Protein and Micronutrient Intake After Two Years of Sapropterin Treatment in PKU

Background: Sapropterin allows dietary liberalisation in responsive individuals with phenylketonuria (PKU), increasing natural protein intake and reducing dependence on protein substitutes (PSs). As PSs provide essential micronutrients, dietary liberalisation may increase the risk of nutritional insufficiency. Evidence describing detailed micronutrient intake in sapropterin-treated children remains limited. Methods: This secondary analysis evaluated dietary protein and micronutrient intake after 24 months of sapropterin treatment in 21 responsive children from a prospective longitudinal study. Caregiver-completed three-day food records were analysed for protein, calcium, iron, zinc, vitamin D, and vitamin B12, with micronutrient intakes compared with UK dietary reference values (DRVs). Results: Mean total protein intake was 75 ± 14 g/day, comprising 30 ± 16 g/day natural protein and 45 ± 21 g/day protein equivalent from PSs; natural protein tolerance ranged from 8 to 66 g/day. PSs contributed most micronutrients: calcium 80%, iron 84%, zinc 87%, vitamin D 96%, and vitamin B12 78%. Median micronutrient intakes exceeded DRVs for most children; however, four had intakes below DRVs, almost exclusively when PSs were reduced or omitted. One child consuming > 40 g/day natural protein without PSs had low iron (51%), zinc (90%), and vitamin D (4%) intakes. A non-adherent adolescent had low intakes of calcium (46%), iron (64%), zinc (41%), and vitamin D (60%). Another child receiving 60 g/day protein equivalent from PSs had marginally low vitamin D intake (85%) due to lower fortification. Children maintaining regular PS use met micronutrient requirements. Conclusions: After two years of sapropterin treatment, dietary liberalisation increased natural protein intake but did not consistently ensure adequate micronutrient intake. Micronutrient shortfalls were associated with reduced PS use, emphasising the need for careful dietitian-guided adjustment as diets become more flexible.