Phenylalanine in diet soda: Is it harmful?

Date Updated: 12/23/2020

Answer Section

Phenylalanine isn't a health concern for most people. However, for people who have the genetic disorder phenylketonuria (PKU) or certain other health conditions phenylalanine can be a serious health concern.

Phenylalanine can cause intellectual disabilities, brain damage, seizures and other problems in people with PKU. Phenylalanine occurs naturally in many protein-rich foods, such as milk, eggs and meat. Phenylalanine is also sold as a dietary supplement.

The artificial sweetener aspartame (Equal, NutraSweet), which is added to many medications, diet foods and diet sodas, contains phenylalanine.

Federal regulations require that any beverage or food that contains aspartame bear this warning: "Phenylketonurics: Contains phenylalanine." This warning helps people with PKU avoid products that are a source of phenylalanine.

If you don't have PKU, you probably don't need to worry about harmful health effects of phenylalanine — with certain important exceptions. Aspartame in large doses can cause a rapid increase in brain levels of phenylalanine. Because of this, use products with aspartame cautiously if you:

Take certain medications, such as monoamine oxidase inhibitors, neuroleptics or medications that contain levodopa (Sinemet, Rytary, others)
Have the muscle movement disorder tardive dyskinesia
Have a sleep disorder, anxiety disorder or other mental health condition; phenylalanine may worsen feelings of anxiety and jitteriness

If you aren't sure if phenylalanine or aspartame is a concern for you, talk to your doctor. A blood test to determine if you have PKU is available and is now routinely done as part of newborn screening.