylalanine-free protein supplements called medical foods, made from amino acids or glycomacropeptide. Most European recommendations rest on evidence graded C or D, the lower grades. Three prescription drugs can add to diet, and each has limits set by regulators.

Sapropterin (US approval 2007) is for BH4-responsive PKU, from age 1 month in the US label, used with a phenylalanine-restricted diet. The label says response can be found only by a therapeutic trial.

Pegvaliase (US approval 2018) carries a boxed warning for anaphylaxis and is available only through a restricted program. It requires epinephrine to be prescribed. In the 2026 US label it is for people 12 and older whose level stays above 600 µmol/L despite existing management. The EMA (the EU medicines agency) product information lists patients aged 16 and older for pegvaliase.

Sepiapterin (US approval 2025) is for sepiapterin-responsive PKU from age 1 month, with a phenylalanine-restricted diet. Treatment is run by a metabolic team.

Pregnancy is the highest-stakes case. The National Academies call maternal hyperphenylalaninemia (high phenylalanine in the mother) a recognized human teratogen, something that causes birth defects. It is linked to intellectual disability, small head size, growth delay and congenital heart defects in the baby. The ACMG guideline strongly recommends a level of 360 µmol/L or lower before conception.

People who carry one PKU variant are a different case. In a trial in 48 carriers, aspartame at 45 mg per kg a day raised blood phenylalanine only within the normal range.

Aspartame and the “contains phenylalanine” warning

Aspartame is the chemical 1-methyl N-L-α-aspartyl-L-phenylalanine, and about 50% of it by weight is phenylalanine, according to the National Academies. In the US, 21 CFR 172.804 requires the label of any food containing it to say “PHENYLKETONURICS: CONTAINS PHENYLALANINE”. In the EU, Regulation 1169/2011 requires “contains a source of phenylalanine”. When only the E number is listed, it requires “contains aspartame (a source of phenylalanine)”.

The warning is for people with PKU. For everyone else, a trial in 48 healthy adults found no effect on thinking, brain waves or behavior. They took aspartame at 45 mg per kg a day for 20 days.

In July 2023 the WHO’s expert committee on food additives (JECFA) kept its acceptable daily intake of 0 to 40 mg per kg a day. The cancer agency IARC classified aspartame as possibly carcinogenic to humans (group 2B) on limited evidence, mainly for liver cancer. By the WHO’s example, a 70 kg adult would need 9 to 14 cans of diet soda a day to pass the limit. Each can holds 200 to 300 mg of aspartame.

• Levodopa. In 9 people with Parkinson’s disease, phenylalanine at 100 mg per kg reversed the effect of infused levodopa. The US labels for sapropterin and sepiapterin list a levodopa interaction: seizures, over-stimulation or irritability may occur. Ask your doctor.
• Tardive dyskinesia. This is involuntary movement, often caused by antipsychotic drugs. In 312 psychiatric patients, men with tardive dyskinesia had higher blood phenylalanine after a 100 mg per kg test dose than men without. Women did not show this. That shows a link, not a cause, and we found no trial of phenylalanine pills in the condition.
• Doses used in trials. Trials used 50 to 100 mg per kg a day by mouth in vitiligo and 10 g once for appetite. They used 250 mg four times a day of D-phenylalanine for pain and 150 to 200 mg a day of DL-phenylalanine for depression. These are trial doses, not recommendations.
• What we did not find. We found no trial of phenylalanine pills taken with MAO inhibitors (a class of antidepressants), antipsychotics or blood pressure drugs. We found none in pregnancy, and no study of long-term use in healthy adults.
• Who should ask a doctor. People with PKU restrict phenylalanine for life, so a phenylalanine pill is the opposite of their treatment. Anyone who is pregnant, takes levodopa, or has a movement disorder or another inherited metabolic condition should also ask first.

How to get enough phenylalanine, or less if you have PKU

• Eat enough protein from varied foods. The mean US intake of 3.4 g a day is about twice the WHO figure for phenylalanine plus tyrosine (our arithmetic). One chicken breast gives 1.03 g and one cup of soybeans 1.49 g, and staple proteins carry well over the requirement pattern.
• Skip the pills unless a doctor suggests them. EFSA found no cause and effect link for mood, pain, alertness or memory. The vitiligo, appetite, pain and depression trials are small, old or negative.
• If you have PKU, follow your metabolic team’s plan. The guideline target is a blood level of 360 µmol/L or lower. The foods that supply most phenylalanine are the protein-rich ones listed above, and products with the aspartame warning add to the total.

History of phenylalanine

Phenylalanine was first found in etiolated lupin sprouts (grown in the dark) by E. Schulze and Barbieri in 1879, according to a 1914 physiological chemistry textbook. Chemists including Erlenmeyer and Fischer later made it by synthesis.

The disease came to light before the amino acid’s role did. In 1934 the Norwegian physician and biochemist Asbjørn Følling examined two children with severe intellectual disability. He proved by organic chemistry that they excreted phenylpyruvic acid in their urine, and he then found it in eight more patients.

He described a new inborn error of metabolism, later called phenylketonuria. He showed that it follows an autosomal recessive pattern and was probably caused by a block in phenylalanine metabolism. He also found that carriers could be detected by phenylalanine loading. It was the first inborn error of metabolism shown to affect the mind.

Treatment followed in 1951 in Birmingham, UK. Three hospital professionals, Evelyn Hickmans, John Gerrard and Horst Bickel, worked with a two-year-old girl named Sheila Jones and her mother Mary. Together they produced a diet and showed that it worked. Newborn screening began only after it was shown that people with PKU could be treated with a modified diet.

In 1963 Guthrie and Susi published a simple blood phenylalanine method for screening newborns in large populations.

The requirement was harder to pin down. The WHO report credits the pioneering nitrogen balance work of William Rose for early amino acid requirements. The 1973 and 1985 FAO/WHO values for phenylalanine plus tyrosine were 14 mg per kg a day.

The 2007 report set 25 after some tracer studies suggested higher values of 22 to 39 mg per kg. It noted that all the estimates carry considerable uncertainty. Drugs arrived later, with sapropterin in the US in 2007, pegvaliase in 2018 and sepiapterin in 2025.

Frequently asked questions about phenylalanine

Is phenylalanine bad for you?

Not for most people. It is an essential amino acid, and the National Academies found no data to set an upper limit for supplements. They list no adverse effects from single doses of 10 g in 13 men. The exception is PKU, where phenylalanine builds up in blood and brain. Left untreated, it causes severe intellectual disability, epilepsy and behavioral problems, according to the European guidelines. People with PKU follow a low-phenylalanine diet from birth.

What does “contains phenylalanine” on a label mean?

It is a warning for people with PKU. In the US, 21 CFR 172.804 requires foods with aspartame to say “PHENYLKETONURICS: CONTAINS PHENYLALANINE”, and EU rules require “contains a source of phenylalanine”. Aspartame is about 50% phenylalanine by weight. For others it is not a danger sign. In 48 healthy adults, 45 mg of aspartame per kg a day for 20 days did not affect thinking, brain waves or behavior.

How is PKU found and treated?

Newborns are screened with a blood phenylalanine test, and an out-of-range result is confirmed with plasma amino acids and genetic testing. Treatment is a lifelong diet low in phenylalanine, with phenylalanine-free protein supplements. The ACMG guideline aims for a blood level of 360 µmol/L or lower. Sapropterin, sepiapterin and pegvaliase can lower blood phenylalanine in some people, each with limits in its US and EU labels. A metabolic team manages all of it.

How much phenylalanine do you need a day?

The WHO sets 25 mg per kg of body weight a day for phenylalanine plus tyrosine, about 1,750 mg for a 70 kg adult. The US recommended allowance for the pair is 33 mg per kg, about 2,310 mg, with 44 mg per kg in pregnancy. The 70 kg figures are our arithmetic. Mean US intake was 3.4 g of phenylalanine a day.

Which foods are highest in phenylalanine?

Per serving, soybeans lead our USDA ranking with 1.49 g per cup. Then come chicken breast (1.03 g in 4 oz), chicken drumstick (0.94 g), adzuki beans (0.92 g) and pinto beans (0.91 g). Per 100 g, pumpkin seeds lead with 1.73 g, then hemp seeds (1.45 g), peanuts (1.38 g) and sunflower seeds (1.17 g). Per 100 kcal, cod (0.85 g) and tuna (0.83 g) lead.

Does phenylalanine help depression, pain or vitiligo?

The evidence is thin. EFSA found no cause and effect link for mood, pain relief, alertness or memory. DL-phenylalanine at 150 to 200 mg a day scored the same as imipramine in 40 depressed patients, but the trial had no placebo group. D-phenylalanine at 250 mg four times a day did not beat placebo for chronic pain in 30 people. For vitiligo, oral L-phenylalanine with UVA light repigmented skin by up to 77% in an open trial and 60% in a small blinded trial. The best dose seemed to be below 50 mg per kg a day.

What is the difference between phenylalanine and tyrosine?

Phenylalanine is essential, and the liver makes tyrosine from it. That is why the National Academies call tyrosine conditionally indispensable. Requirements are set for the pair, 25 mg per kg a day by the WHO. Tyrosine is the preferred starting material for dopamine. A Cochrane review of 3 trials in 56 people with PKU found no benefit of tyrosine supplements beyond a higher blood tyrosine level.

Is aspartame safe?

In July 2023 the WHO’s expert committee JECFA kept its acceptable daily intake of 0 to 40 mg per kg a day. The cancer agency IARC classified aspartame as possibly carcinogenic (group 2B) on limited evidence, mainly for liver cancer. By the WHO’s example, a 70 kg adult would need 9 to 14 cans of diet soda a day to pass the limit. Each can holds 200 to 300 mg of aspartame. The label warning exists for people with PKU.