Diet drinks and any other products which contain aspartame must bear the following health warning: “Phenylketonurics: Contains Phenylalanine.”
This has led many to jump to the conclusion that phenylalanine is poisonous. Why else would they put a warning about it on a label? Well, because it is dangerous to some people who have a very rare disease known as phenylketonuria.
What is Phenylketonuria (PKU)?
Phenylketonuria, or PKU for short, is an inherited condition that affects about one in 15,000 people. It is usually diagnosed at birth by a heal prick test known as the Guthrie test. People with the condition lack the enzyme, phenylalanine hydroxylase, necessary to deal with the amino acid phenylalanine.
Normally, the phenylalanine hydroxylase enzyme acts to convert phenylalanine to tyrosine. Lacking this enzyme, those with PKU can build up high levels of phenylalanine in their blood. The elevated levels of phenylalanine can cause damage to the developing brain of a child, resulting in brain damage and mental impairment. Also, the fact that the amino is not converted to tyrosine causes a lack of tyrosine in the body. Tyrosine is needed to make melanin, which is the dark brown pigment in the body (skin, eyes). PKU babies will tend to have blond hair, blue eyes, and fair skin.
PKU babies are normal at birth because the mother’s body handles the phenylalanine. They start developing symptoms when they are fed and phenylalanine enters their system. The results, down the road, can be devastating. IQ’s of less than thirty are common and they can have a lack of interest in other people, along with other cognitive and physical problems. Seizures can occur and they often suffer from bad eczema.
Once screened and detected, which usually occurs after 5 days or so, a baby with PKU must be put on a special low phenylalanine diet, which initially is a special formula made from beef serum. Later, high protein foods must be restricted, along with certain other phenylalanine-containing foods, which includes anything containing aspartame. Levels of the amino are monitored and the special diet can often be discontinued in adolescence unless high levels are detected again or symptoms are noticed. With proper diet, those with PKU can live normal lives.1
Is Phenylalanine Dangerous for Those Without PKU?
Phenylalanine poses no danger to anyone without PKU. However, in order to develop PKU, an individual must inherit the defect from both parents. Those who only inherit the defect from one parent are PKU carriers and known as PKU heterozygotes. These individuals have an impaired ability to metabolize phenylalanine but otherwise are normal. It is not recommended that PKU heterozygotes consume a special diet. However, there has been fear that aspartame could represent a particular danger to these individuals. Studies have not shown there to be any danger from aspartame for PKU heterozygotes, either short or long term.2,3
Most high protein foods contain much much more of the amino than a diet soda or other diet drink does. There is a particular concern among parents about the supposed harm of phenylalanine from aspartame in their children’s diet. Keep in mind that since children are rapidly growing, they already take in a much higher proportion of phenylalanine than adults do. For all of us, there is a significantly higher amount of phenylalanine, aspartic acid, and methanol from the rest of our diet than there is from aspartame.
See the next article in the aspartame series: Exaggerated Claims About the Harmful Effects of Aspartame
1. Jacoby, David B. Encyclopedia of Family Health. New York: Marshall Cavendish, 1998.
2. Hayes, A. W. “Artificial Sweeteners: A Special Issue of the Journal Comments on Toxicology.”
Comments on Toxicology III (1989)
3. Tulchinsky, Theodore H., and Elena Varavikova. The New Public Health. Amsterdam: Elsevier / Academic, 2009.