The main treatment for phenylketonuria (PKU), a rare genetic disorder that causes an amino acid phenylalanine to build up in the body, is a low-protein diet. The aim of an PKU diet is to avoid protein-rich foods like meat, eggs, and dairy products while limiting your intake of foods like potatoes and cereals that contain significant amounts of phenylalanine.
Phenylalanine is essential to building chemicals like dopamine and epinephrine that the brain and body needs to function normally. But, the inability to break down phenylalanine can cause the amino acid to build up, leading to behavioral problems, intellectual disabilities, and even seizures.
A PKU diet typically includes a phenylalanine-free medical formula or shake to meet your daily nutritional needs along with carefully measured amounts of fruits, vegetables, breads, pasta, and cereals to ensure you obtain just the right amount of phenylalanine to function normally.
This article describes the aims and benefits of a PKU diet. It also lists the foods that you can eat, cannot eat, or need to limit if you or someone you love has PKU.
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.
How It Works
The PKU diet works by providing:
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.
- 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
- 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 70% 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
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.
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
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
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
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.
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.
Calculating Phenylalanine
To determine the amount of phenylalanine in a food, follow these steps:
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. However, breast milk should only make up around 25% of a nursing baby’s overall intake. 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.
