In studies conducted on mice, the guts of obese mice had higher levels of firmicutes and lower levels of bacteroidetes. In normal mice, the opposite was true. Transplanting the gut bacteria from obese mice to normal mice caused the normal mice to gain weight.
By analyzing mice poop, scientists reached the conclusion that firmicutes are better at extracting energy from food when compared to bacteroidetes. This means that if you have a strong firmicutes population in your gut, more of your food will be converted to energy (calories) for the body to use or store as fat.
What are some practical implications of this research? Basically, modulating the baseline of your gut microbiome population to include a higher proportion of bacteroidetes compared to firmicutes. This can be achieved by eating – no surprises – foods high in fiber as well as foods containing probiotics and prebiotics.