Principle of the PKU Diet
In phenylketonuria (PKU), the body cannot break down the amino acid phenylalanine (Phe) because the enzyme phenylalanine hydroxylase (PAH) is absent or severely impaired. Since phenylalanine is a component of all natural proteins — whether from animal or plant sources — every protein intake must be carefully controlled.
The goal of the PKU diet is to keep blood Phe levels consistently within the individual target range, preventing damage to the nervous system. The diet rests on three pillars:
- Strict restriction of all naturally protein-rich foods (meat, fish, dairy, eggs, nuts, legumes, grains)
- Daily intake of a phenylalanine-free amino acid formula, supplying all essential amino acids except Phe and ensuring adequate protein nutrition
- Preference for fruit, vegetables, and specially formulated low-protein products as the dietary base
Individual Phe tolerance varies considerably by PKU severity:
| PKU type | Daily Phe tolerance | Target blood Phe |
|---|---|---|
| Classic PKU (severe) | < 200 mg Phe/day | 120–360 µmol/L |
| Moderate PKU | 200–400 mg Phe/day | 120–360 µmol/L |
| Mild PKU | > 400 mg Phe/day | 360–600 µmol/L |
| Mild hyperphenylalaninaemia (MHP) | Usually no strict diet | < 600 µmol/L |
The exact individual daily limit is adjusted regularly by the metabolic team based on current blood values. PHE Buddy stores your personal limit centrally and deducts every meal automatically.
Foods to Avoid
All naturally protein-rich foods contain phenylalanine — since Phe is part of every protein. The following food groups must be completely avoided or reduced to minimal amounts on the PKU diet:
Meat, Fish & Poultry
All varieties: beef, pork, chicken, turkey, lamb, fish, seafood, processed meats. Very high Phe content (600–900 mg/100g).
Milk & Dairy Products
Cow's milk, cheese (Gouda approx. 1,700 mg/100g, Parmesan even higher), yoghurt, quark, cream. Even butter contains trace amounts of Phe.
Eggs
One hen's egg (approx. 60g) contains around 440 mg Phe — exceeding the entire daily limit for classic PKU. Must be completely avoided.
Nuts & Seeds
Peanuts (~1,300 mg/100g), almonds, walnuts, cashews, sunflower seeds, pumpkin seeds, sesame. All must be strictly avoided.
Legumes
Lentils, beans (kidney, white, black), chickpeas, peas, soya and all soya products (tofu: ~980 mg/100g). Soya milk must also be avoided.
Grains & Flour Products
Wheat flour (~640 mg/100g), regular bread, pasta, oats, cornflakes, rice in larger amounts. Use PKU specialty products as replacements.
Aspartame (E951)
Artificial sweetener made from phenylalanine. Required EU labelling: "Contains a source of phenylalanine". Commonly found in diet drinks, sugar-free gum, light yoghurts, and some medications.
Quinoa & Amaranth
Despite their healthy reputation, both have a very high protein content and therefore significant Phe levels. Not suitable as grain substitutes in PKU.
Allowed Foods
The low-Phe diet offers plenty of options despite its restrictions. The following foods may be eaten — within the individual daily limit:
Fruit & Berries
Almost all fruit: apples (~3 mg/100g), pears, oranges, bananas (~30 mg/100g), strawberries, blueberries, grapes, melon. Very low Phe content throughout.
Vegetables
Lettuce, tomatoes (~18 mg/100g), cucumber, courgette, peppers, carrots (~24 mg/100g), beetroot, aubergine. Most varieties are well tolerated.
PKU Specialty Products
Low-protein bread, pasta, flour, crackers, and biscuits — specially formulated for PKU. Available from pharmacies and specialist online retailers, often on prescription.
Fats & Oils
Plant-based oils (olive, rapeseed, sunflower), Phe-free margarine. Little to no phenylalanine — important calorie source for the PKU diet.
Sugar & Sweets
Table sugar, honey, jam (check for aspartame), fruit gums (check for E951), lollipops. Use in moderation and always check for Phe content or aspartame.
Phe-Free Drinks
Water, pure fruit juices, many soft drinks without aspartame, tea, coffee. Always check labels of sweetened drinks for E951/aspartame before buying.
Limited — Allowed in Small Amounts
These foods contain moderate amounts of phenylalanine and may be consumed in small portions within the individual daily limit. Always confirm exact portion sizes with your dietitian:
- Potatoes (~70–80 mg Phe/100g): Permitted in small to moderate portions where the daily limit allows. Often the most important carbohydrate source in the PKU diet.
- Rice (white rice: ~65 mg/100g cooked): In very small amounts. Speciality low-protein rice with an extremely low protein content is preferable.
- Maize & maize products (~40–50 mg/100g): Plain popcorn and unfilled corn tortillas in small amounts are feasible.
- Higher-Phe vegetables: Broccoli (~105 mg/100g), spinach (~90 mg/100g) — can be included in small portions. Peas (~280 mg/100g) are better avoided.
- Mushrooms (~55–85 mg/100g depending on variety): In small amounts as a flavouring ingredient.
All values refer to raw foods unless otherwise stated. PHE Buddy calculates the exact Phe content automatically based on the portion entered and the BLS 4.0 database.
Phenylalanine Content of Common Foods
The following table shows the phenylalanine content per 100g for common foods, based on the BLS 4.0 database. PHE Buddy contains 7,140 entries with clinically validated values for the DACH region.
| Food | Phe per 100g | Status |
|---|---|---|
| Chicken breast fillet | ~780 mg | Avoid |
| Gouda cheese (45% fat) | ~1,700 mg | Avoid |
| Hen's egg (1 egg, ~60g) | ~730 mg | Avoid |
| Peanuts (roasted) | ~1,300 mg | Avoid |
| Tofu (plain) | ~980 mg | Avoid |
| Wheat flour (Type 405) | ~640 mg | Avoid |
| Broccoli (raw) | ~105 mg | Limited |
| Potato (raw) | ~70 mg | Limited |
| Carrot (raw) | ~24 mg | Allowed |
| Tomato | ~18 mg | Allowed |
| Apple | ~3 mg | Allowed |
| Olive oil | 0 mg | Allowed |
Source: BLS 4.0 (German Nutrient Database, 4th version). PHE Buddy uses 7,140 validated entries from the BLS 4.0 database for instant, precise Phe look-up — fully available offline.
Low-Protein PKU Specialty Products
Specially formulated low-protein products are one of the most important quality-of-life tools for people with PKU. They replace protein-rich standard foods and enable a varied diet without exhausting the daily Phe budget.
- PKU bread and rolls: Specialist bread with an extremely low protein content (often below 0.4g protein/100g, compared with 8–10g in regular bread). Soft texture, similar to regular white bread.
- Low-protein pasta: PKU spaghetti, penne, and fusilli with very low Phe content. Cooking behaviour and taste closely resemble conventional pasta.
- PKU flours and baking mixes: Starch-based (cornstarch, potato starch) with minimal protein. Enable home baking of bread, cakes, and biscuits.
- Low-protein rice and crackers: As a side dish and snack. Crackers are practical for school, work, and travel.
- PKU sauces and condiments: Ready-made sauces without Phe-relevant ingredients — for easy everyday cooking.
Where to buy PKU specialty foods in the DACH region:
- Pharmacies — often on prescription and reimbursable by statutory health insurance
- Specialist online retailers: Loprofin, Vitaflo, PKU Perspectives, Nutricia online shop
- Directly through metabolic centres and dietetic clinics
- Some health food stores and organic supermarkets with special dietary sections
Reimbursement: In Austria and Germany, statutory health insurance covers many PKU specialty products when prescribed by a doctor. Contact your metabolic centre for details on what is covered in your country.
PHE Buddy & Digital Dietary Management
Daily weighing, looking up values, and calculating Phe — PKU dietary management is demanding. PHE Buddy handles this work and makes the low-Phe diet significantly more manageable.
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