Positive Impact of Breastfeeding on Nutritional Status and Metabolic Control in Infants with PKU: A Retrospective Study

Background/Objectives: Dietary treatment in phenylketonuria consists of a phenylalanine-restricted diet supplemented with a phenylalanine-free medical formula (Phe-FF). During the first six months of life, phenylalanine requirements can be met with breast milk (BM) or infant formula (IF). Despite all the benefits breastfeeding confers, it is often discontinued upon diagnosis of phenylketonuria, so more evidence is needed to support it. This study aimed to compare the assessments of nutritional status and metabolic control in infants with hyperphenylalaninemia/phenylketonuria who received BM, IF, or a combination of both as sources of intact protein, in addition to Phe-FF. Methods: A retrospective observational study was conducted in hyperphenylalaninemia/phenylketonuria patients between 0 and 6 months of age. Three groups were compared depending on the source of intact protein ingested: (1) BM + Phe-FF; (2) IF + Phe-FF; (3) mixture of BM and IF (BM + IF + Phe-FF). At each clinic visit, an anthropometric assessment and phenylalanine blood levels were analyzed. Results: 185 nutritional and metabolic assessments were included. The lowest median phenylalanine blood concentration was observed in the BM + Phe-FF group (129 µmol/L, interquartile range [IQR]: 39.5–232.5). In the BM + Phe-FF group all assessments were classified as eutrophic: −0.09 (SD ± 0.78); a statistically significant difference was observed between the BMI Z-Score of BM + Phe-FF and BM + IF-Phe-FF (p = 0.036). No statistically significant differences were observed in length/age Z-Score. Conclusions: Our results indicate that BM is the best option as a source of intact protein for children under 6 months of age with hyperphenylalaninemia/phenylketonuria, to maintain an adequate nutritional status and metabolic control.