In the UK, some fruits and vegetables are permitted without measurement in the diets of people with phenylketonuria (PKU). It is proven that fruits and vegetables containing phenylalanine (Phe) up to 75mg/100g (e.g. carrots, sweet potato, tomatoes) do not affect blood Phe control but it is unknown to what extent fruits and vegetables containing Phe from 76-100mg/100g (e.g. cauliflower, broccoli, beansprouts and asparagus) increase blood Phe levels. In addition, there is very little information about how differently animal and vegetable protein affect blood Phe levels. It is believed that plant protein may have less bioavailability and therefore have less impact on blood Phe control. The investigators aim is to perform a randomized controlled study over 21 weeks in children (5-12y) with PKU. Children will be studied during five different time periods, examining the effect on blood Phe control when increasing Phe intake from vegetables/fruits containing Phe from 76-100 mg/100g vs. animal sources. At the end of the study, fruits and vegetables containing 76- 100mg Phe/100g will continue to be given as "exchange-free" for 6 months. This is a practical, cost effective study and should bring benefit to all people with PKU following a very restrictive diet both within the UK and throughout the world.

Inclusion Criteria:

• PKU patients diagnosed by NBS and BH4 deficiencies;
• Early and continuously treated;
• Adherent to their prescribed PKU diet consisting of a protein-restricted diet and Phe-free amino acid based protein substitute;
• In good general health as evidenced by medical history;
• Able to provide written informed consent/assent (parent/child);
• Able to comply with the study protocol and take study product according to the opinion of the PI;
• Patients with good metabolic control (age 5-12y, 120-360μmol/L) with 3 out of 4 levels within range prior to consent (if 2 of 4 above target range but one close to the upper limit, PI to decide if patient can enter the study).

Exclusion Criteria:

• Concomitant diseases / disorders such as renal or gut disease / disorders and diabetes;
• Dislikes fruits and vegetables containing 76-100 mg of Phe/100g e.g.brocolli or cauliflower;
• Treated with BH4 (sapropterin);
• Presence of intercurrent infection;
• Patients with poor metabolic control (age 5-12y, >360μmol/L).