**3. Gut microbiota composition**

The parameters that help us to characterize the microbiome have been classically used in ecology and can be separated into those related to alpha diversity, which is a measure of microbiome diversity that allows us to define the total number of species and their relative contribution applicable to a single sample (Shannon or Chao1 indexes, i.e.); and beta diversity, which is a measure of similarity or dissimilarity between two communities, allowing us to compare ecosystems of different subjects or times (Bray-Curtis or UniFrac metrics, i.e.) [14, 15].

Currently, there is no consensus on the "normal" composition of the microbiota or universal cutoff points for classifying a microbiome as healthy or pathological according to the presence/absence or abundance of certain taxa in the overall ecosystem. It is generally considered that the greater the number of species present, and the more balanced the distribution of species, the healthier and more resilient the ecosystem [16].

The most dominant bacterial phyla of the human gut are Bacillota (formerly Firmicutes), Bacteroidota (formerly Bacteroidetes), Actinomycetota, and Pseudomonadota (formerly Proteobacteria), with *Bacteroides*, *Clostridium*, *Peptococcus*, *Bifidobacterium*, *Eubacterium*, *Ruminococcus*, *Faecalibacterium*, and *Peptostreptococcus* as the most abundant genera. Remarkably, *Bacteroides* family represents approximately 30% of the total bacteria, suggesting an important role in the global metabolism [4].

Beyond composition, the real impact of microbiota on human health is conditioned by their metabolism. This balanced host–microbe interaction can be defined as eubiosis, again habitually linked to high taxa diversity, high microbial gene richness, and stable microbiome functionality [8]. An imbalance in this functionality is defined as dysbiosis, a term typically used when the composition is different from that of healthy individuals; in our view, however, it is more a functional than compositional concept. This disturbance of gut microbiota can also modulate intestinal permeability as well as immune responses, favoring a proinflammatory state [5, 17, 18].
