The World Health Organization defines obesity in adults as a body mass index (BMI) ≥30 kg/m2 and overweight as a BMI ≥25 kg/m2.1 Worldwide, approximately 1.9 billion adults are overweight, with 650 million of those classified as obese.1 This global epidemic is responsible for economic drain and is associated with numerous chronic disorders and comorbidities, including cardiovascular disease, cancer, sleep apnea, liver disease, and infertility. As per the Centers for Disease Control and Prevention, the healthcare cost of obesity in 2008 was $147 billion.2 Effective treatments therefore have the potential to decrease the incidence and ultimately the economic impact of obesity.
While it is known that the human microbiome has an impact on disease, this relationship has not been robustly studied until recently. The human gut microbiota has been identified as a metabolic organ having a significant effect on host metabolism. Lean individuals harbor a diverse microbiota that requires extraction of less energy from food sources. In contrast, an obese individual’s microbiota is less diverse and extracts more energy from food. The obese gut microbiome becomes more diverse with weight loss and exercise. Although it is not yet clear how long this change lasts, increased intestinal microbiome diversity has been found to be associated with increased host metabolism.
Establishing the ideal gut microbiome has yet to be determined. Probiotics and prebiotics may alter this microbiome, although no clear evidence exists suggesting that this alteration can diversify in a way to increase metabolism for the long term. However, it is known that antibiotic use can decrease diversity permanently.3 Most widely known as a successful treatment for Clostridioides difficile colitis, fecal microbiota transplant (FMT) may have potential as an effective therapeutic option for the treatment of obesity. FMT is the administration of a solution of fecal matter from a donor into the intestinal tract of a recipient, directly altering the recipients’ gut microbiome. The procedure is performed via enema, encapsulation, colonoscopy, or nasogastric tube.
The layman’s view that lean individuals have a faster metabolism than their heavier counterparts is far from the truth. Historically, metabolism has been studied using the basal metabolic rate, which was first defined in 1924.4 The basal metabolic rate is derived from a standard calculation using height, weight, age, and gender as the sole factors affecting metabolism. This tool determines total daily energy expenditure or the number of calories burned at rest. Throughout history, researchers have looked at factors in addition to BMR that have the potential to affect metabolism such as genetics, race, biochemical parameters, environmental factors, health status, and diet composition.5 Identifying the gut microbiota as a metabolic organ combines both old and new information. Scientific evidence of the human microbiome and its impact on disease has existed since the 1880s. This knowledge has only recently expanded with the National Institutes of Health Common Fund Human Microbiome Project of 2007.6 This project was established with the mission of generating research resources that have been the basis for several studies analyzing the human microbiota and its role in overall health and disease.6 Manipulation of the gut microbiome represents a promising therapeutic approach not only for obesity but also for autoimmune diseases, mood disorders, type 2 diabetes, and dermatologic conditions.