**3. Gut microbiome projects**

*Weight Management*

microbiome.

genome [10–14].

Enterotype 3 (*Ruminococcus*).

has co-evolved in a symbiotic relationship with humans. Its diversity is influenced by several factors including host genetics, mode of birth, age, gender, pregnancy, BMI, diet, medications, and surgery [12, 15–31]. The understanding of the gut microbiome evolves at a rapid pace, but the practical application of this knowledge is still in its infancy. The gut microbiome is essential for the maintenance of human health. It is involved in protection against pathogens and regulation of immune system and metabolism [32]. Profound changes affecting the diversity and the abundance of gut microbiome (dysbiosis) are associated with several disorders including obesity [33]. The prevention and management of obesity may benefit from manipulation of gut

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

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

**2. Gut microbiome description and composition**

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

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**Figure 1.**

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 caused by different factors such as age and diet.

**Figure 2.** *Assessment of gut microbiome (Picture downloaded from the internet).*
