Foods To Eat and Avoid on a Phenylketonuria (PKU) Diet

If your child is living with phenylketonuria (PKU), the most important way to manage their condition is to help them follow a special eating plan. A PKU diet limits phenylalanine (Phe), an amino acid found in many foods that contain protein.

PKU diets are restrictive, and following one can be a challenge. But pediatric medical geneticist Abigail Kacpura, DO, says understanding which foods to avoid and what they can safely eat is key. Doing so will help both of you feel more confident and in control —and keep them healthy in the long-run.

Diet guidelines for people with phenylketonuria

PKU means your body can’t properly break down phenylalanine, causing it to build up in your blood and brain instead. But high levels of Phe are toxic to the brain, which can lead to developmental delays and intellectual disability.

To keep your child safe, a PKU diet limits how much Phe they consume while making sure they get the nutrients they need.

“The goal isn’t to eliminate Phe entirely,” Dr. Kacpura clarifies. “We just want to keep it within a safe range.”

A PKU diet:

• Strictly limits high-Phe foods
• Includes low-Phe foods
• Avoids aspartame
• Monitors Phe levels on an ongoing basis

This isn’t the kind of diet you can figure out on your own. It’s important to work with a dietitian to create an eating plan that’s tailored to your child’s medical needs.

“They’ll be given a daily allowance of Phe — the amount of Phe they can have every day,” Dr. Kacpura explains. “You’ll also learn how to track their Phe intake, and you’ll receive other resources that will help you manage their PKU diet in your everyday lives.”

What foods do you need to avoid?

When your child has PKU, even small amounts of certain foods can raise their Phe levels. These foods should be carefully restricted or avoided altogether — but again, always under a dietitian’s careful guidance.

Let’s take a closer look at foods that aren’t part of a standard PKU diet.

High-protein foods

Foods that are high in protein are often high in Phe, too. This includes:

• Meat, including beef and pork
• Poultry, including chicken and turkey
• Fish and seafood
• Dairy products, including milk, cheese and yogurt
• Eggs
• Legumes, including beans and lentils
• Nuts and seeds
• Peanut butter and other nut butters
• Soy products, including tofu
• Whole grains, like barley, oats, quinoa and wheat
• Protein powders and protein bars

All that said, our bodies still need protein to function. So, you’ll work with your child’s healthcare team to figure out exactly what they can and can’t eat, which may include small, measured amounts of the foods on this list.

Foods with aspartame

“It’s important for people with PKU to avoid aspartame, an artificial sweetener that breaks down and becomes Phe when digested,” Dr. Kacpura says. “It’s often in products that are labeled ‘diet’ or ‘sugar-free.’”

Aspartame is found in low-calorie sweeteners with the brand names Nutrasweet®, Equal®, Canderel® and Sugar Twin®. Other foods and products that contain it include (but aren’t limited to):

• Diet soda
• Reduced-sugar condiments and yogurts
• Certain cereals
• Sugar-free gum
• Flavored sparkling water
• Chewable vitamins
• Cough drops
• Some medications, both prescription and over-the-counter

In the U.S., products that include aspartame legally have to say so on their packaging. To know which products to avoid, look for the warning, “Phenylketonurics: Contains phenylalanine.”

Foods to eat

Although the PKU diet limits many common foods, there are still many options that add balance, variety and proper nutrition. Dr. Kacpura explains what foods can be safely eaten while following a PKU diet.

Foods low in phenylalanine

Some foods naturally don’t contain much phenylalanine. Low-Phe foods include:

• Most fruits and vegetables
• Low-protein starches, like cassava flour, arrowroot and tapioca
• Drinks like coffee, tea and fruit juice
• Fats, like butter, ghee and vegetable oil
• Herbs and seasonings
• Sugars and honey

“Some foods, including many fruits and vegetables, do contain Phe,” Dr. Kacpura points out. “But it’s typically in low enough amounts that you don’t need to measure or track them.”

Protein substitution formulas

Specialized medical formulas are another key part of the PKU diet.

“They provide protein without adding phenylalanine,” Dr. Kacpura explains. “They also provide other vitamins and minerals that may be harder to get through diet alone.”

Newborns with PKU start on Phe-free formula as soon as they’re diagnosed, and many people with PKU continue using ready-to-drink or powdered options throughout their lives.

Low-Phe specialty foods

Some companies make low-protein foods — including bread, pasta, rice and baking mixes — that are designed to replace high-Phe foods and keep Phe intake low.

“These products typically contain small, manageable amounts of phenylalanine,” Dr. Kacpura notes. “So, it’s important to count and carefully track your child’s intake to be sure they’re consuming safe amounts.”

Some vegan-friendly products

Following a PKU diet isn’t like being a vegan. Veganism is often a choice, while PKU is a medical condition. And people with PKU can’t eat many of the foods that vegans can, including legumes, nut butters and soy products.

That said, some vegan-friendly products are safe under a PKU diet, like certain vegan cheeses, milks and desserts.

“They may help you incorporate everyday foods that you or your child couldn’t otherwise have,” Dr. Kacpura says. “But as is always the case with PKU, it’s very important to read labels carefully and thoroughly.”

Some recipes targeted at vegans may also be helpful as you’re looking for PKU-friendly dishes to make at home. They often need to be tweaked less than regular recipes.

Final thoughts

There’s no way around it, so we might as well say it: Following a PKU diet can feel incredibly restrictive, and all of that planning and tracking can be both challenging, exhausting and isolating.

But you don’t have to do it alone.

“Working closely with a dietitian and the rest of your child’s healthcare team can help make the diet more manageable,” Dr. Kacpura says. “Following a PKU diet helps ensure that your child is meeting all of their nutritional needs and protecting their brain health for the long term.