**2. Gut microbiome description and composition**

Gut microbiome is a complex community of microorganisms living in the digestive tract, mainly in the colon (**Figure 1**). Variable pH and oxygen concentration affect the abundance of gut microbiome across the gastrointestinal tract. Gut microbiome represents up to 60% of the dry mass of feces (biomass around 1.5 kg), has more cells than host somatic cells and at least 100 times more genes than human genome [10–14].

Gut microbiome is established within the few first years of life and contains up to 100 trillion microbes, mainly bacteria (more than 1,000 species) but also fungi, protozoa, archaea, and viruses. The taxonomic ranking of gut microbiome includes species, genera, families, orders, classes, and phyla. Most of the species belong to Actinobacteria, Bacteroidetes, Firmicutes, Proteobacteria, and Verrucomicrobia phyla. The predominant phyla (90%) are Firmicutes (e.g., *Ruminococcus* and *Lactobacillus* genera) and Bacteroidetes (e.g., *Bacteroides* and *Prevotella* genera). Three enterotypes with functional differences have been defined based on variation in the level of genera: Enterotype 1 (*Bacteroides*), Enterotype 2 (*Prevotella*), and Enterotype 3 (*Ruminococcus*).

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*Gut Microbiome in Obesity Management DOI: http://dx.doi.org/10.5772/intechopen.91974*

caused by different factors such as age and diet.

**4. Gut microbiome metabolism**

*Assessment of gut microbiome (Picture downloaded from the internet).*

**5. Gut microbiome changes**

(**Figure 3**) [12, 15–31].

**4.1 Nutrition sources**

**4.2 Metabolites**

**Figure 2.**

There are two major gut microbiome projects: European Megagenomics of the Human Intestinal Tract and US Human Microbiome Project [11]. For gut microbiome studies, multiple fecal collections of the same subject are recommended. The assessments are DNA-based methods (16S rDNA sequencing, whole genome shotgun sequencing) (**Figure 2**) [32, 34, 35]. The challenges in the assessments of gut microbiome are due to the diversity and the inter/intra-individual variability

The sources for nutrition of gut microbiome are ingested dietary components (carbohydrate, protein, lipid) and host-derived components (shed epithelial cells, mucus).

Several metabolites are produced by gut microbiome. They include short-chain fatty acids (following carbohydrate fermentation), phenolic substances (following

Gut microbiome is diverse, varies between individuals, and can fluctuate over time. Western gut microbiome has less species than non-Western gut microbiome. In addition to several pathological conditions that can alter gut microbiome composition, multiple factors are responsible for the changes in gut microbiome

protein fermentation), and vitamins (vitamin B, vitamin K).

**3. Gut microbiome projects**

**Figure 1.** *Gut microbiome is mainly located in the colon (Picture downloaded from the internet).*
