Two phyla of bacteria, bacteroidetes and firmicutes, make up the bulk of the gut microbial species. (Quick high school biology taxonomy recap: animal organisms are classified in descending order of specificity as kingdom —> Phylum —> class —> order —> family —> genus —> species.)
As it turns out, individuals with more firmicutes tend toward obesity, while those with predominantly bacteroidetes tend to be leaner. This is because firmicutes can break down complex sugars that bacteroidetes can’t, turning them into simple sugars (and therefore absorbable calories). In other words, someone with more firmicutes can absorb more calories from a meal than would the same individual who had predominance of bacteroidetes instead.
As I mentioned in this article, people who consume more indigenous diets have greater biodiversity in their gut flora compared to those who consume the Standard American Diet—but, as it turns out, the same is true for leaner individuals compared to obese individuals, as well.
So two things appear to be related to staying lean, as far as your gut flora is concerned:
- greater variety, and
- more variety within the bacteroidetes phylum.