**1. Introduction**

Currently, obesity is defined as a medical condition characterised by an energy imbalance as a consequence of energy intake bigger than energy expenditure [1]. The US Centers for Disease Control and Prevention (CDC) defines it as 'excessive weight that is considered unhealthy for a given height'. It was not until 1997 that the World Health Organization (WHO) described obesity as an excessive accumulation of body fat that can be harmful to people's health [2].

One of the criteria to diagnose obesity is through the calculation of body mass index (BMI); the result, according to the WHO, must be equal to or bigger than 30 kg/m2 [2].

However, the Asian population including Korea, given their increased risk of developing comorbidities, such as cardiovascular diseases and type 2 diabetes, agreed to an overweight category with a BMI between 23 and 24.9 kg/m2 and obesity with a BMI > 25 kg/m2 (**Table 1**) [3].

Worldwide obesity is a public health problem, and despite all the strategies designed and implemented to reverse the situation, these have not been effective. According to the World Health Organization, globally, in 2016, more than


#### **Table 1.**

*Comparison of the World Health Organization (WHO) and Asia-Pacific body mass index (BMI) classifications in COPD patients [3].*

1900 million adults were overweight, over 650 million were obese, and 41 million children under the age of 5 were overweight. Fifty-seven percent of the world's population lives in countries where being overweight and obesity cause more deaths than being underweight. Furthermore, each year at least 2.8 million people die worldwide due to obesity or being overweight, irrespective of whether the country is economically developed or not [1, 2].

Various studies have shown that patients with illnesses related to the gastrointestinal tract, such as type 2 diabetes, obesity and cardiovascular diseases, among others, present dysbiosis or an imbalance within the gut microbiota (GM).

A hypothesis of the positive influence of microbiota on fat storage exists, and several studies carried out in mice have proven the role of certain fatty acids in the diet in the prevention of obesity, an effect mediated through changes in the composition of gut microbiota. Studies in humans have also shown a relationship between a decrease in *Bacteroidetes* in obese individuals and a greater proportion of these in the faecal microbiota in subjects who have undergone hypocaloric diets and who have managed to reduce their weight [5, 7].

An important role of the regulation of gut microbiota has been proven as the ingestion in the diet of prebiotics and probiotics that modulate the growth of diverse families of bacteria that could influence the nutritional status of individuals.

### **2. Obesity as a multifactorial condition**

Obesity is a medical condition of multifactorial causes, ranging from the influence of the nearby environment on the acquisition of predisposing habits, such as the consumption of foods high in refined carbohydrates and saturated fats and a sedentary lifestyle. Genetic predispositions are also among the principle causes [5]. On the other hand, socioeconomic development, transport, urban planning, environmental, agricultural, educational, processing, distribution and marketing policies on food influence dietary habits and preferences, as well as people's level of physical activity [5, 6].

Research concludes that obesity occurs with greater prevalence in women, older people (25 years and above), those with a lower school level and those with a low socioeconomic income. Geographic location also has an influence, but it occurs in an opposite way in men and women. There is greater obesity in women living in rural areas, but in men rurality is a protective factor since obesity is more prevalent in those living in urban areas. This is most likely explained by the higher levels of physical activity of men living in rural areas [6]. In terms of lifestyles, obese people have lower levels of physical activity, a bigger consumption of salt and low levels of sleep [7].

Another bibliographical review relates meal times with the prevalence of obesity. This study, which is based on an analysis of the circadian rhythm, a biological function regulated by the anterior hypothalamus, reveals clinical observations

**95**

*Gut Microbiota and Obesity: Prebiotic and Probiotic Effects*

and experiments on animals that relate the times of food intake to metabolism and body weight. It is proposed that eating at times when the body is prepared for rest increases body weight. At rest there are low levels of hormonal activity, a low body temperature and low activity of nutrient absorption which alters energy homeostasis and facilitates the accumulation of energy, increasing body weight [5, 8].

A factor recently studied as a cause of obesity is gut microbiota, defined as the communities of living microorganisms that colonise the intestines. It is known that the gut microbiota contributes significantly to the two main functions of the

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

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].

adulthood, they remain stable in the individual (**Figure 1**) [11–14].

response, nutrition and metabolism [15].

allergies and autoimmune diseases [16].

responding to the gut microbiota [17].

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

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

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

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

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

*DOI: http://dx.doi.org/10.5772/intechopen.86672*

gastrointestinal tract: nutrition and defence.

linked to beneficial health effects [9].

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

*Gut Microbiota and Obesity: Prebiotic and Probiotic Effects DOI: http://dx.doi.org/10.5772/intechopen.86672*

*Oral Health by Using Probiotic Products*

*classifications in COPD patients [3].*

**Table 1.**

is economically developed or not [1, 2].

have managed to reduce their weight [5, 7].

**2. Obesity as a multifactorial condition**

physical activity [5, 6].

1900 million adults were overweight, over 650 million were obese, and 41 million children under the age of 5 were overweight. Fifty-seven percent of the world's population lives in countries where being overweight and obesity cause more deaths than being underweight. Furthermore, each year at least 2.8 million people die worldwide due to obesity or being overweight, irrespective of whether the country

Underweight <18.5 <18.5 Normal 18.5–24.9 18.5–22.9 Overweight 25–29.9 23–24.9 Obese ≥30 ≥25

*Comparison of the World Health Organization (WHO) and Asia-Pacific body mass index (BMI)* 

**WHO (BMI) Asia-Pacific (BMI)**

Various studies have shown that patients with illnesses related to the gastrointestinal tract, such as type 2 diabetes, obesity and cardiovascular diseases, among

A hypothesis of the positive influence of microbiota on fat storage exists, and several studies carried out in mice have proven the role of certain fatty acids in the diet in the prevention of obesity, an effect mediated through changes in the composition of gut microbiota. Studies in humans have also shown a relationship between a decrease in *Bacteroidetes* in obese individuals and a greater proportion of these in the faecal microbiota in subjects who have undergone hypocaloric diets and who

An important role of the regulation of gut microbiota has been proven as the ingestion in the diet of prebiotics and probiotics that modulate the growth of diverse families of bacteria that could influence the nutritional status of individuals.

Obesity is a medical condition of multifactorial causes, ranging from the influence of the nearby environment on the acquisition of predisposing habits, such as the consumption of foods high in refined carbohydrates and saturated fats and a sedentary lifestyle. Genetic predispositions are also among the principle causes [5]. On the other hand, socioeconomic development, transport, urban planning, environmental, agricultural, educational, processing, distribution and marketing policies on food influence dietary habits and preferences, as well as people's level of

Research concludes that obesity occurs with greater prevalence in women, older people (25 years and above), those with a lower school level and those with a low socioeconomic income. Geographic location also has an influence, but it occurs in an opposite way in men and women. There is greater obesity in women living in rural areas, but in men rurality is a protective factor since obesity is more prevalent in those living in urban areas. This is most likely explained by the higher levels of physical activity of men living in rural areas [6]. In terms of lifestyles, obese people have lower levels of physical activity, a bigger consumption of salt and low levels of sleep [7]. Another bibliographical review relates meal times with the prevalence of obesity. This study, which is based on an analysis of the circadian rhythm, a biological function regulated by the anterior hypothalamus, reveals clinical observations

others, present dysbiosis or an imbalance within the gut microbiota (GM).

**94**

and experiments on animals that relate the times of food intake to metabolism and body weight. It is proposed that eating at times when the body is prepared for rest increases body weight. At rest there are low levels of hormonal activity, a low body temperature and low activity of nutrient absorption which alters energy homeostasis and facilitates the accumulation of energy, increasing body weight [5, 8].

A factor recently studied as a cause of obesity is gut microbiota, defined as the communities of living microorganisms that colonise the intestines. It is known that the gut microbiota contributes significantly to the two main functions of the gastrointestinal tract: nutrition and defence.
