Have you ever wondered why two people can eat the same amount of food, but weigh differently? This may be due to the gut microbiome, the taxa of microorganisms residing in the gut and their genes.

Many people have tried to find correlations (and remedies) to obesity, a health problem that affects 13 percent of the global population. Studies from chili peppers to the amount of sleep, to vitamin C, have all tried to find probable cause and prevention of obesity. But there is another factor. Research of microbiomes might reveal a new mechanism for weight gain, beyond diet, lifestyle and genetics.

How the Gut Microbiome Shapes Obesity

The gut microbiome is the taxa of microorganisms residing in the gut (the microbiota) and their genes. The gut microbiota consists of trillions of microbes, including bacteria, archaea, viruses and single-celled eukaryotes. Current estimates reveal around 1,000 distinct bacterial species in the gut, with the most common phyla being Bacteroidetes and Firmicutes.

Research in recent years has highlighted the key function of the microbes in the gut for digestion and uptake of nutrients. Microbes play vital roles in the fermentation of unused energy substrates, maintenance of the immune system and synthesis of vitamins and enzymes. Furthermore, several studies have established strong links between the gut microbiome and certain diseases, including obesity, diabetes, and metabolic syndrome.

In obesity, it is proposed that the fermentation activity of the gut microbiome is increased. Through increased metabolism of ingested polysaccharides, the bioavailability of these molecules is also increased. This leads to a greater proportion of the energy in food being stored in adipose tissue as fat, thus resulting in weight gain. In other words, individuals harboring certain microbes in their gut will gain more weight from the same amount of food than a person lacking these microbes.

Enterotypes and Weight Gain

A milestone study, carried out at the European Molecular Biology Laboratory (EMBL) in Germany, proposed that there are three distinct ‘types’ of gut microbiome. Similar to blood types. These ‘enterotypes’ were defined by the specific species of microbes present and were named according to the most dominant genus; Bacteroidetes, Prevotella, and Ruminococcus.

This finding was significant as it changes how microbiome variation is perceived. Rather than being a continuum, where the composition is sensitive to differences in lifestyle, diet, and genetics, it was instead clustered into a limited number of enterotypes, seemingly independent of geography or differences in lifestyle, diet, and genetics.

This would have major implications for understanding how people with different enterotypes would respond to pharmaceutical drugs and diet and could mark the first step towards tailored medications and/or supplements.

More specifically, this could have implications for obesity treatments. Of the types, Bacteroidetes, Prevotella and Ruminococcus, it is known that Bacteroidetes are particularly good at metabolizing carbohydrates. Thus, people who fit into this type may be more prone to gaining weight from carbohydrate rich foods. Ruminococcus are associated with increased absorption of sugars, which may also contribute to weight gain through sugary foods. These links remain somewhat speculative, but certainly, raise some interesting and potentially highly disruptive avenues for personalized and rationally designed obesity treatments.

How Do Lifestyle and Diet Factor Into the Enterotype Theory?

Since the initial postulation of gut enterotypes, further studies have questioned the over-simplification of the Enterotype theory. At first, the number of types was queried, with many studies finding evidence only for the Bacteroides and Prevotella dominated types. Also, with more data from broader samples, the gut microbiome has been more closely linked with geographical location and lifestyle factors. This is in disagreement with the original proposal that enterotypes were somewhat independent of such factors. Most recently, it has been suggested that the concept of gut microbiome types be re-defined as ‘biomarkers’ for diet, lifestyle and disease state.

A recent study found that the communities of microbes on computer keyboards and mice could be correlated to the skin microbiomes of the hands of their owners, with 95% accuracy. It was postulated that the microbial signature could be viewed much like a fingerprint. While the skin microbiome is exposed to very different conditions compared with the gut microbiome, it is possible that a similar concept could be applied. In this case, an individual would have a gut microbiome signature which is shaped by their enterotype, but also diet, lifestyle and genetic influences. In this case, enterotypes would still be useful for understanding a patient’s gut microbiome at a high level. However, in order to define an individual by their microbiome, as well as to use this to design tailored medicines, it would be crucial to understand their microbial signature at a higher resolution.

Conclusion

The association between the gut microbiome and obesity is well-supported in the literature, whereas, the existence and definition of distinct enterotypes are less clear. One thing is certain, that the gut microbiome is a highly complex system which may hold the key to effective, tailored treatments for obesity.

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