You may see warnings on diet soda bottles about the risks of phenylalanine. An essential amino acid used to make proteins as well as brain chemical and some hormones, phenylalanine occurs naturally in many foods, including meats, dairy products and fish. Phenylalanine poses a health risk only if you have a health disorder called phenylketonuria or if you take high doses of phenylalanine supplements.

Phenylketonuria and Phenylalanine

If you have a genetic disorder known as phenylketonuria, you should not consume phenylalanine. People with PKU lack an enzyme, phenylalanine hydroxylase, that breaks down phenylalanine into other substances the body needs and uses. If you have PKU, levels of phenylalanine build up in your body, which can cause mental retardation and developmental delays. Around one in 25,000 babies born in the United States has PKU, according to the March of Dimes. In the United States, all babies undergo testing for PKU before leaving the hospital. A special diet that limits phenylalanine can prevent damage from PKU.

Supplemental Phenylalanine

Supplemental phenylalanine may help treat vitiligo, a disease that causes patches of skin to lose their pigment. Some practitioners use phenylalanine to treat chronic pain or depression. Take supplemental phenylalanine only under your doctor's supervision, the University of Maryland Medical Center cautions. This supplement can cause a severe rise in blood pressure if taken with medications such as monoamine oxidase inhibitors, an older class of antidepressants. Phenylalanine might also interfere with levodopa, a medication taken to treat Parkinson's disease. High doses of this supplement, over 5,000 mg per day, can cause nerve damage, warns UMMC. In lower doses, nausea, headaches and heartburn may occur.

Aspartame and Phenylalanine

Aspartame, an artificial sweetener, contains phenylalanine. No evidence exists to support claims that aspartame causes a number of health problems. Studies show that aspartame does not cause cancer or nerve damage when taken in doses found in foods, according to a report published in the 2007 issue of "Critical Reviews in Toxicology" by The Burdock Group, an independent food safety and regulatory compliance consulting firm.

Pregnancy and Phenylketonuria

Around 3,000 women in the United States were treated for PKU as children and then stopped the PKU diet in childhood, following doctor's recommendations at the time. If you fit this description and get pregnant, high levels of phenylalanine could cause mental retardation or small head size, called microcephaly, in your baby. Restarting the PKU diet at least three months before getting pregnant and continuing throughout pregnancy can prevent damage from phenylalanine, the March of Dimes reports.