What to Eat When You Have PKU

Dietary Recommendations for Better Management

Vegetables are safe for a PKU diet.
Debby Lewis-Harrison / Getty Images

A PKU diet is the key treatment strategy if you have the genetic disorder phenylketonuria (PKU), in which you are missing an enzyme that allows for the conversion of phenylalanine to tyrosine (both amino acids). Because this normal process doesn't occur, phenylalanine collects in the brain and causes cognitive and/or behavioral problems. A PKU diet centers around foods that are low in phenylalanine to keep this build-up to a minimum.

Avoiding protein-rich foods is one of the basics of the PKU diet, as these choices are a major source of phenylalanine. Of course, your body still needs protein, so specially formulated medical foods help fill that gap.

Following a low phenylalanine PKU diet will not cure the disorder, but it can help keep your symptoms under control.

Benefits

Following a PKU diet will help you live your best life and prevent or alleviate cognitive issues related to the condition, such as memory problems, headaches, anxiety, depression, hyperactivity, and psychosis.

A study published in the Journal of Inherited Metabolic Disease looked at the neurological and psychological function of adults who were treated with a PKU diet since childhood. Researchers found that there were some deficits in neurological and social function. However, they weren't significant, and most adults had a normal IQ and good quality of life compared to healthy individuals.

It's especially important for young children with PKU to follow the diet carefully because their brains are developing quickly. High levels of phenylalanine in a child's body can cause seizures and irreversible damage to their brain, resulting in permanent intellectual disability.

Researchers identified detrimental changes in white matter in the brains of children who had both high and variable levels of phenylalanine throughout their lifetime. Their research, published in Molecular Genetics and Metabolism, stresses the importance of strictly adhering to a PKU diet and not liberalizing the diet with age.

Other side effects of uncontrolled levels of phenylalanine for all ages include eczema, body odor, and poor socialization.

According to the National PKU Alliance, following the PKU diet for life is associated with higher IQ levels, while having higher levels of phenylalanine in the blood over one's lifetime is linked to more problems with brain functioning and neurological impairment.

How It Works

The PKU diet works by providing:

  • Adequate calories for proper growth (in children) or to maintain a healthy weight (in adults)
  • Enough protein and phenylalanine to meet but not exceed your essential amino acid needs, which helps prevent a build-up
  • The right nutrients to keep you healthy

Phenylalanine is present in varying amounts in different foods. Protein foods are the highest sources, so they should be avoided. Other foods like grains, starchy and regular vegetables, and some fruits have smaller amounts of phenylalanine, so they can be eaten in moderation.

To make up for the protein and other nutrients you'll be missing from your diet, you'll need to add a specially designed, phenylalanine-free medical nutrition food. In fact, about 75% to 85% of your protein will come from such supplements, like Phenyl-Free. The amino acids they supply are very rapidly oxidized compared to the amino acids in whole foods, so protein needs are higher for someone relying on these medical foods for protein.

Children (over the age of 4) and adults require 120% to 140% of the reference daily intake of protein for their age. The protein needs for infants are dependent upon their age but average around 2.5 grams of protein per gram of body weight. For children up to age 4, protein needs are 1.5 to 2.1 grams per kilogram of body weight.

Your healthcare provider can tell you how much protein you need each day and what your daily phenylalanine goal should be.

Duration

The PKU diet is a life-long diet for anyone with PKU. It's essential that infants and children follow it strictly. In the past, adolescents and adults were advised that they could go off of the diet, but this is no longer recommended.

If you go off the PKU diet, you may notice that you have headaches, mood issues, memory problems, or other cognitive concerns. If you resume it, these should improve.

What to Eat

Compliant Foods

  • Medical foods, shakes, powders designed for PKU

  • Most fruits and fruit juices

  • Most non-starchy vegetables (and juices)

  • Low protein breads and pastas

  • Fats like olive oil, butter, margarine, coconut oil, ghee

  • Sweeteners like sugar (white or brown), honey, maple syrup, molasses

  • Small amounts of heavy cream

  • Non-dairy milk or yogurt (rice, almond, coconut, etc.)

  • Arrowroot, corn flour, tapioca starch

  • Herbs, spices, vinegar, mustard, salt, pepper

  • Coffee, tea

  • Alcohol

Non-Compliant Foods

  • Foods that contain the artificial sweetener aspartame

  • Meats (beef, pork, veal, goat, etc.)

  • Poultry (chicken, turkey, etc.)

  • Fish and seafood

  • Eggs

  • Soy foods (tofu, tempeh, soy meats, cheeses, etc.)

  • Legumes (lentils, chickpeas, kidney, black, pinto beans, etc.)

  • Artichokes


  • Asparagus


  • Avocado


  • Broccoli


  • Butternut squash

  • Corn


  • Green beans


  • Kale


  • Peas


  • Potatoes and French fries

  • Raisins


  • Rice, whole grains (oats, quinoa, cracked wheat, etc), regular pasta

  • Sweet potatoes

  • Nuts and nut butters

  • Dairy: Milk, cheeses, and yogurt

With the exception of fats, oils, and sugar, all foods have some amount of phenylalanine. So, even if you choose foods from the compliant list, you still have to be mindful of your portions.

If you have PKU, it's important to work with a dietitian or your healthcare provider to tailor the diet to your specific needs, which will be based on the phenylalanine levels in your blood, your protein and other dietary goals, your food preferences, and your lifestyle.

General Guidelines

Aspartame: This artificial sweetener is made from phenylalanine, so it should be avoided. It's commonly sold as NutraSweet or Equal, but it's listed as aspartame in diet, lite, or sugar-free foods like soft drinks, desserts, gelatins, puddings, yogurts, etc.

Fruits: Most fresh or frozen fruits are low in phenylalanine, with the exception of raisins, which are high and should be limited, and bananas and honeydew melon, which are considered medium phenylalanine fruits. Keep your portion of these to half a cup and eat them in moderation. Keep in mind if you eat dried banana chips (or most dried fruits), it's easy to eat large portions, so make sure you limit those to about half a cup as well.

Vegetables: Starchy vegetables like white, sweet, purple potatoes (and fries), winter squash, corn, and peas should be eaten sparingly. Most other fresh or frozen vegetables are low in phenylalanine with the exception of the ones on the non-compliant list. Carrots, zucchini, and cauliflower have a medium amount of phenylalanine, but in small portions, cauliflower rice can be a good substitute for regular rice; spiralized carrots or zucchini works instead of pasta. If you buy frozen vegetables, make sure there's no cream or cheese sauce, which will add additional protein and phenylalanine.

Grains: Most regular grains, bread, pasta, and baked goods made with wheat flour have a fair amount of protein, so they should only be eaten in moderation. Look for low-protein bread, cereals, pasta, and baked goods, which are better options. Ask your dietitian for a good resource.

Dairy: Regular milk, cheese, yogurt, and foods made from these are high in protein and phenylalanine, so look for non-dairy options. Very high-fat dairy foods like heavy cream or full-fat cream cheese can be used in moderation.

Nuts, nut butters, seeds: These can contribute large amounts of phenylalanine if eaten even in average amounts. Avoid peanut butter or other nut butters, and if using whole nuts or seeds, limit them to less than a handful. Ask your healthcare provider for more specific guidelines.

Seasonings: Herbs, spices, salt, pepper, mustard, plain or flavored vinegar, soy sauce, Worcestershire sauce, and barbecue sauce are low in protein and fine to use.

Calculating Phenylalanine

To determine the amount of phenylalanine in a food, follow these steps:

  1. Check the serving size on the label.
  2. Multiply the number of servings you'll eat by the amount of protein per serving to figure out the total amount of protein.
  3. Multiply the total amount of protein by 50 to get the amount of phenylalanine in that food.

Recommended Timing

Because it's important to moderate the amount of phenylalanine in your blood, you should do your best to spread out your meals and snacks over the course of the day. This will also help keep you more full and satisfied.

Since many of your portions will have to be small to stay within your phenylalanine goals, you'll probably find it helpful to eat five or six small meals per day.

Infants and Babies

It was once thought that babies with PKU had to be fed with a special formula only, but it's now known that breast milk is low in phenylalanine, and breastfeeding is encouraged. Babies with PKU who are not breastfed must only receive a special formula that doesn't contain phenylalanine.

In a small study on breastfed babies with PKU published in the Journal of Pediatric Nursing, researchers found that in their first year of life, breastfed infants with PKU had lower phenylalanine levels and better weight gain than infants with PKU who were exclusively formula-fed.

Considerations

Monitoring

Your blood should be drawn monthly to make sure that phenylalanine isn't accumulating. If it is, you'll need to further cut back on foods that contain phenylalanine and add more of a nutrition supplement.

In the United States and most other countries, infants are screened for PKU at birth with a blood test. Blood levels of phenylalanine and tyrosine are monitored closely in infants with PKU to ensure that they're receiving enough nutrition for growth while keeping the amount of phenylalanine in their diet within a target range.

General Nutrition

If followed properly, the PKU diet should be nutritionally adequate. Nonetheless, because your food choices are limited, it may be low in some vitamins, minerals, and fiber, especially if you take less than the recommended amount of your medical food.

Practicality

The PKU diet is easiest to follow when you're at home and preparing your own meals. If you're away at college, traveling for work or vacations, attending parties or celebrations, or gathering with friends and family for the holidays, planning proper meals and sticking with the diet will be more challenging. It can help to have some go-to take-a-long meals and snacks in mind for when you need them.

Flexibility

There are a variety of medical foods available for people with PKU, so you should be able to find a few that you like. You can also flavor them in different ways for more variety. If there are many low phenylalanine fruits and vegetables that you don't like, your diet may be pretty limited.

Cost

A major barrier to the PKU diet is the cost of your medical foods. If you have health insurance, they will be treated as necessary medications by your carrier, but the amount of coverage and specific products covered may vary.

Support

Having to follow such a strict diet can be socially isolating at times. It's important to get help from a dietitian or your healthcare team to help with meal planning as well as emotional support if needed.

You may also find it helpful to connect with a PKU support group or join the National PKU Alliance, which is a great resource for information, tips, and encouragement.

A Word from Verywell

Many teens and adults go off the PKU diet because they miss the variety in their diet. Doing this increases your risk of cognitive and behavioral problems, which can impact your school, work, or relationships. The good thing is, you can always go back on your diet, and even making some simple swaps can be helpful. If you've been "off diet" for a while, talk to your healthcare provider about the risks and benefits and how you can go back on the PKU diet.

Was this page helpful?

Article Sources