Phenylalanine is an essential amino acid (building block of protein) that the body doesn’t produce, meaning it must be obtained through food. It is found in protein-rich foods.

Your body uses phenylalanine to make tyrosine, which helps produce hormones and neurotransmitters, including melanin, dopamine, norepinephrine, and thyroxine. These support your mood, motivation, attention, cognitive function, and metabolism. They also protect your skin and eyes from light. 

While most people need phenylalanine, those with a rare genetic disorder called phenylketonuria (PKU) have to limit their intake. Their bodies cannot break it down.

There are three different types of phenylalanine: L-phenylalanine, D-phenylalanine, and DL-phenylalanine. Each one has a slightly different chemical structure.

• L-phenylalanine: L-phenylalanine naturally occurs in protein-containing food. This amino acid is a building block for protein and the precursor to tyrosine.
• D-phenylalanine: This is the synthetic form of L-phenylalanine. It has similar uses to L-phenylalanine, although scientists don’t yet know if it has the same effects on the body as L-phenylalanine.
• DL-phenylalanine: This is a combination of 50% L-phenylalanine and 50% D-phenylalanine.

Phenylalanine can benefit your body in a number of ways. Here are some of the more common benefits:

Supports Nervous System Functioning

Phenylalanine helps your body produce important brain chemicals like dopamine, epinephrine, and norepinephrine. These chemicals, known as catecholamines, help your nervous system work properly.

Catecholamines play an important role in your body’s response to stress. They help control your blood pressure, heart rate, breathing, digestion, and urination. They also affect mood and focus, helping you stay alert.

Improves Vitiligo Symptoms

Vitiligo is an autoimmune condition that occurs when your body’s immune system mistakenly attacks melanocytes, skin pigment-producing cells. This causes patches of skin to lose color and turn milky-white. 

Phenylalanine helps your body produce melanin, the pigment that gives skin its color. Some research suggests that taking phenylalanine, combined with ultraviolet (UV) light therapy, may help treat vitiligo. It can be taken as a supplement or applied directly to your skin, typically in doses of 50-100 milligrams per kilogram of body weight. 

May Improve Symptoms of Depression

Some studies suggest that phenylalanine may help reduce symptoms of depression, though results are mixed. Part of this relationship is thought to be due to dopamine, which plays a role in feeling pleasure. Anhedonia (a lack of pleasure from life) is a common characteristic of depression.

Several studies have found that people with major depressive disorder tend to have lower levels of phenylalanine in their blood. While older studies found some benefits of taking phenylalanine for depression, newer studies have not. More research is needed to know if taking phenylalanine helps manage depression.

Phenylalanine content in foods is usually not labeled, but PKU guidelines estimate that animal and cereal proteins are about 5% phenylalanine. In other words, each gram of protein contains about 50 milligrams of phenylalanine.

The Dietary Reference Intake recommendations include total phenylalanine and tyrosine intake per day, since phenylalanine is turned into tyrosine in your body. Recommendations include:

• Infants 3–4 months: 125 milligrams per kilogram of body weight (mg/kg)
• Children age 2: 69 mg/kg
• Children age 10–12: 22 mg/kg
• Adults: 14 mg/kg

During early pregnancy, 15 milligrams per kilogram of body weight is an ideal phenylalanine intake; during late pregnancy, it is 21 milligrams per kilogram. The amount of phenylalanine most people consume from foods is generally safe, but when taking a supplement, be careful about overdoing it.

Studies show that up to 100 milligrams per kilogram of body weight of L-phenylalanine daily for three months may be safe for adults. D-phenylalanine may be safe in doses up to 1 gram per day for up to four weeks, or in a single dose of 4-10 grams. 

People with PKU, on the other hand, must greatly limit their phenylalanine intake to prevent a dangerous buildup of it in the blood. The specific amount depends on the individual’s tolerance, but most have to limit their intake to less than 500 milligrams daily.

While the specific phenylalanine content of foods can be hard to find, high-protein foods tend to be highest in phenylalanine. Good sources of phenylalanine include:

• Meat
• Chicken
• Fish
• Eggs
• Cheese
• Nuts and seeds
• Whole grains
• Soy
• Tempeh
• Legumes

Aspartame, gelatin, and plant algae like spirulina are other sources of phenylalanine.

Supplements

Phenylalanine, typically in L-phenylalanine form, is available as a dietary supplement. Other supplements, such as protein powders, usually have high amounts of phenylalanine since it is an essential amino acid.

As a result, those with PKU should avoid taking protein powders or green powders that may have added protein, gelatin, or spirulina.

When in doubt, speak with a healthcare provider before starting a new supplement, especially if you have a medical condition, are pregnant or breastfeeding, or are taking any prescription medications.

Phenylalanine is generally well-tolerated in standard dosages. The most common side effects are anxiety, constipation, headaches, insomnia, nausea, and sedation. 

Excess phenylalanine is most risky for those with PKU, whose bodies cannot convert phenylalanine to tyrosine. This causes a buildup of phenylalanine in the blood and brain, which can lead to intellectual disabilities, seizures, developmental delays, behavioral issues, and psychiatric disorders.

Phenylalanine Deficiency 

Phenylalanine is one of many amino acids found in protein-rich foods. Because of this, a lack of phenylalanine usually results from insufficient protein consumption rather than a specific deficiency.

If you’re getting enough protein in your diet, you’re also getting enough phenylalanine. However, not eating enough protein can lead to symptoms like pale, dry skin, hair loss, frequent illness, muscle loss, and weak bones. This is why following a strict diet for PKU can be challenging—phenylalanine is found in many common foods. 

Drug Interactions

Phenylalanine supplements can interact with a few types of drugs, including:

• Baclofen: Phenylalanine may reduce the absorption of this muscle relaxing medication. 
• Levodopa: Levodopa is a dopamine replacement drug for people with Parkinson’s disease. High doses of phenylalanine can reduce levodopa’s effectiveness.
• Monoamine oxidase inhibitors (MAOIs): Taking phenylalanine and MAOIs may increase your risk of a dangerous, rapid increase in blood pressure called hypertensive crisis.

If you are taking one of these medications, speak with a healthcare provider before adding supplemental phenylalanine to your diet.

Phenylalanine is an important amino acid that may benefit your nervous system and help with vitiligo. There isn’t enough proof that it has other health benefits, especially in supplement form.

People with a rare genetic disorder called phenylketonuria, or PKU, need to avoid too much of it.

Eating foods with phenylalanine is generally safe, but if you’re considering taking a supplement, it’s best to talk to your healthcare provider first.