**5.4 Gender**

*Weight Management*

**5.1 Host genetics**

**Figure 3.**

relations [15].

**5.3 Age**

**5.2 Mode of birth**

There are possible relations between host genetic profile and gut microbiome composition, but additional studies are needed for a better understanding of these

*Multiple factors can impact gut microbiome (Picture downloaded from the Internet).*

Mode of birth has an important influence on gut microbiome composition [12, 16]. With vaginal delivery, infants are colonized by maternal vaginal bacteria (dominated by *Lactobacillus* and *Prevotella* genera) while following C-section delivery, infants are colonized by maternal skin bacteria (dominated by *Staphylococcus*,

Age is associated with important changes in gut microbiome [12, 17]. The changes occur mainly before 20 and after 70 years. The diet plays a significant role. In preterm infants, there is a predominance of Proteobacteria phylum. There are marked changes in infants. The choice of diet after birth (breast milk or formula milk) affects the colonization process in the newborn. With age, the introduction of solid food from 2 years and the production of sex hormones from puberty (to menopause in females) bring additional richness and complexity to gut microbiome. There is a relative stability of gut microbiome between 20 and 70 years (predominance of Firmicutes phylum). In elderly subjects, Bacteroidetes phylum is

*Corynebacterium*, and *Propionibacterium* genera).

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predominant.

Gender specificity of gut microbiome appears at puberty with the production of sex hormones [12, 18]. There is a lower abundance of Bacteroidetes phylum in women (role of estrogen).

#### **5.5 Pregnancy**

Elevated levels of estrogen and progesterone observed during pregnancy have important impact on gut microbiome [12, 16, 19]. The changes are characterized by a decrease in richness of gut microbiome, a higher abundance of Proteobacteria and Actinobacteria phyla, a decrease in *Faecalibacterium* genus, and an increase in Firmicutes-to-Bacteroidetes phyla ratio.

## **5.6 BMI**

BMI is associated with gut microbiome composition particularly in women [18]. Bacteroidetes phylum is less abundant in subjects with high BMI. The role of estrogen has been proposed.

#### **5.7 Diet**

Diet has an important influence on gut microbiome composition [20–25]. However, there is a high interindividual variability. A diet high in fat (≥ 55% of total macronutrients) is associated with increased Firmicutes and Proteobacteria phyla and decreased Bacteroidetes phylum while a diet rich in fiber (≥ 30 g/day) has the opposite effect. The changes in gut microbiome (composition and functionality) induced by diet can be observed as early as 2 days. However, major changes in gut microbiome require long-term change in dietary habits.

Important differences in gut microbiome have been reported in children between Europe and rural African village of Burkina Faso (polysaccharide-rich diet) with Firmicutes-to-Bacteroidetes phyla ratios of 2.8 and 0.5, respectively [26].

Diet may also contribute to the seasonal variations of gut microbiome likely due to different availability of fresh produce containing complex carbohydrates [27].

#### **5.8 Medication**

Several medications (e.g., antibiotics, nonsteroidal anti-inflammatory drugs, proton pump inhibitors, and metformin) affect gut microbiome [28–30]. The use of antibiotics is associated with increased Firmicutes phylum, with higher sensitivity in infants.

The impact of prebiotics and probiotics on gut microbiome is presented in Section 8.

#### **5.9 Surgery**

Since colon is the main reservoir of gut microbiome, surgical removal of colon may affect gut microbiome [31]. Indeed, right hemicolectomy for colorectal cancer has been reported to be associated with a decrease in diversity and richness of gut microbiome.

The impact of bariatric surgery on gut microbiome is presented in Section 8.
