The Gut Microbiome and Obesity

Previous research has established the gut microbiome as a potential mediator of weight gain and obesity. The proposed mechanism for this is the extraction of energy from dietary sources by gut flora, with obese-associated gut microbiomes extracting more energy than average from food intake. This excess energy is then stored as adipose tissue, which contributes to increased fat mass.

This concept was established about a decade ago by experiments in which an obese-associated gut microbiome was transplanted into mice with controlled gut microbiota. Results showed that after transplantation, these mice gained more overall body mass and more body fat than those transplanted with a lean-associated gut microbiome. This phenomenon has since sparked further research into the relationship between the gut microbiome and obesity, with a focus on how gut flora can be utilized for personalized nutrition recommendations in weight loss.

Enterotypes

Early on in gut microbiome and obesity research, the ratio of the two bacteria Firmicutes and Bacteroidetes was hypothesized to be a measure of predisposition to obesity. However, further studies showed that the balance between these two organisms was not correlated with energy harvest from diet. As a result, this ratio is no longer used as a relevant measure of gut microbiome-related obesity. 

Instead, a system of categorizing individuals based on a single predominant bacterium was developed and is currently in use. This method divides individuals into one of three groups, or enterotypes, based on the main organism that is present in their gut microbiome. The three organisms used as categories are prevalent in individuals globally and are relatively stable over time, making them suitable for grouping purposes. 

The reason why these organisms are such good markers for obesity is because they are associated with different nutrient degradation capacities. In other words, individuals in which one of these three microbes is predominant may break down carbohydrates, proteins, or fats more efficiently than other enterotypes. This shows potential for personalizing nutrition recommendations based on one’s gut microbiome. In addition, according to a 2022 study, enterotypes differ in energy extraction capacity, which explains their relationship to obesity and metabolism. 

Interestingly, this study revealed that these enterotypes correlated with body weight, supporting the hypothesis that the predominant organisms have a role in energy metabolism. The three enterotypes, along with their relationship to body weight and nutrient metabolism, include:

1. B-type, characterized by predominant Bacteroides

B-type subjects were found to have significantly higher body weights when compared to R-type in particular. B-type microbiota also have a higher capacity to metabolize carbohydrates and proteins than other groups, indicating that diets high in whole grains and fiber are not conducive to weight loss in these individuals. 

2. R-type, characterized by predominant Ruminococcaceae

R-type individuals were found to be the leanest of the three enterotypes. They were good metabolizers of proteins, but less efficient metabolizers of carbohydrates and fats. 

3. P-type, characterized by predominant Prevotella

P-type individuals were an intermediate between R and B-types in terms of body weight. Those with P-type microbiome composition were poor metabolizers of whole grains and fiber, so they tended to lose more weight on diets rich in carbohydrates than other groups. 

Impact of Stool Energy Density 

The impact of these enterotypes on body weight and their metabolic role can be explained by stool energy density. Low stool energy density implies greater efficiency of the gut microbiome in harvesting energy from food. On the other hand, higher stool energy density means that some calories from food are excreted rather than taken up by the gut bacteria. This excreted energy is not taken up by the body and therefore cannot contribute to fat storage and weight gain. Therefore, obesity-related gut microbiome compositions tend to be associated with a lower stool energy density than lean-associated microbiota. 

This holds true for the enterotypes previously discussed. In fact, B-type individuals have both the lowest stool energy density and the highest body weight of the three groups. This indicates that the propensity of the predominant gut microbiota to extract energy from the diet determines predisposition to obesity. 

Effect of Transit Time

Transit time, or the time it takes for food to pass through the intestines, was also related to the gut microbiome’s metabolic role. Intuitively, one might think that slower stool transit time is associated with a lower stool energy density because there is more time for the gut microbiota to extract energy from stool. However, the study by Boekhurst et al. showed that the opposite was true. 

In reality, a positive association exists between transit time and stool energy density. B-type individuals with lower stool energy density and higher energy extraction by the gut microbiome actually had more rapid transit times. In contrast, R-type individuals, who had high stool energy density and low energy extraction had high transit times. As expected, P-type individuals were in between B-type and R-type in terms of transit time as well.

Impact of Diet on Microbial Diversity

Given this information, you might be wondering if there is a way to change one’s gut microbiome and what factors influence enterotype. Interestingly, an individual’s enterotype is strongly influenced by long-term dietary patterns, as shown by a 2011 study. Perhaps unsurprisingly, B-type gut microbiota, which have the least microbial diversity and are the most associated with obesity, are related to a Western long-term dietary pattern. P-type gut microbiota are correlated with an intermediate body weight and a more Mediterranean style diet rich in complex carbohydrates such as fruits, vegetables, and legumes. On a positive note, diet can induce changes in enterotype state, meaning that there is a way to improve gut health through long-term eating habits. 

Summary/Conclusions

Overall, gut microbiome can be categorized into three enterotypes, B-type, P-type and R-type, based on a single predominant organism. This classification lends insight into the role of the gut microbiome in metabolism and energy extraction from the diet. 

The B-type microbiome is most associated with weight gain because it is most efficient at extracting energy from food, while the R-type microbiome is found in leaner individuals because of its lack of efficiency at energy extraction. In addition, B-type individuals are better metabolizers of carbohydrates and proteins. R-type people are most efficient at breaking down proteins, and those in the P-type group are particularly poor metabolizers of carbohydrates. These differences in energy processing and metabolism suggest a potential role of personalized nutrition in weight loss.

Importantly, enterotype is not a permanent state and is strongly influenced by long-term dietary patterns. For example, a B-type microbiome is strongly associated with a Western diet and obesity, while a P-type microbiome is linked to an intermediate body weight and a more Mediterranean diet. This implies that one’s nutrition can greatly influence both metabolic health and body weight, which are related through the health and diversity of the gut microbiome.