PKU Diet: Foods to Avoid and Sample Menu

ByGavin Van De Walle, MS, RDN, LN

Phenylketonuria (PKU) is a genetic disorder in which harmful concentrations of the amino acid phenylalanine accumulate in the body.

The primary treatment for PKU is restricting phenylalanine from the diet.

This article explains what to avoid and limit with PKU and provides a 3-day sample PKU diet menu.

PKU diet

What is PKU?

PKU is a condition in which a genetic mutation in phenylalanine hydroxylase (PAH) reduces or prevents the breakdown of phenylalanine to tyrosine.

This mutation may decrease the amount of PAH that the body produces or limits the enzyme’s ability to efficiently break down phenylalanine.

Consequently, harmful levels of phenylalanine accumulate in the blood, which can cause brain dysfunction and behavioral problems (1).

Depending on the mutation, the resulting PKU may range from mild to severe.

PKU is commonly diagnosed at birth through newborn screening programs.

However, it can go undetected if the mother is discharged too soon or if the newborn hasn’t consumed any protein prior to the screening test (1).

It occurs in about 1 of every 10,000 newborns and is more common in white in Native American populations (2).

Ealy identification and treatment of PKU is key for reducing or preventing its signs and symptoms, which may include developmental delays, seizures, hyperactivity and behavioral problems, and loss of skin color.

These signs and symptoms occur due to the harmful accumulation of phenylalanine in the blood but also from reduced tyrosine levels (1).

How to follow a PKU diet

The primary treatment for PKU involves maintaining low phenylalanine levels through dietary phenylalanine restriction.

A low phenylalanine diet generally consists of less than 500 mg per day (3).

However, the amount of phenylalanine that one can tolerate before it leads to harmful concentrations will vary from person to person, depending on the severity of PKU or the use of medications for its management, among other factors.

As such, it’s important to work with a dietitian and monitor blood phenylalanine levels regularly to determine what your daily allowance of phenylalanine should be based on your tolerance level.

Once you know your tolerance level, you should track your intake of phenylalanine from food and drinks to stay within your daily limit.

Food manufacturers aren’t required to list the amount of phenylalanine their products contain, so it can be difficult and overwhelming to try and track your intake.

Fortunately, there are several ways to calculate your dietary phenylalanine intake.

One system is called the 1-gram protein exchange, which is based on the general rule that animal and cereal protein sources provide 50 mg of phenylalanine per one gram of protein.

For example, if an egg contains 6 grams of protein, it would provide about 300 mg of phenylalanine (6 x 50 = 300).

There are also apps you can download that use this same method.

Foods to eat and limit

A better approach is to reduce your protein intake since phenylalanine is an amino acid and therefore a component of protein.

High-protein foods to limit include (3):

  • Poultry: eggs, chicken, turkey
  • Meats: beef, goat, lamb, pork
  • Seafood: fish and shellfish
  • Dairy: cheese and milk
  • Legumes: beans, peanuts, peas, lentils
  • Grains: barley, oats, rye, quinoa, wheat
  • Nuts and seeds: almonds, Brazil nuts, cashews, flax seeds, hazelnuts, pine nuts, sesame seeds, etc.
  • Supplements: protein powders including both animal- and plant-based products and spirulina

In addition to these protein-rich foods, you should also limit foods and drinks that contain aspartame as it’s converted to phenylalanine in the body.

The exception is Neotame, an artificial sweetener derived from aspartame, that only releases small amounts of aspartame.

You can identify which products contain aspartame by reading the ingredient list.

Here is a list of items that commonly contain aspartame:

  • diet soda
  • sugar-free ice cream
  • gum
  • yogurt
  • sugarless candy
  • tabletop sweeteners

Foods to eat

Here is a list of foods that you can eat without having to calculate phenylalanine content or restrict how much you consume:

  • Fruits: apples, apricots, bananas, berries, cherries, melons, oranges, pears, pineapple, plums, etc.
  • Vegetables: beetroot, cabbage, cucumbers, garlic, green beans, lettuce, peppers, tomatoes, etc.
  • Starches: arrowroot, cassava flour, corn starch, and sweet but not white potatoes
  • Fats: butter, ghee, margarine, and vegetable oils
  • Beverages: aspartame-free drinks, including coffee, tea, and fruit juice, and low-protein plant-based milk
  • Herbs and spices: allspice, basil, cinnamon, chili powder, cumin, garlic, ginger, nutmeg, oregano, paprika, pepper, etc.
  • Aspartame alternatives: acesulfame K, saccharin, stevia, sucralose, fructose, sucrose, maltodextrin, mannitol, sorbitol, and xylitol

Since a PKU diet limits protein, phenylalanine-free protein supplements can help you meet your protein targets.

Some examples include:

Evenly spread your intake of protein from these supplements throughout the day.

The supplements usually contain vitamins, minerals, and essential fatty acids, but you may have to supplement with these nutrients if they’re not supplied by the product.

Your dietitian can help determine what your protein needs are.

Breast milk contains phenylalanine but it’s still encouraged due to its myriad benefits for both mother and baby (3).

However, it’s recommended to still reduce breastfeeding by giving a measured amount — between 30 to 60 mL — of a phenylalanine-free infant formula before each breastfeeding to reduce the infant’s appetite and therefore suckling (3).

3-day sample PKU diet menu

Here’s a 3-day sample PKU diet menu that provides less than 500 mg of phenylalanine per day:

Day 1

  • Breakfast: phenylalanine-free protein supplement and low-protein toast with butter and tomatoes
  • Lunch: phenylalanine-free protein supplement, home-made vegetable soup, low-protein bread, and one apple
  • Snack: low-protein muffin and berries
  • Dinner: phenylalanine-free protein supplement, low-protein risotto, and salad

Day 2

  • Breakfast: phenylalanine-free protein supplement and low-protein toast with avocado
  • Lunch: phenylalanine-free protein supplement and low-protein pasta salad
  • Snack: low-protein toast with honey
  • Dinner: phenylalanine-free protein supplement and veggie tacos with cassava tortilla

Day 3

  • Breakfast: phenylalanine-free protein supplement, cassava flour pancakes, and berries
  • Lunch: phenylalanine-free protein supplement, sweet potato soup, and a side salad
  • Snack: cinnamon roasted butternut squash
  • Dinner: phenylalanine-free protein supplement, tomato soup, and low-protein bread with butter

The bottom line

PKU is a genetic disorder that reduces or prevents the breakdown of the amino acid phenylalanine.

Consequently, phenylalanine can accumulate to harmful levels in the body and cause damage to the brain and nervous system.

Restricting phenylalanine to less than 500 mg per day is the primary treatment for PKU, but some people may be able to tolerate more.

Because the PKU diet restricts protein, it’s important to supplement with a phenylalanine-free protein formula, which may contain other essential vitamins and minerals necessary for good health.

Gavin Van De Walle is a registered dietitian nutritionist and holds a master's degree in human nutrition and bioenergetics. He specializes in nutrition for older adults and long-term care regulations as they relate to food and nutrition.

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