# A Review of the Relationship between Gut Microbiome and Obesity
## 2 The Relationship between Gut Microbiota and Obesity  
Obesity is still a rapidly growing public health issue worldwide, responsible for more than sixty percent of deaths associated with high body mass index (BMI) [10]. If the increasing trend continues, it is estimated that by 2025, the prevalence of obesity will rise to 18% in men and over 21% in women worldwide [11]. Obesity is considered a complex disease where we must take several factors in account, mainly due to genetic, behavioural, socioeconomic, and environmental risk factors, but the role of gut microbiota in obesity is one of the most promising discoveries of the last decade [2]. The first assumption of a connection between obesity and the gut microbiota was established after studies in germ-free mice. A subgroup of germ-free mice was reared in a sterile environment, in comparison to mice reared in a normal environment. The body fat percentage of mice raised in a conventional environment was 40% higher and the fat percentage around the reproductive organs 47% higher than of the germ-free mice, despite consuming lower amount of food compared to their germ-free counterparts. Transplantation of the gut microbiota from regular mice into the germ-free mice resulted in a sixty percent raise in body fat within fourteen days without increasing their food intake significant changes in energy expenditure. The finding suggests that gut microbiota influences the phenotypic characteristics of the host associated with obesity. The transplanted microbiota simultaneously increased the availability of energy from dietary plant polysaccharides, but also modified genes carbohydrate response element binding protein (ChREBP) and sterol response element binding protein 1 ( SREBP-1) in the host that affect energy storage in adipocytes [12]. Obesity may alter the gut microbiota structurally and functionally [5], and gut microbiota can also modulate nutritional status [13; 14; 15]. The abundant and diverse quantity of certain bacteria may facilitate energy storage and metabolic pathways leading to obesity [4; 5]. This indicates that altercating the gut microbiota by dietary or other means may provide beneficial effects by restoring functional integrity of the gut and reversing the dysbiosis that characterizes obesity [5; 16]. Animal studies show favorable results in obese models based on changes in physical and biochemical parameters, metabolic and inflammatory markers (e.g., increased IL-10 secretion, increased AMPK, reduction of acetyl-CoA carboxylase, fatty acid synthase), and gut microbiota diversity, whereas results in humans are limited and controversial [6,7]. The microbiota can influence both aspects of the energy balance of the host organism; namely, as a factor affecting energy harvest from the diet and as a factor influencing host genes affecting the deposition of energy (e.g., fasting-induced adipocyte factor), regulating energy expenditure and storage. Gut microbiota composition is characterised by constant variability, and it can be influenced by several dietary components, such as probiotics, including fermented foods, or prebiotics, such as inulin, other oligosaccharides, lactulose, and resistant starch, suggesting the possibility that manipulation of the gut microbiota may promote weight loss or prevent obesity in humans [17]. Relationships between Dietary Patterns, Gut Microbiota Composition, and Obesity in Certain Populations  
Long-term follow-up of a given type of diet leads to different microbiome communities in different ethnic groups/populations, and in some cases, this may be associated with obesity. In the hunter-gatherer Hadza tribe of Tanzania, the prevalence of obesity is very low, explained by the researchers in terms of their microbiome diversity and diet. During the African rainy season, they maintain a predominantly plant-based diet dominated by roots, baobab, and wild honey, consuming meat very rarely [18].