SIBO and weight
SIBO and weight
Medically reviewed by:
Dr John Freeman
SIBO can contribute to both weight gain and weight loss, and the outcome varies widely between individuals. The reason lies in which microbes are present, how they ferment food, and how they influence digestion, motility, inflammation, and nutrient absorption.
How SIBO can lead to weight gain
Fermentation occurring in the wrong place
Under normal conditions, carbohydrate fermentation occurs in the large intestine, which is well equipped to handle gas production. In SIBO, fermentation shifts into the small intestine, a narrow organ not designed to hold gas.
Gas trapped in the small intestine:
Causes visible abdominal distension
Creates a “bloated” or larger appearance
Can make people feel heavier even without true fat gain
Increased calorie extraction from food
In SIBO, excess bacteria in the small intestine ferment carbohydrates — including some fibres — before they reach the colon.
This abnormal fermentation can allow bacteria to:
Extract energy from foods humans normally cannot digest
Increase calorie availability from the same diet
Contribute to gradual weight gain
This effect is more commonly seen in methane-dominant and hydrogen-sulfide SIBO, although it is not exclusive to these types.
In simple terms:
Humans cannot extract energy from grass — cows can. In SIBO, bacteria may enable the body to absorb calories that would otherwise pass through unused.
Methane SIBO, slow transit, and fat storage
Methane gas has been shown to slow intestinal transit. When food remains in the gut longer than normal:
More calories may be absorbed
Constipation becomes more likely
Weight loss becomes more difficult
Slow transit also gives bacteria additional time to ferment food and extract energy, reinforcing the cycle.
Studies have found that SIBO is associated with increased visceral fat accumulation, which is linked to higher risks of insulin resistance, cardiovascular disease, and diabetes.[1]
SIBO, energy levels, and metabolism
Nutrient deficiencies
The small intestine is responsible for nutrient absorption. In SIBO, bacteria can consume or interfere with nutrients before the body has a chance to absorb them.
Common deficiencies include:
Iron (low ferritin)
Vitamin B vitamins
Vitamin D
Iron deficiency, in particular, is associated with fatigue, reduced exercise tolerance, and low motivation, all of which may indirectly contribute to weight gain.
Chronic inflammation caused by SIBO further increases energy demands and can drive cravings for quick, high-sugar foods.
Metabolic disruption
SIBO may affect hormones involved in appetite and fat storage, including insulin and leptin. Disruption of these pathways can:
Reduce metabolic efficiency
Promote fat storage
Make weight loss more difficult even with dietary changes
Sleep disruption
SIBO symptoms — especially bloating and discomfort — often worsen in the evening and can interfere with restful sleep.
Research shows that reduced sleep duration, particularly in women, is associated with:
Increased calorie intake
Reduced energy expenditure
Greater risk of weight gain[2]
How SIBO can lead to weight loss
Not all SIBO cases result in weight gain. Some individuals experience unintentional weight loss due to impaired digestion and nutrient absorption.
Premature fullness and reduced intake
Gas pressure in the small intestine can cause:
Early satiety
Painful reflux
Reduced appetite or fear of eating
Over time, this can significantly reduce calorie intake.
Bacterial competition for nutrients
In some cases, bacteria consume nutrients before the body can absorb them, leading to:
Energy loss
Vitamin and mineral deficiencies
Muscle loss and malnutrition
Nausea and vomiting
Severe gas pressure and fermentation may trigger nausea or vomiting, further reducing energy intake and contributing to weight loss.
Key takeaway
SIBO can drive weight gain, weight loss, or a combination of both, depending on the microbial profile, intestinal transit time, and degree of inflammation. No two cases are identical, and many individuals have overlapping bacterial patterns.
Addressing the underlying bacterial imbalance, motility issues, and nutrient deficiencies is essential for restoring healthy weight regulation.
1 Fialho et al., 2016
2 Sharma and Kavuru, 2010
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