Where to Find Phenylalanine and Side Effects

Phenylalanine in diet soda

Phenylalanine is an essential amino acid, or building block for protein. When used in excess, potentially severe phenylalanine side effects can occur, especially in people with certain health conditions.

Where Phenylalanine Is Found

In order to understand and avoid potential side effects of phenylalanine, you need to understand where the substance is found.

Natural and Synthetic

Naturally occurring phenylalanine is called L-phenylalanine. The synthetic form of phenylalanine, or man-made in a lab, is D-phenylalanine. A mixture of both types is DL-phenylalanine.

Phenylalanine is essential to your body, but you must get it from outside sources in the food you eat or supplements. Some foods containing naturally occurring phenylalanine are milk, yogurt, cottage cheese, bananas, nuts, beef and poultry.


The artificial sweetener aspartame contains phenylalanine in fairly high quantities. Any food containing aspartame or phenylalanine are required to state this on their product label: "Phenylketonurics: Contains Phenylalanine."

Aspartame is used in over 6000 food items including diet sodas and sugar free beverages, chewing gum, breath mints, low calorie yogurt and ice cream, sugar free snacks and artificial sweeteners. Some over-the-counter medications and personal care products also contain aspartame.


The recommended daily allowance of phenylalanine is approximately 15 mg per pound of body weight. People who consume many artificially sweetened beverages and foods on a daily basis may exceed the recommended daily allowance. Unwanted side effects may be the result.

Potential Side Effects

Although phenylalanine is a necessary substance for optimum health, too much of it is not a good thing. High quantities may cause adverse reactions. Negative reactions may also occur when phenylalanine is combined with certain medications. If you're considering taking phenylalanine supplements, speak to your physician and be aware of the potential side effects.

Allergic Reaction

Some people may experience allergic reaction to phenylalanine. Signs of an allergic reaction include:

  • Itching
  • Swelling of hands or feet
  • Trouble breathing
  • Mouth tingling

Anxiety, Jitteriness or Hyperactivity

Children ingesting DL-phenylalanine may experience increased anxiety, jitteriness or hyperactivity, according to National Health Institutes.

Worsening Phenylketonuria

According to the National Library of Medicine's website, phenylketonuria occurs when a person is born without the ability to break down phenylalanine appropriately. As a result, high levels of phenylalanine may occur in the body causing:

  • Delayed learning
  • Larger than normal head size
  • Hyperactivity
  • Jerking movements
  • Tremors
  • Seizures
  • Pigmentation issues in the skin and hair

To avoid worsening symptoms or acute side effects, it's critical that people with phenylketonuria avoid ingesting foods, medicines or supplements containing phenylalanine.

Increased Risk During Pregnancy

According to an abstract from the American Academy of Pediatrics (AAP), elevated levels of phenylalanine -- even mildly elevated levels -- during pregnancy may increase the risk of developmental delays and growth delays, birth defects and microencephaly. As a result, the AAP recommends women with phenylketonuria who are pregnant or considering conception should receive nutritional counseling.

Although the Food and Drug Administration states that aspartame containing phenylalanine is safe to use, the findings remain controversial. The AAP's findings may be interpreted as reason for pregnant women to avoid aspartame. If you are pregnant or lactating, talk to your doctor about the risks of using aspartame.

High Blood Pressure

According to the University of Maryland's (UMM) website, phenylalanine should not be used in combination with Monoamine Oxidase Inhibitors (MAOIs), a class of anti-depressants. Combining phenylalanine with MAOIs may cause dangerously high blood pressure, possibly leading to a stroke or heart attack.

To avoid additional adverse drug side effects, UMM also recommends avoiding phenylalanine if you are taking any of the following medications:

  • Baclofen (Lioresal)
  • Levodopa (Sinemet)
  • Anti-psychotic or neuroplectic drugs including Dilantin, Depakene, and Tegretol

Nerve Damage

UMM's website also states that phenylalanine can be toxic in doses over 5,000 mg per day and may lead to nerve damage.

Digestive Disturbances and Headache

Phenylalanine may cause digestive disturbances such as heartburn, nausea and constipation as well as headaches or dizziness, especially when taken in the form of aspartame. This may be due to the rapid increase of phenylalanine levels to the brain that occurs when high levels of aspartame are ingested.

Use Caution with Phenylalanine

Phenylalanine consumption needs to be within reasonable dosages for healthy individuals. While most people stay within this reasonable dose, if you consistently use artificial sweeteners or drink a lot of diet beverages -- especially if you regularly eat foods high in phenylalanine -- you may experience uncomfortable symptoms.

Do not take phenylalanine supplements to treat any health condition without first consulting your doctor. Be sure to discuss your risks of developing phenylalanine side effects during your consultation.

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Where to Find Phenylalanine and Side Effects