Introduction

In 2016, a scientific article published in The Lancet put forth the shocking finding that the numbers of obese people now outnumber those that are under-weight. Further, health risks associated with obesity are among the leading causes of death.

While there are many ways that fat accumulation can be influenced, one of the more recent areas that has received considerable attention is the microbiome. What is exciting about the relationship of a host’s microbiome to obesity is that there are many ways to influence the microbiome and therefore, potentially, affect the prevalence and severity of obesity. 

Adding to the excitement in this research area are advances in techniques that enable genetic ID and characterization of many types of gut organisms.  Further, researchers are taking advantage of these technique advances to characterize factors such as diet, age, gender, etc., on the microbiome.  Following will be a discussion of “target” organisms understood to be related to the prevalence of obesity.  Future articles will focus on how to influence these target organisms in ways that lessen obesity.

Important Microorganisms

In 2005, a significant study analyzed over 5000 bacterial RNA sequences from genetically lean and obese mice and reported that the Firmicutes to Bacteroidetes ratio was increased in the obese mice (Ley et al., 2005).  This altered ratio was due to a reduced prevalence of Bacteroidetes.  Similarly, Guo et al. (2008)found that obese pigs, when compared to lean pigs, had reduced Bacteroidetes populations which resulted in an increased Firmicutes to Bacteroidetes ratio.  Ley et al. (2006)went on to confirm that Bacteroidetes levels were lower in obese compared to lean humans.  

While many studies focus on the microbiome of the intestinal environment, Craig et al. (2018)studied the relationship between the oral microbiota composition and its association with rapid infant weight gain, a strong risk factor for childhood obesity.  Researchers found that rapid infant weight gain occurred when the oral microbiota composition of children was decreased in the level of Bacteroidetes resulting in an increase in the Firmicutes to Bacteroidetes ratio.  

However, this is not a consistent finding.  Schwiertz et al. (2010)found the opposite to be true – increased BMI was associated with a lower Firmicutes to Bacteroidetes ratio.  A recent review (Castaner et al., 2018) of several human studies involving obese individuals as well as bariatric surgery patients further suggests that the association between obesity and Bacteriodetes level and(or) Firmicutes to Bacteroidetes ratio is inconsistent.  Another recent review (Idiani et al., 2018) of risk factors for childhood obesity utilized scrutinous inclusion criteria for which studies to review. Findings from the Idiani et al. (2018)review cautiously affirmed that decreased Bacteriodetes and increased Firmicutes spp. was associated with an increased obesity (ie, body mass index; BMI).  The reasons for these discrepancies between reviews are unclear.  However, the analytical technology used to identify and categorize different strains of bacteria continues to evolve and may play a role.

Another bacterial species that appears to have some relationship with obesity is Akkermansia.  Increased obesity, type 2 diabetes, body weight, and fasting blood glucose is associated with individuals having Akkermansia levels lower than in normal weight humans and mice (Tilg and Moschen, 2014).  A study in Columbian individuals with high levels of Akkermansia-Bacteroidales and Rumino coccaceae-co-associated bacterial groups had reduced risk of cardiometabolic disease and obesity (Cuesta-Zuluaga et al., 2018) compared to those with lower levels of those bacterial groups.  While a quick search indicated that no commercial supply of Akkermansia exists, there are a number of food ingredientsthat can help promote its growth including fructooligosaccharides (FOS), polyphenols, inulin, arabinoxylans, fish oil and navy beans (Tilg and Moschen, 2014; Naito et al., 2018; Monk et al., 2016; Costantini et al., 2017; Van den Abbeele et al., 2011).

Yet another “microorganism” association with obesity appears to be a “diversity index” related to the relative diversity of the existing microbiota population.  Increased diversity implies a more robust population.  In some obesity related studies (Riva et al., 2017; Cuesta-Zuluaga et al., 2018), a reduced diversity index has been associated with an increase in obesity.  The oral microbiota study in children reported by Craig et al. (2018)also found reduced diversity with increased risk of obesity (ie, rapid infant weight gain).  However, the review by Castaner et al. (2018)points out that a reduced diversity index is not always associated with obesity. 

Conclusions

Several aspects of the microbiota have been discussed as they relate to obesity.  Overweight/obese individuals tend to have less diverse intestinal microbiota populations. The Firmicutes:Bacteroidetes relationship generally is increased due to decreased Bacteroidetes populations in individuals that are overweight or obese.  Increased amounts of Akkermansia populations appear to be associated with more healthy weight individuals.  Of course, not all studies/reviews consistently report these effects.  And, while consuming a probiotic may be an obvious approach to altering the above microbiota populations, other approaches, such as diet, may be just as important, if not more so, in achieving desired microbiota populations.  Finally, inconsistent findings from study-to-study indicate that future discoveries will be needed to better understand the ideal microbiota composition and its relationship to obesity.

A future article will explore possible dietary interventions on the microbiome to address obesity.

References

Noted throughout article.