Notably, the vitamin B12 differences remained statistically significant even after the older studies were excluded, which supports a strong and consistent association, while the folate levels showed a non-significant result that was close to significance but did not quite reach it (random-effects model, SMD: 0.729, 95% CI: −0.032, 1.490, p = 0.061).
The meta-analysis found no significant differences between the adherent and non-adherent groups, which were defined based on their phenylalanine levels, but showed higher vitamin B12 levels (fixed-effects model, SMD: 1.080, 95% CI: 0.754, 1.405, p < 0.0001) and a near-significant trend toward higher folate levels (random-effects model, SMD: 0.729, 95% CI: −0.032, 1.490, p = 0.061) in regular formula consumers.
When older studies were excluded, the comparison of folate levels between adherent and non-adherent PKU individuals revealed a trend toward higher folate levels in the adherent group, although this difference did not reach statistical significance (random-effects model, SMD: 0.431, 95% CI: −0.043, 0.906, p = 0.075).