Firmicutes / Bacteroidetes Ratio

The Firmicutes/Bacteroidetes (F/B) ratio is a classic metric that assesses the balance between the two dominant bacterial groups in your gut (which together account for over 90% of your microbiota). Historically, this number has been used as a "metabolic thermometer": a high ratio is associated with greater efficiency in extracting calories from food and a predisposition to weight gain, while a balanced ratio favors the maintenance of a healthy weight.

In your gut, imagine the Firmicutes as the "savers." They are extremely efficient at breaking down fats and complex sugars into absorbable calories. If you have too many, it's like having a "too efficient" digestive system: you absorb more energy from the same apple than another person. Conversely, Bacteroidetes are the "spenders" or fiber specialists. They prefer to feed on plants and are not as focused on accumulating energy reserves.

The goal is not to eliminate Firmicutes (many are vital, such as Faecalibacterium), but to maintain a healthy proportion where neither excessively dominates the other. A diet high in processed foods and low in fiber tends to dangerously tip the scales towards Firmicutes.

This ratio acts as a high-level indicator of your metabolic phenotype:

• Weight Control and Obesity: Multiple studies have observed that individuals with obesity tend to have a significantly higher F/B ratio than lean individuals. Upon weight loss, this ratio typically normalizes, with an increase in Bacteroidetes.
• Dietary Response: A high ratio may explain why some individuals find it harder to lose weight than others while eating the same amount. Their microbiome is "programmed" not to waste a single calorie.
• Inflammation: An extreme imbalance (dysbiosis) in this ratio has also been linked to increased gut permeability and low-grade systemic inflammation.

The calculation is performed by summing the relative abundance of all taxa belonging to the phylum Firmicutes and dividing it by the sum of the phylum Bacteroidetes, obtained by Shotgun metagenomic sequencing.

Although it is a useful metric for an overview, in the era of modern metagenomics it is considered a "macroscopic" marker that should be interpreted with nuances. A high F/B ratio can be due to an excess of Clostridium (negative) or an excellent abundance of Lactobacillus and Faecalibacterium (positive), both Firmicutes. Therefore, this data is always cross-referenced with detailed species analysis.