Body mass index in adult patients with diet-treated phenylketonuria

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There is an increasing number of adults with phenylketonuria (PKU) on a low phenylalanine diet. In the general population, an increasing body mass index (BMI) in the UK is a major problem with associated co-morbidities. The present study aimed to identify whether patients with diet-treated PKU have obesity rates comparable to those in the general population.


Two hundred and thirty-six PKU subjects (49% male, 51% female), aged >16 years, who were diagnosed by newborn screening and were receiving a low phenylalanine diet, were identified from seven metabolic centres in the UK. Retrospective data were collated on age, sex, BMI and mean phenylalanine concentration over the previous 12 months.


Mean (SD) phenylalanine concentration for all 236 subjects was 789 (311) μm; mean (SD) BMI was 26 (5.4) kg m−2 [males 25 (4.3) kg m−2, females 27 (6.2) kg m−2]; mean (SD) age was 26 (7) years; and 55% had a BMI > 25 kg m−2. The percentage of subjects with a BMI >25 kg m−2 and >30 kg m−2, as well as increasing obesity with age, was similar to the UK population. A correlation was observed between increasing BMI and a higher phenylalanine concentration (r = 0.243, P = 0.001).


The number of overweight and obese patients with diet-treated PKU in the UK is a concern. This could lead to other obesity-related complications increasing the complexity of diet and the cost of their care. There is a need to educate patients with respect to adopting a healthy, low phenylalanine diet and lifestyle to prevent further rises in BMI.

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