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The main treatment for PKU is a rigorous, regimented diet with very limited levels of phenylalanine. This special diet can prevent mental retardation if it's started just after birth. Doctors formerly recommended discontinuing the diet in adolescence, but today, doctors recommend sticking to the diet for life.

The amount of phenylalanine allowed in the PKU diet differs for each individual, depending on age, height, weight and a person's ability to metabolize this amino acid. The "right" amount is determined by a doctor or nutritionist through regular checking of diet records, growth charts and blood phenylalanine levels, and it may change frequently, especially during childhood growth spurts and pregnancy. In general, the idea is to supply only the amount of phenylalanine that's necessary for normal growth and body processes, but no more. In people with PKU, this tends to be about 200 to 500 milligrams of phenylalanine a day — less than 10 percent of the normal intake.

Because this number is so low, it's crucial for people with PKU to avoid all high-protein foods, which are naturally made up of amino acids, including phenylalanine. This means no milk, ice cream, nuts, beans, eggs, chicken, steak or fish. Foods and medications made with aspartame (NutraSweet, Equal) are another big no-no, because this artificial sweetener contains a large amount of phenylalanine.

However, eating a low-protein diet and avoiding diet soda isn't good enough. The amount of phenylalanine allowed in the PKU diet is so small that it usually limits regular pasta, rice, bread, cookies, and even some fruits and vegetables. For instance, one ear of corn on the cob contains more than 200 milligrams of phenylalanine — which can be nearly half or all of a person's allotment for the day. Even approved foods add up. To stick within the recommended rations, a person with PKU can't gorge on big plates of lower-phenylalanine foods such as potatoes, apples, cranberries and certain cereals. The amount of phenylalanine in all foods must be considered when planning the diet.

Focus on formula

In fact, the major source of calories and nutrients in the PKU diet is actually a synthetic formula, which is high in protein and essential nutrients but contains little or no phenylalanine. Babies with PKU are promptly put on a phenylalanine-free formula just days after birth. Parents introduce solid foods with low levels of phenylalanine to children with PKU on the same schedule used for normal infants. But older children and adults still drink several glasses of formula each day, as directed by a doctor or dietitian. The formula for older kids and adults is not the same as the one used for infants, but it works on the same principle. It acts as a nutritional substitute for forbidden foods and is continued for life.

This focus on formula and the limited list of allowed foods certainly make the PKU diet challenging. It's unlike diets for other diseases that center on making "smart choices" from a wide range of regular foods or making simple substitutions for outlawed ingredients. Still, the PKU diet is the only way to prevent mental retardation and serious problems in kids with PKU. So families need to commit to it whole-heartedly, realizing that it may be difficult but not impossible.

Applying the PKU diet

If you or your child is "on-diet," you'll need to keep records of the amount of phenylalanine eaten every day to be sure you're sticking to the specific, individualized dietary guidelines recommended by your dietitian. To do so, use a book or computer program that lists the amount of phenylalanine in baby foods, solid foods, PKU formulas, and common baking and cooking ingredients. To be as accurate as possible, measure portions, using standard measuring cups and spoons and a kitchen scale that reads in grams.

You also may want to invest in some of the many low-protein products available through specialty food retailers. You'll find low-protein rice, pasta, pizza crusts, tortillas, bagels, breads, cookie dough and baking mixes, as well as egg replacers and imitation cheeses. These products provide some dietary variety, and they allow people with PKU to eat lunches and dinners that more closely resemble what everyone else is eating. Like the PKU formulas, these products can be extremely expensive, but you might consider splurging on a few favorites with the money you save on dairy and meat products.

Whether you buy special low-protein products or not, you can also be creative with foods you can find at your local grocery store. Here are some ideas for serving up three square meals with low amounts of phenylalanine:

• Breakfast. In the morning, dish up lower-phenylalanine cereals, such as corn flakes or puffed rice, swimming in nondairy creamer diluted with water. Or whip up a smoothie by blending fresh or frozen fruit with ice cubes, fruit juice, and almond or vanilla flavoring.

• Lunch. Pack a lunch bag with small portions of rice cakes, grapes, applesauce, lemonade and jellybeans. Or slurp up a low-protein pumpkin or vegetable soup.

• Dinner. Veggie stir-fries and salads make great evening meals. So do vegetable shish kabobs, soaked in a tangy vinaigrette or Asian marinade. You can also try eggplant curry, baked potatoes topped with broccoli or mushrooms, and peppers stuffed with sweet potatoes or carrots.

You can transform the same basic lower-phenylalanine vegetables into a whole menu of different dishes, using a little creativity — and a lot of seasonings. Herbs and flavorings such as basil, cilantro, lemon juice, soy sauce, sesame oil, maple syrup and honey are low in phenylalanine, but they pack a flavorful punch. Just remember to measure and count every ingredient and adjust ideas to your individualized diet. Talk to your doctor or dietitian if you have any questions.

Women with PKU can prevent their children from developing serious birth defects by sticking to — or returning to — a low-phenylalanine diet during pregnancy. Even women with mild cases of PKU may place their unborn children at risk by not following the special PKU diet. If you're a woman with a history of PKU, talk to your doctor before you start trying to conceive.

If you have a history of PKU, a close relative with PKU or a child with PKU, you may also benefit from pre-conception genetic counseling. A doctor who specializes in medical genetics can help you better understand how PKU is passed through your family tree. He or she can also help you determine your risk for having a child with PKU and assist with family planning.

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