Bacteroidetes

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What Are Bacteroidetes?

Bacteroidetes are one of the two major bacterial phyla that dominate the human gut microbiome, alongside Firmicutes. These rod-shaped, Gram-negative bacteria are abundant in the human intestine and also occur widely in nature—found in soil, seawater, and on animal skin.

In the digestive tract, Bacteroidetes play a central role in breaking down complex carbohydrates and plant fibers that humans cannot digest on their own. Through this process, they produce short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate—compounds that nourish intestinal cells, reduce inflammation, and support immune balance.


Why Bacteroidetes Matter for Gut and Metabolic Health

Bacteroidetes are essential to a healthy, balanced gut ecosystem. Their activity supports multiple aspects of human health:

  • Fiber digestion and SCFA production: Bacteroidetes help convert dietary fiber into beneficial fatty acids that strengthen the gut lining, regulate inflammation, and provide energy for intestinal cells.

  • Metabolic regulation: Studies show that people with a higher relative abundance of Bacteroidetes tend to have a leaner body composition. This bacterial group is less efficient at extracting calories from food than some others, which may help protect against weight gain and support metabolic health.

  • Immune system support: Bacteroidetes interact with immune cells in the gut to promote tolerance toward beneficial bacteria while defending against pathogens.

  • Gut barrier protection: By maintaining SCFA production, these bacteria help preserve the integrity of the intestinal lining, reducing “leaky gut” and systemic inflammation.

  • Microbial stability: The Bacteroidetes phylum is considered one of the most stable and resilient components of the gut microbiota, helping to anchor overall microbial balance.


Scientific Insights and Research Highlights

Research consistently links Bacteroidetes to healthy metabolism, inflammation control, and gut homeostasis:

  • Dietary fiber and immune protection: In a Nature Medicine study (Trompette et al., 2014), mice fed a high-fiber diet showed increased Bacteroidetes populations and higher SCFA levels, which protected them from allergic airway inflammation.

  • Lean vs. obese microbiota: A Science study (Ridaura et al., 2013) found that germ-free mice receiving gut microbes from lean human twins—whose microbiota contained more Bacteroidetes—remained lean and metabolically healthy, while those colonized with microbes from obese twins gained weight and developed insulin resistance.

  • Human associations: Observational studies suggest that lean individuals tend to have higher proportions of Bacteroidetes, while people with obesity often show lower levels, especially when consuming diets low in fiber and high in fat and sugar.

  • Inflammation and SCFA links: Research shows that diets promoting Bacteroidetes growth enhance SCFA production, which has been linked to lower inflammation and improved gut barrier integrity (Koh et al., Cell, 2016).

Together, these findings highlight the critical role of Bacteroidetes in maintaining digestive, metabolic, and immune health.


How to Support a Healthy Bacteroidetes Balance

Your gut microbiome naturally adapts to what you eat and how you live. To encourage healthy levels of Bacteroidetes:

  • Increase dietary fiber: Eat a variety of plant-based foods—vegetables, fruits, legumes, whole grains, and nuts—to fuel beneficial bacteria.

  • Include prebiotic foods: Garlic, onions, leeks, asparagus, and chicory root help nourish Bacteroidetes and other fiber-loving microbes.

  • Limit processed and high-fat foods: Diets heavy in refined sugars and fats can disrupt microbial balance and reduce Bacteroidetes diversity.

  • Exercise regularly: Physical activity has been shown to enhance microbial diversity and support a balanced Firmicutes-to-Bacteroidetes ratio.

  • Manage stress and sleep: Chronic stress and poor sleep can alter gut microbial composition through the gut-brain axis.

  • Avoid unnecessary antibiotics: Overuse can deplete beneficial bacterial groups, including Bacteroidetes.


In Summary

Bacteroidetes are beneficial gut bacteria that help digest fiber, produce anti-inflammatory compounds, and support a healthy metabolism. They are closely associated with lean body composition, improved gut barrier integrity, and lower inflammation.

A strong, balanced population of Bacteroidetes—alongside other key bacterial groups like Firmicutes—is a hallmark of a healthy, resilient gut microbiome that supports optimal digestion, energy balance, and immune function.


References

  1. Trompette A. et al. (2014). Gut microbiota metabolism of dietary fiber influences allergic airway disease and hematopoiesis. Nature Medicine, 20(2):159–166. [PubMed: 24390308]

  2. Ridaura V.K. et al. (2013). Gut microbiota from twins discordant for obesity modulate metabolism in mice. Science, 341(6150):1241214.

  3. Koh A. et al. (2016). From dietary fiber to host physiology: Short-chain fatty acids as key bacterial metabolites. Cell, 165(6):1332–1345.

  4. Schwiertz A. et al. (2010). Microbiota and SCFA in lean and overweight healthy subjects. Obesity, 18(1):190–195.

  5. Arifuzzaman M. et al. (2024). Nutritional regulation of microbiota-derived metabolites and their impact on host health. Frontiers in Immunology.

  6. Makki K. et al. (2018). The impact of dietary fiber on gut microbiota in host health and disease. Cell Host & Microbe, 23(6):705–715.

  7. Houtman T.A. et al. (2022). Gut microbiota composition in relation to body mass index throughout childhood. Scientific Reports, 12:2347.

What does it mean if your Bacteroidetes result is too high?

An elevated Bacteroidetes level on your GI-MAP test means that bacteria from this phylum make up a higher-than-normal share of your gut microbiome. Bacteroidetes are essential, beneficial bacteria that play key roles in digesting dietary fiber and maintaining metabolic balance. However, when their relative abundance becomes too high, it can indicate a microbial imbalance, or dysbiosis, within the gut ecosystem.

Because gut bacteria constantly interact with diet, metabolism, and immune function, the significance of elevated Bacteroidetes depends largely on the overall microbiome balance, particularly the Firmicutes-to-Bacteroidetes ratio.


The Firmicutes-to-Bacteroidetes Ratio and What It Means

To understand elevated Bacteroidetes results, it helps to look at their relationship with another major bacterial group, Firmicutes. Together, these two phyla dominate the human gut, typically making up over 90% of its bacterial population.

The ratio between them has been widely studied as a marker of metabolic and digestive health:

  • A higher Bacteroidetes-to-Firmicutes ratio is often associated with lean body composition, fiber-rich diets, and efficient energy metabolism.

  • However, when Bacteroidetes become too dominant, it may signal an overactive breakdown of carbohydrates, leading to increased gas production, gut irritation, or nutrient malabsorption.

This ratio is dynamic—it changes with diet, stress, medications, and lifestyle factors—so interpreting elevated levels should always consider the individual’s symptoms, habits, and overall health context.


Possible Causes of Elevated Bacteroidetes

Several factors can contribute to higher Bacteroidetes levels on a GI-MAP test:

  • High-fiber or plant-based diets: Bacteroidetes thrive on complex plant fibers. Diets rich in fruits, vegetables, legumes, and whole grains can raise their levels.

  • Low-fat or calorie-restricted diets: Reduced fat intake can shift gut composition toward Bacteroidetes dominance.

  • Fasting or rapid weight loss: Shifts in nutrient availability can temporarily favor Bacteroidetes species.

  • Recent microbiome changes: After antibiotic use or a restrictive diet, certain bacterial groups may rebound faster than others, leading to temporary imbalances.

  • Probiotic or prebiotic supplementation: Some supplements that enhance fiber fermentation can increase Bacteroidetes activity.

In most cases, elevated Bacteroidetes reflect dietary adaptation rather than disease, but when accompanied by digestive symptoms, they may indicate functional dysbiosis.


Symptoms or Issues Linked to High Bacteroidetes

Although Bacteroidetes are beneficial, excessive levels can sometimes contribute to or correlate with digestive discomfort or imbalance:

  • Gas, bloating, or loose stools, especially after high-fiber meals

  • Reduced nutrient absorption or fat digestion efficiency

  • Inflammation or gut irritation, if overgrowth disrupts microbial equilibrium

  • Weight loss or low BMI, in cases where food energy extraction is reduced

Rarely, certain Bacteroidetes species—such as Bacteroides fragilis—can become opportunistic and contribute to infection or inflammation when the gut barrier is weakened. This typically occurs only when the overall microbial balance is significantly disturbed.


When Both Bacteroidetes and Firmicutes Are Elevated

If both Bacteroidetes and Firmicutes are elevated on your GI-MAP test, it generally suggests an overall increase in bacterial load or activity within the gut, rather than a single imbalance. These two phyla make up the majority of intestinal bacteria, so simultaneous elevation often reflects microbial overgrowth—potentially due to diet, slowed gut motility, or excessive fermentation of carbohydrates.

While not always harmful, high levels of both groups can indicate gut dysbiosis, leading to symptoms such as bloating, gas, or irregular stools. This pattern may occur after antibiotic recovery, during high-calorie or high-fiber diets, or in cases of small intestinal bacterial overgrowth (SIBO).

Restoring balance typically involves reducing fermentable carbohydrate intake, diversifying fiber sources, and supporting motility and digestive function to bring both groups back into optimal proportion.


Balancing Elevated Bacteroidetes Levels

If your GI-MAP shows elevated Bacteroidetes, the goal isn’t to eliminate them—they’re beneficial and necessary—but to restore microbial balance. Practical ways to support a healthier ratio include:

  • Diversify your diet: Balance high-fiber foods with moderate amounts of healthy fats (avocados, olive oil, omega-3 sources) and proteins to prevent over-fermentation.

  • Monitor fiber intake: Excessive fiber—especially from supplements—can promote bloating or loose stools in those with elevated Bacteroidetes.

  • Include polyphenol-rich foods: Berries, green tea, cocoa, and olive oil support microbial diversity and help moderate overgrowth.

  • Limit refined carbohydrates: Simple sugars can fuel imbalances even within beneficial bacterial groups.

  • Support overall gut health: Probiotic foods, stress management, regular exercise, and hydration help maintain microbial stability.


When to Discuss Results with a Clinician

Elevated Bacteroidetes are not inherently harmful and may reflect a healthy, fiber-adapted microbiome—especially in individuals following plant-based or Mediterranean-style diets. However, if you’re experiencing bloating, diarrhea, or nutrient absorption issues, it’s best to discuss your results with a clinician experienced in functional medicine or gastroenterology.

They can help interpret your GI-MAP findings in context and recommend targeted interventions—such as dietary modifications, stool diversity testing, or prebiotic/probiotic therapy—to restore microbial harmony and digestive comfort.


Key Takeaway

Elevated Bacteroidetes levels often reflect a gut microbiome adapted to a high-fiber or plant-based diet and may correlate with lean metabolism. However, excessive dominance can signal gut imbalance or digestive irritation, particularly when paired with symptoms.

Balancing fiber intake, diversifying your diet, and supporting microbial diversity are key to ensuring that Bacteroidetes continue to benefit—not disrupt—your gut health.

What does it mean if your Bacteroidetes result is too low?

- Reduced abundance of the Bacteroidetes in some cases is associated with obesity.

- Lower level of Bacteroidetes in the gut microbiota is associated with IBD (irritable bowel disease) patients.

Possible ways to increase Bacteroidetes in the gut: 

Whole grains can promote the growth of Bacteroidetes in humans. Whole grains contain non-digestible carbs that can promote the growth of beneficial bacteria within the gut microbiota. These changes to the gut flora may improve certain aspects of metabolic health.

More ways to generally improve gut health:

- Eat Lots of Vegetables, Legumes, Beans and Fruit (high in fiber).

- Eat Fermented Foods

- Avoid artificial sweeteners

- Eat prebiotic foods

- Eat a plant-based diet

- Eat foods rich in polyphenols (blueberries, broccoli, dark chocolate, green tea)

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