According to a recent report from the Organisation for Economic Co-operation and Development (OECD), over half of all adults in the OECD area are overweight, and 18% of adults are obese.1 Environmental factors may be a major factor underlying the obesity ‘epidemic’, prompting researchers to harness the potential of metagenomics to comprehensively examine the communities of microorganisms inhabiting the human gut (known as the microbiota) as a novel environmental factor associated with obesity. Metagenomics is the genomic analysis of microbial DNA that enables culture-independent investigation of microbial communities in complex sample material, and the rationale is primarily that differences in the structure and function of the microbiota are key to individual differences in energy harvest and storage.
Recently, Hu et al. carried out an in-depth analysis of the gut microbiota from 67 obese and 67 normal-weight Korean adolescents.2 The researchers sequenced bacterial ribosomal genes (16S) obtained from stool samples in order to obtain data on the gut bacterial composition of the two groups according to taxonomic rank. Operational taxonomic units were identified using the much-used software QIIME, and taxonomic abundance was calculated using the RDP Classifier.
Firmicutes/Bacteroidetes (F/B) ratios have often been calculated and used to identify potential differences in the microbiota of obese and non-obese individuals. The authors of the present study were not able to identify differences in F/B ratios among obese and non-obese adolescents; to this end, no differences could be detected between the two groups at the phylum level. Meanwhile, significant differences were noticed at the family and genus levels: the proportion of Bacteroides in normal-weight and obese adolescents was 45% and 25%, respectively. Conversely, the proportion of Prevotella in normal-weight adolescents was 16%, but in obese adolescents it was 35%. The authors developed an algorithm that enabled prediction of obesity versus non-obesity based on the relative composition of the genera Bacteroides, Alistipes, Prevotella, Faecalibacterium, and Oscillibacter.
While the title of the article appears to imply that the development of obesity drives gut microbiota in a particular direction, the team does not actually show this. The data presented here are cross-sectional, and no longitudinal data are presented. This means that we do not know whether certain changes in the microbiota lead to obesity or obesity leads to certain changes in the microbiota. Safe to say, however, is that in this particular study cohort, obesity was associated with a certain microbiota profile at the family and genus level.
Regardless of any shortcomings, the work is fascinating. In the event that methods (e.g. sampling methods, DNA extraction from stool, and taxonomic analysis) can be standardized, it may prove useful to simply look at the distribution of bacteria in stool samples in order to identify microbiota differences between disease phenotypes—in this case obesity versus non-obesity—with a view to manipulating the gut flora towards a healthy microbiota. However, there is still a need to identify to which extent the composition of bacterial communities present in stool in fact reflects the relative distribution of these bacteria in the digestive system. Probably even more importantly, it should be investigated to which extent different bacteria share functional properties. Hence, we need to know whether the function of one type of microbiota can be more or less the same as a different type of microbiota due to the potential ability of bacteria to adapt to various ecological and physiological situations. Furthermore, variation in the structure and function of the microbiota due to differences in genetic factors (age, gender, ethnicity, etc.) should be investigated.
While studies aiming to link microbiota profiles to obesity are booming, we still need large amounts of standardized data in order to answer the question as to if and how we may be able to prevent obesity and metabolic disorder by manipulating gut microbial communities (for instance by prebiotics and/or probiotics), or—potentially—more bespoke cocktails of bacteria. We also need to develop a deeper understanding of the potential short-term and long-term effects of changes in diet on our gut flora. The article by Hu et al. cites a good many papers that have been central to the research linking the gut microbiota with diet and metabolism, so if you’re new to the area, this is a good place to start!
- OECD. Obesity update. June 2014.
- Hu H-J, Park S-G, Jang HB, et al. Obesity alters the microbial community profile in Korean adolescents. PLoS ONE 2015 10: e0134333.