Dietary conflicts with standard clinical procedures for colonoscopy in Phenylketonuria: a case report

Colonoscopy is a diagnostic technique used for gastrointestinal tract pathologies that presents additional challenges when used in individuals with phenylketonuria (PKU). The dietary requirements for colonoscopy preparation conflict with a phenylalanine (Phe) restricted diet essential in classical PKU, and aspartame containing laxatives must also be avoided. This case study highlights the complexities of applying conventional colonoscopy protocols to patients with PKU and emphasis the importance of personalized preparation strategies that consider strict dietary requirements.

A 24-year-old White British woman with classical PKU, with a Phe tolerance of 250 mg/day (5 g/day natural protein) and a consistent maintenance of blood Phe < 360 µmol/L, required an urgent colonoscopy/gastroscopy for investigation of bowel cancer. In the 3 days preceding the procedure, for the first 48 h she followed a low fibre/residue diet (avoiding fruit/vegetables), followed by 24 h of clear fluids only, omitting the Phe-free amino acid supplements. On the evening before the procedure, the first dose of a bowel preparation solution (Plenvu^®, containing Macrogol 3350 but aspartame-free) was taken. Immediately, she was lightheaded, had a headache with extreme fatigue. Three hours later, she took the second dose of Plenvu^®, containing aspartame (0.88 g/493 mg Phe). Her headache became intense, she felt exhausted and was unaware of her surroundings. It was unclear if the symptoms were caused by side effects associated with the bowel preparation solution or probable high Phe levels following 3 days of a very low energy diet with cessation of protein substitute, together with a high Phe intake associated with the aspartame load.

Awareness of potential dietary conflicts with standard procedures, together with effective communication within and between healthcare teams, is essential to ensure patient-centered care. Developing practical guidelines and specific warnings could significantly support both patients and healthcare providers.