Bacteroidetes/Firmicutes ratio

The Bacteroidetes/Firmicutes ratio was prominently proposed in the mid-2000s as a marker of your gut microbiome's health. It came from observations in obese mice and small human studies: obesity went with relatively fewer Bacteroidetes and more Firmicutes, and weight loss reversed the pattern. The hypothesis was that a high-Firmicutes gut extracts more energy from the same food, promoting fat gain. But this tidy story has not held up across diverse human cohorts. Larger studies found the ratio varies a lot with the sequencing method, diet, geography, and who is in the cohort. Many found no consistent direction of association with obesity or metabolic health. So the field has largely set the ratio aside as a clinically meaningful metric. Splitting the whole gut microbiome into just two phyla throws away the huge functional diversity within each one, and species- or gene-level analyses are far more informative. The ratio lives on mostly in popular science writing and supplement marketing, where it is often overstated.

Last reviewed: June 22, 2026

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