Your microbiome is the collective term for all the bacteria that populate your gut—all 100 trillion of them. Even though your microbiome is not technically “you,” it’s so important to your body’s function that it’s been characterized as another organ.

Among other things, your microbiome helps you break down your food. The more diverse your diet, and the more replete it is with prebiotics (food for the bacteria), the more diverse the populations in your gut will be.


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:

  1. greater variety, and
  2. more variety within the bacteroidetes phylum.


What you feed, grows. If you feed the bacteroidetes population, it will grow… but if you feed the firmicutes population, it will grow!

What bacteroidetes eats is fiber (veggies, legumes, fruits, 100% whole grains). What firmicutes eats are refined carbs and sugars. 

Disclaimer here: it is certainly true that two people on the exactly same high fiber, low refined carb diet but who have a different ratio of bacteroidetes to firmicutes will not lose the same amount of weight… but all else being equal, the person with high firmicutes who begins to feed their bacteroidetes population instead of their firmicutes population will cause the ratio between the two to shift… for the better!


  • Cut out the refined carbs and sugar.
  • Increase your intake and variety of high fiber foods (veggies, legumes, fruits, 100% whole grains). These are prebiotics—food for the good bacteria.
  • Avoid unnecessary antibiotics, as this can create low biodiversity, shifting the ratio in favor of firmicutes.
  • Take a good probiotic, unless you eat fermented foods (those naturally high in probiotics, such as yogurt, kefir, kombucha, and sauerkraut) on a regular basis!