Mixed Epidemiological Evidence Linking Gut Microbiota with Obesity

Within the last decade, there has been a revelation that the complex assemblage of microflora residing in the large intestine of humans contribute to metabolic health. ¹ Previously the role assigned to these microorganisms was primarily the fermentation of undigested material in our diets, mostly fiber. Recent landmark papers have implicated the actions of microbiota in the functioning and health of various tissues, including the intestine (inflammation and proliferation), the liver (glucose metabolism), adipose tissue (energy balance), and the brain (satiety and mood).^2–6 These studies have shown that gut microbiota can impact the partitioning of energetic nutrients, through absorption in the intestine, as well as through catabolic versus anabolic pathways. Furthermore, a link between gut microflora and human obesity has been made.^7,8 These data are supported by evidence that changes in the gut microbiome can be altered following weight loss induced by bariatric surgery. ⁹ However, causal mechanisms linking microbial populations and the development of obesity have not yet been delineated.

In a study by Lin and Massey, ¹⁰ using epidemiological approaches, the authors assessed associations between gut microbiota and body mass index (BMI). A previous study had demonstrated a positive relationship between methane on breath test, which is considered a surrogate marker for intestinal Methanobrevibacter smithii colonization, and BMI. ¹¹ For the current study, the authors retrospectively analyzed data from patients (N=535) who underwent a breath glucose test for evaluation of gastrointestinal symptoms, including diarrhea, bloating, abdominal distension, abdominal pain, and excess gas. This is a standard clinical test used to diagnose bacterial overgrowth; methane gas excretion was used as a marker of fermentation and thus the presence and size of the methonogenic bacteria population. The findings of this study were that presence of methanogenic bacteria was not associated with higher BMI. However, in a subgroup of patients who were positive for methanogenic bacteria, higher methane production was negatively associated with BMI. This study's findings add to the already mixed epidemiological evidence relating measures of gut microbiota with adiposity in populations.^12,13 Strengths of the study include the large sample size and collection of longitudinal data on obesity. Future studies will need to establish the validity of breath tests as a quantitative measure of bacterial populations, as well as the generalizability of studies of patients who present with symptoms of bacterial overgrowth, to the general population. If specific gut microbiota are determined to play a role in the etiology of obesity, pharmacological targets can be developed to decrease the prevalence of obesity in at risk populations.