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Dietetic research papers: The fruit and vegetable story continues! (MacDonald)

Dietetic research papers: The fruit and vegetable story continues! (MacDonald)

In 2012, two European papers were published that produced supporting evidence to suggest that many fruits and vegetables can be allocated in a low phenylalanine diet for patients with PKU without measurement. These papers are important. A low phenylalanine diet can clearly create many practical difficulties for any patient and their family. Having to weigh all fruits and vegetables that are eaten on a daily basis is a huge burden and difficult for anyone to accept. In addition, fruits and vegetables are relatively low in phenylalanine (vegetables approximately 4%; and fruit 3%), and with the increasing concerns about overweight and obesity in PKU, it would be beneficial to be able to encourage these as part of a healthy diet in the same way as for the general population.  Also many fruits and vegetables are a natural source of micronutrients including antioxidants, and are likely to be better utilized than chemical supplements added to L-amino acids. 

The first paper was by Rohde et al, leading a team of co-workers from Leipzig in Germany. They studied a group of 14 children with PKU, aged between 2 and 10 years. It was a four-week, randomised, crossover study were subjects either allowed to eat fruits and vegetables <75 mg/100g without measurement for two weeks, or they  continued to calculate the phenylalanine from these fruits and vegetables into their daily phenylalanine allowance for 2 weeks. Daily blood phenylalanine estimations were conducted.  Permitting these fruits and vegetables increased phenylalanine intake by 58 mg daily and it did not adversely affect blood phenylalanine concentrations.  They concluded that long-term multicentre trials were necessary before complete liberation of fruits and vegetables can be recommended in the PKU diet.

The second study was by Zimmermann et al, from a group representing Zurich, in Switzerland.  They transferred 50 patients from a diet where fruit and vegetables were calculated into the phenylalanine allowance to a diet whereby all fruits and vegetables containing <100 mg/10g were allowed without measurement. They described this to be simplified diet. The new diet did not adversely affect blood phenylalanine control irrespective of the severity of the PKU or the age of the child.

Although the findings of these papers are not new, there has been slow acceptance throughout Europe that many fruits and vegetables can be incorporated into the diet without loss of blood phenylalanine control and dietary practices are highly variable (Ahring et al 2009).  It was in 2003 that the first evidence to support the concept of ‘free’ fruit and vegetables was published by MacDonald and co-workers, and in the UK there is a 40 year history of safely incorporating many fruits and vegetables without measurement in a low phenylalanine diet.   This type of approach is easier for the patient and it is likely to help long-term adherence.  It is hoped that publication of two papers in quick succession on this topic will help practitioners consider the day-to-day diet they prescribe for their patients with PKU. A lot is written about the difficulties associated with diet therapy and how patients struggle with adherence, so it is important that we do more to help by re-examining the dietary approaches used in PKU. 


Rohde C, Mütze U, Weigel JF, Ceglarek U, Thiery J, Kiess W, Beblo S. Unrestricted consumption of fruits and vegetables in phenylketonuria: no major impact on metabolic control. Eur J Clin Nutr. 2012, 66: 633-8.

Zimmermann M, Jacobs P, Fingerhut R, Torresani T, Thöny B, Blau N, Baumgartner  MR, Rohrbach M. Positive effect of a simplified diet on blood phenylalanine control in different phenylketonuria variants, characterized by newborn BH4 loading test and PAH analysis. Mol Genet Metab. 2012, 106: 264-8.

Anita Macdonald

Dietetic Department
The Children’s Hospital
Birmingham, United Kingdom