Aetiology

Student guest post by Liz Stepniak

In the United States, the obesity epidemic is rapidly spreading. Since 1980 the prevalence of obesity has increased over 75%. Currently, over half the population is overweight, and nearly 1 in every 3 adults is clinically obese. Research has also been proliferating, exploring a plethora of possibilities to better understand and treat this growing epidemic. One of the recent trends in obesity research has been investigating the role of the microbiota in the gut and differences in the composition of these bacteria between obese and non-obese individuals. Could this be a potential treatment for obesity? Like many things in research, not all studies come to the same conclusion that there is an important role of gut microbiota in obesity.

Obesity is a result of alterations in the body's regulation of energy intake, expenditure, and storage. During a time when a large portion of the population worked in manual labor and it was necessary to conserve calories for long periods of time; this was an efficient mechanism. Currently, the majority of people do not have manual labor positions, so our energy expenditure is much lower but our bodies are still conserving those calories leading to an imbalance in regulating energy intake and expenditure and an overall increase in weight gain.

An important site to explore the regulation of energy intake and storage is the digestive system, specifically: the gut. The human gut has over 5,000 species of bacteria. There are three main phyla found in the gut: Firmicutes, Bacteriodetes, and Actinobacteria. One of the roles that bacteria plays in the gut is to help extract calories from what we eat, help store these calories for later use, and provide energy and nutrients for the production of new bacteria to continue this job. So, it is biologically plausible that if this was the primary job of one of these groups of bacteria or a certain bacteria phyla were more proficient in this job; that this could help explain a possible link to obesity.

The field of understanding metabolic balance and its role in obesity expanded with a December 2006 paper which discovered that an obesity-associated gut microbiome had an increased capacity for energy harvest. This study demonstrated overt differences between microbiomes of obese and non-obese mice and also indicated that this trait was transmissible among mice. Research has also shown that maintaining a proper ratio between Firmicutes and Bacteriodetes is necessary to maintain good health. Another study found that obese people have higher amounts of Firmicutes and lean people had higher amounts of Bacteriodetes. This study also found that the composition of bacteria shifted as the obese subjects lost weight. Enter: the probiotics push. An attempt to alter the composition of the microbiota in the gut.

This idea was spread further in April of 2008, when the Mayo Clinic published a paper which examined the role that bacteria play in the human gastrointestinal tract for regulating weight and obesity. Through experiments using mouse models, the investigators suggested that the manipulation of gut microbiota could be a useful strategy for regulating energy balance in obese people. They clearly state that this would not be a substitute for proper diet and exercise; this was to be approached as a novel tactic to treating obesity.

However, not all research has agreed upon a link between gut microbiota and obesity. A group of investigators at the University of Aberdeen Rowett contradicted this research and claimed there was no link between gut bacteria and a person's BMI level. This was a small study, only 33 obese participants and 14 participants of average weight and they only changed the carbohydrate portion of the diet. They further comment on this finding to say that they're not ruling out the possibility that a more detailed analysis of the gut bacterial community may reveal differences between obese and normal weight people in some different bacteria species that make up the Bacteroides and Firmicutes groups, which is directly linked to the diet. Essentially, the authors question whether there is an important difference linked to obesity in the composition of gut microbiota or if the composition is merely altered by diet.

Recently, I have seen several mentions of probiotics in the news, articles on maintaining proper digestive health, and frequent literature regarding gut microbiota and obesity. Although there have been multiple studies showing an association between gut microbiota and obesity, there is a significant amount of research needed to confidently classify the link between the role of gut bacteria and obesity. The big question remains whether this is just another short-lived trend in obesity treatment or if it has the ability to be sustained and make a positive impact in the obesity epidemic.

Maybe one day the answer to solving the obesity epidemic will be uncovered....

Until then... in the words of Michael Pollan: "Eat Food. Not Too Much. Mostly Plants." And exercise!!

Works Cited:

CDC: Obesity and Overweight.

Mayo Clinic (2008). Could Changing The Bacteria In Your Digestive System Be An Obesity Treatment? ScienceDaily.

Duncan SH et al. (2008). Human colonic microbiota associated with diet, obesity and weight loss. International Journal of Obesity 32, 1720-1724

Turnbaugh PJ et al. An obesity-associated guy microbiome with increased capacity for energy harvest. Nature 444, 1027-1021.

Grimes M. Jan 26, 2010 NaturalNews.com: Bacteria in the Gut Shown to Reduce Obesity

DiBaise JK et al (2008). Gut microbiota and its possible relationship with obesity. Mayo Clin Proc 83: 460-469.