Phenylalanine is an essential amino acid, which your body needs to build proteins. Your body cannot manufacture this amino acid, so you need to get it from foods such as poultry, pork, fish, milk, yogurt, eggs, cheese, soy products, bananas, certain nuts and seeds. Aspartame, an artificial sweetener used in many diet sodas, is high in phenylalanine, according to HealthVitaminGuide.com. Phenylalanine in general is not a problem to most people and a vast majority do not report any side effects. However, people born with phenylketonuria, a rare metabolic disorder, cannot process phenylalanine and should avoid taking it.
Hypertension and Migraine
If you have PKU, or are sensitive to phenylalanine, and take more than 5000 milligrams of DL-phenylalanine a day, a lab-created mirror-image molecule of phenylalanine, you may experience nausea, heartburn and headaches, HealthVitaminGuide.com notes. You may also experience hypertension and migraine. The risk of this happening becomes especially high and serious if you are using antidepressants that are monoamine oxidase inhibitors. These medications, combined with phenylalanine, may cause a spike in your blood pressure that could lead to a heart attack or stroke, the University of Maryland Medical Center reports.
Neurological Side Effects
Children who have been exposed to high doses of DL-phenylalanine may show signs of anxiety, jitteriness and hyperactivity, reports the University of Maryland Medical Center. Phenylalanine also may worsen the symptoms of people suffering from tardive dyskinesia, a disorder that arises from taking antipsychotic drugs, such as phenytoin, valproid acid or carbamazepine, to treat schizophrenia over a long period of time. Tardive dyskinesia causes its victims to involuntarily move parts of their body such as the tongue, lips, face, trunk and limbs. In addition to this, phenylalanine may also make some of the symptoms of Parkinson's disease worse as well as interfere with the absorption of levodopa, a medication used to treat the disease, according to the University of Maryland Medical Center. At high doses, phenylalanine acts as a neurotoxin that excites the neurons of the brain to the point of cellular death, reports HealthVitaminGuide.com.
Damage to Infants
Women who have phenylketonuria and who are pregnant need to be particularly careful about ingesting phenylalanine either from food or from artificial sweeteners. High levels of phynylalanine ingested during pregnancy can cause birth defects ranging from failure of developing organs to form properly in the first trimester to faulty brain formation during the last trimester, according to a report from the Louisiana State University Health Sciences Center.