In my practice, I see many patients with SIBO and obesity. It is not uncommon for an individual to come to my clinic with both SIBO and unexplained weight gain at the same time. When this happens, my patients always want to know if SIBO might be the cause of their weight gain.
The truth is that SIBO can absolutely cause weight gain in some cases. That being said, SIBO is rarely the sole cause of weight gain. The full picture is much more complex.
In this article, I will explain what SIBO is, how it can cause weight gain, and what you can do about it.
What is SIBO?
Understanding Small Intestinal Bacterial Overgrowth (SIBO)
SIBO occurs when microorganisms (bacteria, fungi, and archaea) overgrow in the small intestines. This means that more microorganisms are living in the small intestines than are supposed to be there.
As a result, these microorganisms begin to ferment the foods we eat and create a bunch of waste products in our intestines. These waste products include gas and microbial toxins. This causes symptoms like bloating, diarrhea, constipation, brain fog, and potentially weight gain.
Common Symptoms and Risk Factors
SIBO symptoms vary heavily from person to person. This depends on the specific microorganisms causing the overgrowth along with person-to-person variability. The Mayo Clinic outlines the most common symptoms as follows:
- Reduced appetite
- Stomach discomfort or cramping
- Feeling nauseous
- Excessive bloating
- Persistent fullness after meals
- Loose stools or diarrhea
- Unexplained weight loss
- Nutrient deficiencies and malabsorption
Is SIBO a Cause or Effect of Weight Gain?
It is abundantly clear that there is a relationship between SIBO and being overweight. Obese individuals are 11 times more likely to have SIBO than people at a healthy weight (1). The real question is whether SIBO causes weight gain or if weight gain causes SIBO.
The Case for SIBO Causing Weight Gain
There is a handful of data supporting the claim that SIBO can cause weight gain. Below are the most common arguments for this theory.
Increased caloric extraction from food
It has been shown that certain bacteria are capable of extracting calories from food more efficiently than others. These bacteria tend to be more common in overweight individuals and may explain weight gain to some degree.
It is possible that people with SIBO can have an overgrowth of these energy-extracting bacteria. As a result, a SIBO infection could increase caloric intake by upwards of 100 calories per day. Cumulatively, this can equate to gaining a few extra pounds per month compared to someone else eating the same diet.
Gut hormone disruption
Gut hormones have gotten a lot of attention lately due to the popularity of GLP-1 analogs, like Ozempic and Wegovy. These drugs have clearly shown that hormones, usually released from intestinal cells, directly play a role in weight regulation and weight loss.
Research has also shown that individuals with dysfunctional microbiomes, like those with SIBO, do not produce high levels of certain gut hormones (2). These hormones include peptide YY (PYY), and the aforementioned GLP-1. Since both of these hormones regulate weight, it is reasonable to assume that SIBO may cause hormonal weight regulation issues.
Methane-dominant SIBO and slow motility
It is also hypothesized that methane-dominant SIBO may cause weight gain more than the other subtypes. Indeed, research shows that obese individuals are more likely to have higher levels of methane gas on a breath test (3).
Methane-dominant SIBO is thought to slow the GI tract, increasing energy extraction from food. This is true in animal models but has not yet been proven to be a cause or effect of obesity in humans (4).
The Case for Weight Gain Causing SIBO
While the evidence above points towards SIBO causing weight gain, I would argue that the bulk of the evidence points towards weight gain causing SIBO.
Obesity-related gut motility issues
It has been shown that the intestines of obese individuals move more slowly (5). In healthy individuals, the intestines perform rhythmic contractions through a mechanism called the migrating motor complex (MMC). This is meant to sweep bacteria down towards the colon.
It is well known that dysfunction in the MMC can be the cause of SIBO. If the intestines are not able to effectively sweep microbes towards the colon, they will start to migrate into the small intestine and cause an overgrowth.
Changes in bile acid metabolism and gut microbiota
Overweight individuals tend to produce and secrete fewer bile acids from their liver into their intestines. Bile acids are antimicrobial in nature and are needed to maintain a healthy microbiome (6). It is possible that SIBO is more common in overweight individuals for this reason.
When Could SIBO Be Driving Weight Gain?
As I see it, it is much more likely that weight gain causes SIBO. However, there can be some clues that might point towards SIBO being the underlying cause of weight gain.
Post-Infectious SIBO and Metabolic Changes
If you started gaining weight following acute gastroenteritis (stomach bug), then it is more likely that SIBO is causing weight gain. Dr. Pimentel at Cedars-Sinai discovered that SIBO can develop after illness due to a specific immune response.
When this happens, it is possible to develop methane-dominant SIBO, which can then cause weight gain through the mechanisms discussed above.
Key Takeaways: Does SIBO Really Make You Gain Weight?
As you can see, the relationship between SIBO and weight gain is quite complicated. Though there are arguments on both sides, it is rarely a straightforward situation.
Most Cases of SIBO Are Secondary to Weight Gain
The bulk of the research shows that weight gain is much more likely to cause SIBO than for SIBO to cause weight gain. This is because being overweight slows down the GI tract and reduces the amount of bile able to keep bacterial populations within the normal range.
SIBO Can Worsen Metabolic Dysfunction but Is Rarely the Primary Cause
Though SIBO may increase calorie extraction from food and disrupt hunger hormones, it is seldom the main cause of weight gain. Instead, it complicates metabolism and makes weight loss harder. It will make it harder to lose weight, but it probably is not the reason you are overweight in the first place.
What to Do If You Suspect SIBO is Affecting Your Weight
Testing for SIBO
SIBO can be diagnosed through breath testing. This is typically done at home under the supervision of a healthcare professional. Breath testing requires a specific diet leading up to specimen collection.
Stool and blood testing cannot diagnose SIBO. Only a positive hydrogen, methane, or hydrogen-sulfide breath test can make a diagnosis of SIBO. Alternatively, an invasive small bowel aspirate can diagnose SIBO under the care of a gastroenterologist.
Be sure to work with a qualified healthcare provider if you think you may have SIBO.
Treatment Options (Diet, Prokinetics, Antibiotics, and Probiotics)
Treating SIBO can be incredibly tricky. Successful treatment is dependent on initiating the following:
Diet: Most SIBO diets are low FODMAP. This stands for low fructose oligosaccharides, disaccharides, monosaccharides, and polyols. These are a set of fermentable carbohydrates in foods that can increase SIBO symptoms.
I personally find that a low FODMAP diet is only needed in very few cases. Usually, a comprehensive elimination diet is more appropriate for identifying food triggers and working on the gut via other mechanisms.
Antimicrobial treatment: SIBO can be treated through the use of conventional antibiotics or herbal antimicrobial agents. Both can treat SIBO and work to decrease overgrowth in the small intestine.
Popular herbal treatments include berberine, allicin, oregano oil, Biocidin, or other blends.
I often find that SIBO can be treated with gentler approaches and does not always need aggressive antimicrobial therapy.
Prokinetics: These are agents that increase the speed of food through the digestive system. This helps sweep bacteria out of the small intestine and prevents overgrowth.
Prokinetics can also include herbs like ginger, artichoke leaf, and gentian.
Probiotics: Though it may seem counterintuitive, there are some probiotics that can help control the overgrowth of other microbes. The best probiotic depends on the subtype of SIBO. These typically include Saccharomyces boulardii, spore-forming bacteria, and lactobacillus.
Final Thoughts
The connection between SIBO and weight is complex. In most cases, excess weight is more likely to contribute to SIBO rather than the other way around. Slower gut motility and reduced bile acid secretion in overweight individuals create an environment where bacterial overgrowth can thrive.
While SIBO can disrupt metabolism by increasing caloric extraction, altering gut hormones, and slowing digestion—especially in methane-dominant cases—it is rarely the primary cause of weight gain. Instead, it can make weight loss more difficult by compounding metabolic dysfunction.
If you suspect SIBO is affecting your weight, proper testing is essential. Breath tests can confirm a diagnosis, and treatment may involve dietary adjustments, antimicrobials, prokinetics, and probiotics.
Addressing both SIBO and its underlying metabolic contributors is key to lasting results. If you’re struggling with unexplained weight gain and gut issues, book a discovery call to work with me as a functional dietitian. Together, we’ll uncover the root causes and create a plan tailored to your needs.
Justin Perr is a Registered Dietitian and Certified Functional Medicine Practitioner specializing in weight loss and gut health. He helps patients uncover root causes and achieve lasting results with science-backed nutrition.
