Changes to taxa within the Firmi-cutes phylum were found, including reduced abundance of potentially beneficial butyrogenic microbes(Roseburia, Faecalbacterium prausnitzii, Lactobacillus, Bifidobacterium and Lachnospiraeceae). Conversely,increased abundance of potentially pathogenic or opportunistic microbes from the Bacteroidetes phylumwas reported, including increases in Alistipes and Bacteroides taxa. However, outcomes were inconsistent,with some trials also showing decreases in Bacteroides taxa and increases in commensal microbiota, suchas Lachnospiraceae and Bifidobacteriaceae. The changes in metabolic parameters observed from baselineto after the ultra-processed very low-energy diets were mostly beneficial or were not significantly altered.Although the selected articles were deemed to have satisfactory methodological quality, to understandthe possible direct effects of these regimens on gut microbiota, further rigorously designed trials, withmore standardised microbiological sequencing techniques and detailed reporting, are required.