If you've spent any time reading about the gut microbiome, you've probably come across the Firmicutes-to-Bacteroidetes (F/B) ratio. It's often presented as a simple number that tells you whether your gut is "healthy" or "unhealthy," with high ratios linked to obesity and low ratios framed as ideal. But the science is far more nuanced than the headlines suggest.
Researchers have examined this ratio in contexts ranging from obesity [3] and irritable bowel syndrome [5] to food preservatives [4] and dietary supplements [7]. What they've found is that the F/B ratio can shift in response to many factors, but it doesn't always tell a straightforward story. In some studies, the ratio didn't differ at all between groups with clearly different health outcomes [8]. Here's what the research actually says.
Key takeaways
- The F/B ratio is a real but incomplete snapshot — it can't capture the complexity of your gut microbiome on its own [3][8].
- The ratio shifts with diet, supplements, preservatives, and other factors, but a change doesn't automatically mean better or worse health [4][5][7].
- Specific bacteria within these phyla, overall diversity, and microbial networks may matter more than the phylum-level ratio [2][3][8].
- In obesity research, the ratio has produced conflicting results and sometimes doesn't differ between groups with different outcomes [3][8].
- Symptoms and how you feel remain an important complement to any microbiome metric [5][7].
What Are Firmicutes and Bacteroidetes?
Firmicutes and Bacteroidetes are two major phyla — broad groupings — of bacteria that dominate the human gut. Together, they make up a large share of the intestinal microbial community, which is why their relative balance has attracted so much scientific attention.
These phyla contain countless individual genera and species, each with different functions. For example, within Firmicutes you'll find families like Lachnospiraceae and Lactobacillaceae, while Bacteroidetes includes genera like Parabacteroides [2]. Because each phylum contains such a wide variety of organisms, a shift in the overall ratio between them doesn't necessarily tell you which specific bacteria are changing or what those changes mean for your health.
The Obesity Connection — and Its Complications
The F/B ratio first gained attention through obesity research. The gut microbiome is now recognized as one of many environmental factors that may contribute to obesity, alongside diet, exercise, and genetics [3]. Some early findings suggested that obese individuals tended to have a higher proportion of Firmicutes relative to Bacteroidetes.
However, a 2022 review noted that the interaction between gut microbiota and obesity is highly complex, and that published studies have produced discordant — sometimes contradictory — findings [3]. The review proposed that it may be more useful to think in terms of collective microbial networks or "guilds" rather than relying on a single ratio [3].
Strikingly, a 2024 fecal microbiota transfer study in mice found that although weight gain differed between groups, the Firmicutes/Bacteroidetes ratio did not differ between them [8]. What did differ were the abundances of specific families and genera, such as Ruminococcaceae, Desulfovibrionaceae, and Lachnospiraceae — some of which correlated positively with body weight [8]. This suggests that looking past the phylum level may be far more informative.
What Actually Shifts the Ratio?
Several studies show that the F/B ratio can change in response to dietary and environmental factors:
- Bread and IBS symptoms: In a pilot study of ulcerative colitis patients with IBS-like symptoms, traditionally elaborated bread suggestively reduced the F/B ratio (p = 0.058), and this decrease appeared associated with symptom improvement [5].
- Food preservatives: Sodium sulfite, a common preservative, produced a significant decrease in the F/B ratio in mouth microbiota samples, along with changes in the relative abundance of Proteobacteria, Firmicutes, and Bacteroidetes [4].
- White kidney bean extract: Supplementation with this nutraceutical decreased Bacteroidetes and increased Firmicutes in healthy adults — effectively raising the F/B ratio — though the changes were not significantly different from control, and no effects on GI inflammation or symptoms were observed [7].
- Exercise: Resistance and endurance training in mice shifted gut microbiota composition, but significant differences appeared mainly at the genus and species level rather than at the phylum level where the F/B ratio lives [2].
These examples highlight that the ratio can move in either direction depending on the intervention, and a shift doesn't automatically mean "better" or "worse."
Why the Ratio Alone Falls Short
One of the biggest takeaways from the research is that the F/B ratio, by itself, is an incomplete picture. Consider these points:
- In the fecal microbiota transfer study, the ratio was identical between lean and heavier mice, yet specific bacterial families clearly differed and correlated with body weight [8].
- In the white kidney bean extract trial, the ratio shifted but produced no measurable changes in GI symptoms or inflammation [7].
- In the bread study, the ratio reduction was only suggestively significant (p = 0.058), and no statistically significant differences were found at any other taxonomic level [5].
- The obesity-microbiome review emphasized that findings across studies are often discordant, likely because the microbiome functions through complex trophic networks rather than simple phylum-level balances [3].
In other words, two people could have the same F/B ratio but very different gut health, depending on which specific bacteria within those phyla are present and what they're doing.
What This Means for Your Gut Wellness Journey
So should you care about the F/B ratio? It can be a useful data point, but it shouldn't be treated as a verdict on your gut health. Here's a practical way to think about it:
- Focus on diversity: Endurance training in mice was associated with higher microbial diversity and evenness — metrics that may be more meaningful than a single ratio [2].
- Consider the whole diet: Green tea has been shown to correct microbial dysbiosis associated with conditions like obesity, potentially through its effects on specific bacterial species and their metabolites rather than through simple ratio shifts [1].
- Pay attention to symptoms: In the bread study, the ratio shift was associated with symptom improvement, suggesting that how you feel matters alongside what your microbiome looks like on paper [5].
- Remember context: Even common food additives like sulfite preservatives can alter the ratio, which means dietary choices well beyond "healthy" or "unhealthy" foods may influence this number [4].
The microbiome is shaped by diet, environment, medications, stress, and countless other factors [3][14]. A single ratio can't capture all of that complexity.
The Bottom Line
The Firmicutes-to-Bacteroidetes ratio is a real and measurable feature of the gut microbiome, and it does respond to interventions like diet changes, supplements, and food additives [4][5][7]. But the research consistently shows that it is not a reliable standalone indicator of health or disease.
Studies in obesity have produced conflicting results [3], and at least one well-designed experiment found no difference in the ratio between groups with clearly different weight outcomes [8]. What matters more may be the specific bacteria and microbial networks within those phyla, the diversity of the overall community, and how those microbes interact with your body [2][3][8].
If you're working on your gut wellness, the F/B ratio is worth noting — but it's one thread in a much larger tapestry.
Frequently asked questions
Is a high Firmicutes-to-Bacteroidetes ratio always bad?
Not necessarily. While a higher ratio has been associated with obesity in some research, findings are discordant and complex [3]. One study found no difference in the ratio between mice with different weight outcomes, suggesting the ratio alone isn't a reliable indicator [8].
Can I lower my F/B ratio through diet?
Dietary changes may influence the ratio. A pilot study found that traditionally elaborated bread suggestively reduced the F/B ratio, and this was associated with improved IBS-like symptoms [5]. Food preservatives like sodium sulfite also altered the ratio [4], showing that many dietary factors can play a role.
Should I get my F/B ratio tested?
The ratio can be measured through 16S rRNA gene sequencing, but researchers caution that it doesn't tell the whole story. Specific genera and families within these phyla, as well as overall microbial diversity, may be more informative than the ratio alone [2][3][8].
Does exercise change the F/B ratio?
Exercise does alter gut microbiota composition, but a study comparing resistance and endurance training in mice found that significant differences appeared mainly at the genus and species level rather than at the phylum level where the F/B ratio is measured [2]. Endurance training was associated with higher overall diversity.
If the F/B ratio isn't the best metric, what should I look at?
Research suggests that microbial diversity, the abundance of specific bacterial families and genera, and the presence of functional microbial networks may be more meaningful than the F/B ratio [2][3][8]. How you actually feel — symptoms, digestion, and overall wellbeing — is also an important measure [5].
References
- Green Tea and Its Relation to Human Gut Microbiome — Pérez-Burillo S et al., 2021, Molecules (Basel, Switzerland)
- Resistance and Endurance Exercise Training Induce Differential Changes in Gut Microbiota Composition in Murine Models — Fernández J et al., 2021, Frontiers in physiology
- Recent insights into the role of microbiome in the pathogenesis of obesity — van der Vossen EWJ et al., 2022, Therapeutic advances in gastroenterology
- Sulfite preservatives effects on the mouth microbiome: Changes in viability, diversity and composition of microbiota — Irwin SV et al., 2022, PloS one
- Impact of bread diet on intestinal dysbiosis and irritable bowel syndrome symptoms in quiescent ulcerative colitis: A pilot study — Lluansí A et al., 2024, PloS one
- Phytonutrients and their neuroprotective role in brain disorders — Adhikary K et al., 2025, Frontiers in molecular biosciences
- White kidney bean extract as a nutraceutical: effects on gut microbiota, alpha-amylase inhibition, and user experiences — Houghton D et al., 2023, Gut microbiome (Cambridge, England)
- Increase in body weight is lowered when mice received fecal microbiota transfer from donor mice treated with the AT<sub>1</sub> receptor antagonist telmisartan — Freschi ML et al., 2024, Frontiers in pharmacology
- Exploring bacterial key genes and therapeutic agents for breast cancer among the Ghanaian female population: Insights from In Silico analyses — Kibria MK et al., 2024, PloS one
- Human placental mesenchymal stem cells ameliorates premature ovarian insufficiency via modulating gut microbiota and suppressing the inflammation in rats — Liu S et al., 2025, PloS one
- <i>Lactobacillus johnsonii</i> HL79 modulates the microbiota-gut-brain axis to protect cognitive function in mice chronically exposed to high altitude — Zhao Z et al., 2025, Frontiers in microbiology
- Spatiotemporal structure and composition of the microbial communities in hypersaline Lake Magadi, Kenya — Kipnyargis A et al., 2024, F1000Research
- Effects of storage conditions on the microbiota of fecal samples collected from dairy cattle — Jaramillo-Jaramillo AS et al., 2024, PloS one
- Therapeutic Remodeling of the Gut Microbiome as a Strategy to Restore Immune Tolerance in Autoimmunity — Boroumand B et al., 2026, MicrobiologyOpen
- Effects of pesticide application on soil bacteria community structure in a cabbage-based agroecosystem in Ghana — Peprah S et al., 2025, PloS one
- LuxS/AI-2 regulates phoP/phoQ by a non-canonical mechanism to enhance acid stress survival in Salmonella Typhimurium — Singh A et al., 2026, PLoS pathogens