In adults, Firmicutes and Bacteroidetes are the most abundant bacterial phyla in the gut. The Firmicutes to Bacteroidetes (F/B) abundance ratio was shown to increase from infancy to adulthood and subsequently to decrease again in the elderly. Elevated F/B ratios have been linked to obesity though the evidence is not conclusive.
What does it mean if your Firmicutes:Bacteroidetes Ratio result is too high?
In adults, Bacteroidetes and Firmicutes are two of the most prevalent phyla present in the gut
Low Firmicutes:Bacteroidetes ratio in gut is generally associated with a healthier metabolism, while increased ratios of Firmicutes:Bacteroidetes are associated with unwanted weight gain.
This ratio is low in infants, increases as we grow up, and decreases again as we enter our elderly years.
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High Firmicutes and low Bacteroidetes (resulting in a high F/B ratio) suggest microbial imbalance which may be related to:
- increased caloric extraction from food,
- fat deposition and lipogenesis,
- impaired insulin sensitivity,
- and increased inflammation.
Potential causes:
- Poor diet
- Dysbiosis
- Maldigestion or hypochlorhydria
Therapeutic Approaches & Considerations
- Balance commensal bacteria using the 5R Protocol (see this Blog article)
- When Firmicutes phyla is high, consider using Bifidobacteria probiotics and Saccharomyces boulardii primarily.
- Lactobacillus spp. and Bacillus spp. (found in probiotics) can elevate Firmicutes
- Optimize the diet; a lower fat diet may help to normalize the F/B ratio
- Address all other imbalances on the BiomeFX panel
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What does it mean if your Firmicutes:Bacteroidetes Ratio result is too low?
A low Firmicutes:Bacteroidetes (F:B) ratio on a BiomeFx panel suggests an imbalance in the two dominant bacterial phyla in the gut. This ratio is often used as a marker of gut health, metabolic function, and inflammation. A low F:B ratio means that Bacteroidetes are more dominant than Firmicutes, which may have several implications depending on the individual's health status.
What Does a Low Firmicutes:Bacteroidetes Ratio Indicate?
-
Potential for Increased Gut Permeability ("Leaky Gut")
- Firmicutes (e.g., Faecalibacterium, Ruminococcus, and Clostridium species) contribute to butyrate production, a short-chain fatty acid (SCFA) that strengthens the gut barrier.
- A lower ratio might indicate reduced butyrate production, potentially leading to increased intestinal permeability and systemic inflammation.
-
Altered Metabolism & Weight Loss Tendencies
- Studies suggest higher Bacteroidetes levels (and a lower F:B ratio) are associated with leanness and difficulty gaining weight.
- A higher F:B ratio has been linked to obesity, while a lower ratio is often found in underweight individuals or those with rapid weight loss.
-
Possible Malabsorption & Nutrient Deficiencies
- Firmicutes help with fiber digestion and SCFA production, which support energy metabolism.
- A low F:B ratio could lead to reduced nutrient absorption, potentially contributing to deficiencies in fat-soluble vitamins (A, D, E, K) and essential fatty acids.
-
Increased Risk of Inflammatory or Autoimmune Conditions
- A lower F:B ratio is sometimes observed in individuals with inflammatory bowel disease (IBD), Crohn’s disease, ulcerative colitis, and autoimmune conditions.
- Higher Bacteroidetes levels can sometimes promote pro-inflammatory responses.
-
Disrupted Gut Microbiome Balance (Dysbiosis)
- Firmicutes include beneficial probiotic species such as Lactobacillus and Clostridium butyricum, which support digestion and immune function.
- A lower ratio may indicate gut dysbiosis, possibly due to dietary factors, antibiotic use, or chronic stress.
How to Support a Healthy F:B Ratio?
-
Increase Fiber Intake
- Consume prebiotic fibers (e.g., resistant starch, inulin, psyllium husk) to support Firmicutes growth.
- Include diverse plant foods (fruits, vegetables, whole grains, and legumes).
-
Consume Fermented Foods & Probiotics
- Yogurt, sauerkraut, kimchi, kefir, and probiotic supplements (e.g., Lactobacillus, Clostridium butyricum) can help boost Firmicutes.
-
Support Butyrate Production
- Butyrate is critical for gut health and produced by Firmicutes.
- Include butyrate-rich or SCFA-promoting foods like ghee, butter, resistant starch (green bananas, cooked-and-cooled potatoes), and soluble fiber.
-
Limit Ultra-Processed Foods & Excessive Sugar
- High sugar intake can favor Bacteroidetes over Firmicutes, leading to an imbalanced ratio.
-
Consider Microbiome Testing & Personalized Probiotic Strategies
- If you have symptoms of gut dysbiosis, metabolic issues, or inflammation, adjusting your microbiome support strategy may help restore balance.
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