Background:
Inborn errors of metabolism (IEMs) are a heterogeneous group of monogenic disorders caused by defects in metabolic enzymes resulting in the accumulation of toxic substrates or a deficiency of essential metabolites. Diet therapies aimed at removal of harmful substances is the main and crucial intervention in some IEMs, such as phenylketonuria (PKU). However, the IEM-induced metabolic dysfunction, as well as the dietary interventions, impact the gut microbiota resulting in exacerbation of the clinical phenotype.

Objectives:
We aim to increase the knowledge of the structure of the microbiota in patients affected by PKU and of how the various therapeutic interventions modify its composition in order to define a personalized dietary interventions on a genetic and metagenetic basis.

Methods:
We performed a systematic review and meta-analysis to examine gut microbiota in PKU patients with dietary treatment. The review was conducted according to the Meta-analysis of Observational Studies in Epidemiology (MOOSE) and Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) criteria.

Results:
A significant reductions in several taxa at the phyla and genus levels were observed in patients with PKU compared to controls. The synthesis of the meta-analysis shows a difference in terms of biodiversity between PKU subjects respect the control population (SMD = -0,067; IC 95%, da -0,113 a .0,021) and performing a meta-regression starting from the data relating to the Firmicutes/Bacteroidetes ratio, it is possible to highlight a role for the type of diet followed by the patients (p = 0.050).

Conclusion:
The increase in knowledge on the variations of the intestinal microbiota in response to treatments in subjects with PKU can be useful to improve therapies but, due to the limited number of studies, further researches are needed.