TL;DR

  • The gut microbiome is an ecosystem that influences health.

  • Early studies linked obesity to more Firmicutes and fewer Bacteroidetes.

  • Newer research shows the connection is more complex and varies by population.

  • Inflammation, gut barrier health, and microbial metabolites may explain how bacteria influence weight.

  • Microbiome diversity seems protective—even among obese individuals without metabolic disease.

The intestine is an ecosystem. There are between four and seven pounds of bacteria in the large intestine. The number of bacterial cells outnumber all the cells in your body (the bacteria cells are much smaller than human cells). Bacteria perform necessary functions, like suppressing pathogens, detoxifying harmful chemicals, and producing vitamins. Dysbiosis refers to a situation where the wrong kinds of microorganisms thrive in the intestine. The wrong microorganisms produce toxins, can irritate the lining of the intestine, and suppress the beneficial bacteria. Researchers are finding that that the kind of bacteria growing in the intestine has a lot to do with many health issues—including obesity.

Research published back in the December, 2006 issue of the journal Nature, indicates that the type of bacteria found in the intestine may be related to whether a person is overweight or not. This was observed in both humans and mice. Obese humans and mice had a higher percentage of a family of bacteria called Firmicutes and less of a type of bacteria called Bacteroidetes. The researchers were unclear whether the obesity is caused by the presence of the bacteria, or if the bacteria are present because of the obesity.

In order to find out if the presence of the bacteria caused obesity, the researchers transplanted Fimicutes into the intestines of lean mice. When the bacteria were transplanted, the mice actually began to take in more calories from the food they ate. The same amount of food actually provided more calories for the mice with the Firmicutes bacteria in the intestine than for those without the Firmicutes bacteria.

Dieting affects the makeup of the bowel bacteria. Bacteroidetes made up about three percent of the gut bacteria in the participants of the study who were obese. But after dieting and losing weight, the subjects had much higher levels of Bacteroidetes—close to 15%.

Since the 2006 study, scientists have learned a little more. What Newer Studies Show

  1. Firmicutes / Bacteroidetes (F/B) Ratio Still Under Study, But Not Always Consistent
    • A 2023 review (“Exploring the Role of Gut Microbiota in Obesity”) notes that many human cohort studies do find a higher F/B ratio in obese individuals versus lean; but also several studies don’t find that difference. Ethnicity, diet, geography, lifestyle all seem to influence it. PMC
    • Another paper (“The Role of Gut Microbiota in the Onset and Progression …”, 2024) found that F/B ratio is strongly correlated with BMI (higher ratio → increased odds of obesity), plus specific species (like Blautia hydrogenotrophica, Coprococcus catus) are more common in obese vs non-obese persons. MDPI
  2. Microbial Diversity and “Metabolically Healthy Obese” (MHO)
    • A 2024 study Gut microbiome signature of metabolically healthy obese individuals found that obese individuals who do not have metabolic dysfunction (insulin resistance, high inflammation, etc.) have different gut microbiome profiles: better diversity, different bacterial taxa (more Akkermansia, Bacteroides) vs those obese with metabolic issues. Nature
  3. Mechanisms: Inflammation, Barrier Function, Metabolites
    • Research is showing more detail about how gut bacteria may contribute to obesity: altered gut barrier, higher intestinal permeability, lipopolysaccharide (LPS)-driven inflammation. Frontiers+2PMC+2
    • Also short-chain fatty acid (SCFA) production, bile acid metabolism, metabolite signaling are getting a lot of attention in newer studies as mediators of how gut bacteria affect energy harvesting, fat accumulation, and systemic inflammation. MDPI+2Frontiers+2
  4. Some Contradictory or Nuanced Findings
    • While older research often assumed a simple model (“more Firmicutes, fewer Bacteroidetes = obesity”), newer data suggest that this isn’t universal. For example, in certain populations or ages (especially young children), the F/B ratio doesn’t always predict obesity. Nature+1
    • Also, “dysbiosis” is more usefully thought of as changes in diversity, composition, function, not just one or two phyla. Which bacterial strains, what metabolites, what diet, etc., matter a lot. MDPI+1

FAQ

Q: Can gut bacteria make you gain weight?
A: Some studies show certain bacteria help harvest more calories from food, possibly influencing weight gain.

Q: Do Firmicutes always cause obesity?
A: Not always. The Firmicutes/Bacteroidetes ratio is higher in some obese groups, but results vary by diet, lifestyle, and population.

Q: What’s more important than just Firmicutes vs Bacteroidetes?
A: Diversity, bacterial species, and their metabolites (like SCFAs, bile acids) may be stronger predictors of metabolic health.

Q: Can changing diet shift gut bacteria?
A: Yes. Studies show weight loss and dietary changes can increase Bacteroidetes and improve microbiome balance.

Q: Are probiotics or prebiotics proven to prevent obesity?
A: Evidence is still early. Some strains show promise, but results depend on diet, strain type, and individual microbiome differences.