Background: Phenylalanine (Phe)-free infant formula is an essential source of safe protein in a Phe restricted diet, but its efficacy is rarely studied. This longitudinal, multi-centre study evaluated the efficacy of a Phe-free amino acid-based infant formula (PKU Start: Vitaflo) in infants with phenylketonuria (PKU).

Methods: The study was conducted in two parts: a 28-day short-term evaluation and a 12- month extension study. Data were collected on dietary intake, growth, blood Phe concentrations, and gastrointestinal tolerance.

Results: Ten infants (n = 8 males, 80%), with a median age of 14 weeks (range 4–36 weeks) were recruited from 3 treatment centres in the UK. Nine of ten infants completed the 28-day follow-up (one caregiver preferred the usual Phe-free formula and discontinued the study formula after day 14), and 7/9 participated in the 12-month extension study (one infant preferred the usual Phe-free infant formula, and n = 1 was lost to follow-up). The Phe-free infant formula contributed a median of 57% (IQR: 12%) energy and 53% (IQR: 33%) of total protein intake from baseline to the end of extension study. Median length-for-age z-scores increased from - 0.3 (IQR: 1.3) at baseline to 0.1 (IQR: 1.0) in the extension study. Median weight for age z- score increased from 0.3 (IQR: 1.9) at baseline to 0.8 (IQR: 0.9) in the extension study. BMI- for-age z-score increased from 0.6 (IQR: 1.7) at baseline to 0.8 (IQR: 1.1) in the extension study. Median blood Phe concentrations remained within the treatment target range of 120-360 μmol/L during the study period. Any early gastrointestinal symptoms (constipation, colic, vomiting and poor feeding) improved with time.

Conclusion: The study formula was well tolerated, helped maintain good metabolic control, and normal growth in infants with PKU. The long-term efficacy of the Phe-free infant formula should continue to be monitored.