Introduction
In recent years, there has been increasing evidence linking gut bacteria to obesity. Manipulation of gut microbiota has emerged as a novel approach to treating obesity. However, little is known regarding the complex connections between obesity, the gut microbiota, and the environment.
What Is Obesity?
Obesity is a complex metabolic disorder. Numerous genetic and non-genetic factors, like environment, can contribute to it. According to the World Health Organization (WHO), obesity is having a body mass index of 30 or more. However, the definition can vary from country to country. A comprehensive study revealed that about thirty percent of the world’s population is overweight, while about ten percent is obese. It is estimated that about 1.2 million people will be obese by 2030.
Obesity has become a primary global concern because of the high-risk complications associated with it. Apart from the outward manifestations, obesity is linked to metabolic problems of fat and glucose, oxidative stress, chronic inflammation, and an elevated risk of many diseases like cardiovascular diseases, cancer, and diabetes. Recent studies have shown that an imbalance in the gut flora can also contribute to obesity.
How Is Obesity and Gut Microbiota Linked?
The gut microbiota is made of symbiotic bacteria, which account for more than ten times the body's cells. The gut microbiota depends on dead cells shed as nutrients, mucous secreted by the gut and the dietary remnants the body cannot digest to maintain its high population. In addition to producing numerous physiologically active compounds like short-chain fatty acids, vitamins, and substances that promote health, like antioxidants, analgesics, and anti-inflammatory agents, the active gut microbiota will also produce potentially harmful compounds like neurotoxins, carcinogens, and immunotoxins. These organisms have the ability to penetrate the bloodstream, influence immunological and metabolic functions in humans, and directly control gene expression. Hence, a healthy gut microbiota is required to preserve the body's metabolism and energy balance. Metabolic problems and increased overall appetite might result from an imbalance in the gut flora. This, in turn, can contribute to obesity.
Does the Composition of the Gut Microbiota Depend on Diet?
Dietary habits can have a significant impact on the general composition of the gut flora. A ‘Western’ style diet, which is high in fat and sugar, was followed in some animal models. Studies show that the diet favored Firmicutes (bacteria that digest the carbohydrates that the body’s enzymes can not digest) over Bacteroidetes in terms of relative abundance. In addition, in gnotobiotic mice colonized with human fecal bacteria, a change in diet for a single day from a low-fat, plant polysaccharide-rich diet to a high-fat and high-sugar ‘Western’ diet caused a change in the microbiota. Human subjects have also shown similar results. Although there is a significant inter-individual variation in the gut microbiota composition of mammals, it is unclear which factor had a greater impact on microbial composition, host genetics, or dietary consumption.
Is It Possible to Prevent Obesity by Altering the Gut Microbiota?
By focusing on the microbiota, therapeutic strategies for the treatment or prevention of obesity and related metabolic problems may become a possibility. These strategies include fecal microbial community transplantation and nutritional therapies using probiotics, synbiotics, or prebiotics.
-
Prebiotics: Prebiotics are food ingredients that the host cannot digest and have positive effects on the host due to the selective stimulation of the gut microbiota's development and/or activity, especially lactobacilli and bifidobacteria. In this field, the nondigestible oligosaccharides have received the majority of interest. Other oligosaccharides, resistant starch, lactulose, and inulin are common prebiotics. Prebiotic characteristics are generally believed to exist in all fermented dietary fibers. To regulate food intake, signals of nutritional and energy status are transmitted from the gut to the central nervous system by gut hormones such as glucagon-like-peptide-1 (GLP-1). Research has indicated that prebiotics upregulate GLP-1 in obese mice, indicating that changes in intestinal microbiota may either promote or inhibit the release of gastrointestinal hormones.
-
Probiotics: The World Health Organization describes probiotics as ‘live microorganisms which, when administered in adequate amounts, confer a health benefit on the host’. Typically, probiotics can be found in dietary supplements or processed meals. The most popular food containing probiotics is yogurt, but other meals that may also include probiotics include cheese, infant or toddler formula, veggies, juices, smoothies, cereal, and unfermented and fermented milk. Three common components of the human gut microbiota, lactobacilli, streptococci, and bifidobacteria, are used in most probiotic supplements available today. However, research is ongoing on the possible probiotic functions of other microorganisms, like yeast (Saccharomyces boulardii), which are not typically present in the digestive system.
-
Synbiotics: A supplement that combines a prebiotic with a probiotic is called a synbiotic. Since synbiotics include both probiotic bacteria and a prebiotic component that promotes probiotic bacteria growth and survival in the gastrointestinal tract, they have the potential to have a more significant impact on the gut microbiota and host health than isolated intake of pre- or probiotics. Research indicates that synbiotics may effectively change the microbiota's makeup.
-
Fecal Microbial Transplant: Patients with Clostridium difficile infections have found fecal microbial transplants to be an effective treatment; however, the benefits of this procedure for other illnesses have yet to be explored well. Moreover, fecal microbial transplants carry risks since viral infections cannot be removed by filtering; therefore, they should only be considered as a last resort for treating human illnesses such as recurrent Clostridium difficile infection. Furthermore, fecal microbial transplants might be harmful to obesity as well.
Conclusion
Current evidence points to a link between obesity and the human gut microbiome. They indicate that lean and obese people have different gut microbiota bacterial compositions and that a Western diet heavy in fat and refined carbohydrates encourages the growth of intestinal bacteria associated with obesity. This begs whether obesity risk may be modulated by changing the microbiota or if individualized diet plans can be created based on an individual's microbiota. The most intriguing evidence supporting the significance of the interaction between a person's microbiome and food may come from a recent study that showed how customized diets for glucose homeostasis may be created using data on a subject's gut microbiota.
