Vitamins D, A and E, and Beta-Carotene in Adherent and Non-Adherent Individuals with Phenylketonuria: Cross-Sectional Study, Systematic Review and Meta-Analysis.

Background/Objectives: The impact of dietary adherence and formula intake regularity on fat-soluble vitamin status in phenylketonuria (PKU) is uncertain. This study assessed whether vitamin A, D, E, and beta-carotene levels differ by dietary adherence and regularity of Phe-free formula intake. Methods: A cross-sectional study included 98 individuals (age 6-41 years) with vitamin D measurements. In a subgroup of 68 patients, vitamin A, vitamin E, and beta-carotene levels were determined. Vitamin levels were compared between adherent and non-adherent groups and between participants with regular vs. irregular formula intake. A subsequent systematic review and meta-analysis of six studies (from PubMed, Scopus, Web of Science, and Cochrane; searched in August 2025) pooled standardised mean differences (SMDs) using fixed-effects and random-effects models. Results: The cross-sectional results showed higher vitamin D in adherent (35.60 [30.39-41.65] vs. 32.90 [26.50-40.00] ng/mL, p = 0.034) and regular formula consumers (35.97 [30.03-42.28] vs. 30.20 [26.08-35.06] ng/mL, p = 0.002). Beta-carotene was elevated with regular intake (74.40 [56.70-98.45] vs. 53.20 [34.10-68.60] ng/mL, p = 0.003). Meta-analysis confirmed higher vitamin D in adherent individuals (fixed-effects model, SMD = 0.290, 95% CI: 0.004, 0.576, p = 0.047) and regular consumers (fixed-effects model, SMD = 0.750, 95% CI: 0.382, 1.118, p < 0.0001). No differences were observed for vitamin E or beta-carotene. Conclusions: Adherence to diet and regular formula intake is associated with improved vitamin D status, underscoring the critical role of fortified formulas in PKU management. The very low certainty of evidence necessitates further research, especially for the other fat-soluble vitamins. Nonetheless, clinical practice should emphasise support for adherence and ongoing nutritional monitoring.