My favorite diet soda has a warning about phenylalanine. What is that? Is it bad for your health?

Answer From Katherine Zeratsky, R.D., L.D.

Phenylalanine is an amino acid, a building block of protein. Most people don't need to worry about it. But it is an issue for people who have a genetic condition called phenylketonuria (PKU). Genetic conditions are passed through families.

In people with PKU, phenylalanine can cause intellectual disability, brain damage, seizures, and skin and other problems. Intellectual disability is slow learning and skill building.

Phenylalanine is in many protein-rich foods, such as milk, eggs and meat. Phenylalanine also is sold as a dietary supplement.

The artificial sweetener aspartame (Equal, NutraSweet) has phenylalanine. Aspartame is added to many medicines, diet foods and diet sodas.

In the United States, any product that has aspartame must carry a warning about the phenylalanine. This warning helps people with PKU avoid those products.

People who don't have phenylketonuria most likely don't need to worry about phenylalanine.

Use products with aspartame with care if you:

  • Take certain medicines. These include monoamine oxidase inhibitors, neuroleptics and medicines that contain levodopa (Sinemet, Rytary, others).
  • Have the muscle movement condition called tardive dyskinesia.
  • Have trouble sleeping, anxiety or another mental health condition. Phenylalanine might make you more anxious, sad or jittery.

If you aren't sure whether to worry about phenylalanine or aspartame, talk to a member of your health care team. A blood test can show if you have phenylketonuria. A blood test also screens newborns for the condition.

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May 24, 2023 See more Expert Answers