What is PKU?

Have you ever looked at the label on a diet soda can and seen the warning: Phenylketonurics - contains phenylalanine? I personally know of several people who refuse to drink diet soda because the word "phenylketonurics" sounds too scary! What does "phenylketonurics" mean? Is it something you can catch from diet soda?


Well, let me take the opportunity to answer these questions. "Phenylketonurics" is the term used to refer to people that have the metabolic disorder Phenylketonuria, or PKU for short. I myself, am one of these phenylketonurics and the warning on diet soda cans is included merely to inform people like me that the product contains aspartame. People that have the disorder PKU cannot consume any product that contains aspartame, for reasons which I will elaborate on below.


Two additional questions that people often ask when they learn of the true meaning of the warning label on diet soda are - Do I have PKU? How do I know that I don't have PKU? Well, if you have reached an age where you have the desire to drink diet soda and you still have the mental capacity to question whether or not you have PKU, the odds are that you don't have the disorder.


Phenylketonuria is a metabolic disorder that results when the PKU gene is inherited from both parents. When babies are born in the United States, a heel stick blood test is done to test for various disorders - PKU is one of the disorders that is tested for. It is very rare, in the US an average of 1 in 15,000 babies is born with the disorder (this number varies from region to region). If a baby is tested positive for PKU at birth, a follow up test is done a few days later. Out of 500 babies which do test positive for PKU on the first test, on average only one will actually have the disorder.


So, what is PKU? What happens to people that have it?


People with PKU have a deficiency of an enzyme which is necessary for the proper metabolism of an amino acid called Phenylalanine (i.e., the component in diet soda included in the warning). Phenylalanine is an essential amino acid and it is found in nearly all foods which contain protein: meat (of all kinds), dairy products, nuts, beans, tofu... the list goes on and on. Additionally, phenylalanine is found in aspartame, the sweetener found in most diet soft drinks and sugar-free candies/gum, hence the need for the warning on the labels of these products. Those of us that have PKU must follow a strict "low protein" diet to avoid all food sources that have high phenylalanine content. To get adequate amounts of other amino acids found in protein (without the damaging phenylalanine), people with PKU also have to take a special formula which is usually in the form of a liquid beverage.


If those of us that have PKU do not follow a strict low protein diet, phenylalanine is not adequately digested in our bodies and it builds up in our blood in the form of an acid called phenylpyruvic acid. High levels of phenylpyruvic acid damage the brain and can cause an array of side effects - mental retardation, the loss of pigmentation in the skin/hair/eyes (people with untreated PKU often have light skin, blond hair and blue eyes), a mousy odor, muscle aches and pains and in extreme cases, even seizures. Depending on the severity of the PKU, babies who were not treated for the disorder and put on the special diet at birth run the risk of suffering irreversable mental retardation within the first year of life.


PKU is certainly a detrimental disorder if not treated properly, but the prognosis isn't nearly as grim as the preceding paragraph suggests. If babies are put on the low protein diet at a young age, and they follow the diet throughout life, there is no evidence that their PKU will be a major hinderance in their accomplishments. In fact, in some cases, (myself included) people with PKU view their disorder as being a positive trait, one which has taught them to be more self-disciplined.


In the past, children were taken off the low protein PKU diet at age 5 or 6 because it was believed that at this age the brain had developed to the point where elevated levels of phenylpyruvic acid in the blood would not cause severe health problems. These children are now adults; many of them have noticed health problems and have decided, of their own accord, to return to the PKU diet. These days, most doctors that provide care for PKU patients preach that the low protein PKU diet should be a way of life and, if we value our health and well-being, those of us with the disorder should never stray from it.


One question that I often get asked by people who read this webpage - "Is phenylalanine harmful to people that don't have PKU?". The answer to this question is a great big resounding NO. Phenylalanine is an essential amino acid that is in nearly all forms of protein - meats, dairy products, eggs, nuts, legumes... etc. Some researchers claim that the amino acid phenylalanine is a natural muscle relaxer and sleep aid. & I've heard a rumor that this could be why turkey, which is a high phenylalanine food, always seems to make people sleepy (in addition to the amino acid tryptophan, which is also a natural sleep aid). For people without PKU or other metabolic disorders, it is very important to eat a balanced diet, and this certainly includes high protein (high phenylalanine) foods.


I get many emails from people saying that they have heard that phenylalanine causes cancer. I want to further emphasize the point: Phenylalanine is absolutely, 100% NOT cancerous! It is in virtually every food you eat, it is one of the 8 essential amino acids that comprise a complete protein. If you don't have PKU, you would become protein deprived and seriously sick if you never ate phenylalanine!


However, the synthesized chemical called ASPARTAME is the artificial sweetener that is used in many kinds of diet soda and other sweets. It has phenylalanine as an ingredient, but is itself a completely different chemical. Apartame can cause a wide range of side effects and is considered to be very unsafe by many people (including medical professionals). Some people complain that they get headaches, cramps, severe vomiting, seizures, etc... Indeed, when aspartame is heated it breaks down into a chemical that is comparable to wood alcohol (Don't ever drink a warm diet soda or consume any warm food that has aspartame in it!). I'm not a medical professional, and because of my PKU I can't consume products that have aspartame in them. So, I have never experienced any of these side effects and have no first hand knowledge of them. For more information, do an internet search on "aspartame toxicity" and see what you get. I'll let you judge for yourself.


Do you have additional questions about PKU that I haven't answered? Please send me an email & I will try to respond in a reasonable time frame.


Last Updated 8/05/02

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