**3. Human gut microbiota: role in people's health**

The human gut is colonised by an enormous quantity of communities of living microorganisms, mainly bacteria that form the GM. As well as participating in processes of digestion and absorption of nutrients, the homeostasis of GM has been linked to beneficial health effects [9].

In the womb, the intestine of the human foetus is sterile, and it is after birth when the newborn acquires its gut microbiota. From birth, several factors influence the composition and initial colonisation of the GM. Natural labour favours the rapid settlement of bacteria that come from the vaginal and faecal microbiota of the mother, associated with a low risk of suffering from diseases and some types of malnutrition through excessive food consumption in the future. On the other hand, children born through caesarean section have a slower colonisation of bacteria, mainly from the hospital environment and healthcare professionals who are in contact with the child. It must be considered that the time taken to establish the GM is a risk factor for the colonisation of *Clostridium*. Despite the fact that GM is established from birth, its colonisation continues to vary, given that there are diverse factors that influence its composition: nutrition, exclusively or predominantly breastfeeding, age, geographic location and intake of supplements and medicines [9, 10].

In the first years of life, when nutrition is acquired through breastfeeding or formula milk, GM is mostly populated by *bifidobacteria*. This is highly adapted to processing oligosaccharides in milk. Later, reaching adulthood, the GM is made up of two main families of bacteria: *Firmicutes* and *Bacteroidetes*. *Firmicutes* are the family found in greater proportion, including more than 200 genders, and the most important are *Mycoplasma*, *Bacillus* and *Clostridium*. Once these are established in adulthood, they remain stable in the individual (**Figure 1**) [11–14].

Various studies support the fact that variations in people's health status are directly related to functions of the GM, highlighting its effect on the immune response, nutrition and metabolism [15].

The immune tissues of the gastrointestinal tract make up the most complex and largest proportion of the body's immune system, which is why alterations of GM are caused by disequilibrium of the immune system, such as the development of allergies and autoimmune diseases [16].

There is a complex interaction between the microbiota, the intestinal epithelium and the gastrointestinal immune system, with many metabolites and microbial components that have a direct influence on the host immunity. The production of metabolites from nutrients or the modification of the metabolites produced by the host has a direct effect on the immune cells and on the integrity and permeability of the intestinal epithelium. The enteric immune system is constantly evaluating and responding to the gut microbiota [17].

Nutritional and metabolic functions of GM involve the synthesis of certain vitamins such as K, B12, biotin, folic acid and pantothenic acid, as well as the

**Figure 1.** *Factors that influence the initial colonisation of gut microbiota.*

fermentation of non-digestible carbohydrates, polysaccharides and oligosaccharides. It makes up an important source of energy for bacterial proliferation and produces short-chain fatty acids that the host can absorb, favouring the recovery and absorption of ions such as calcium, iron and magnesium [18].

As well as the principal functions of GM, recent studies have associated it with the development of obesity and chronic diseases. In order to avoid such conditions, it has been proposed that the role of prebiotics and probiotics in the maintenance of the nutritional status and prevention of diseases should be studied [4].

Undoubtedly, microorganisms present in the guts are fundamental throughout life as they have a direct relationship with health and illness. It is vital to take special care in the factors that make up the initial GM as it is responsible for the general status of future health of individuals.
