Can SIBO cause weight gain?

Small intestinal bacterial overgrowth (SIBO) is a complex condition that is diagnosed when our friendly gut bacteria that usually reside in the large intestine is found to be growing in excess in the small intestine. This overgrowth can lead to uncomfortable digestive symptoms such as bloating and pain, and can even damage the intestinal lining, known as ‘leaky gut’.

Woman leaning on chair looking to camera

With SIBO, some of the symptoms include:

  • loss of appetite
  • abdominal pain and nausea
  • bloating and uncomfortable fullness after eating
  • malnutrition
  • unintentional weight loss

“SIBO is often associated with malabsorption and, as a result, weight loss rather than weight gain. Those with SIBO have impaired digestive function and cannot absorb, use or store the nutrients in the food they eat1,” says Victoria Hamilton, nutritionist and autoimmune disease specialist.  

So, if one of the common symptoms is unintentional weight loss, why do many clinicians see weight gain in patients with diagnosed SIBO? Firstly, we need to look a little deeper at SIBO. Victoria shares that there are two types of SIBO: methane-producing and hydrogen-producing. She says that the type of SIBO you have will be dependent on the type of gas produced by the gut microbiome causing symptoms.

Hydrogen-dominant SIBO

With hydrogen-dominant SIBO, the main symptom is diarrhoea, accompanied by an urgency to go to the bathroom, bloating and fatigue. Hydrogen gas does exist in the large intestine as a byproduct of fermentation but, when excess amounts of it grow in the small intestine, this is when symptoms such as diarrhoea occur, as the gas tends to cause loose stools and fast transit of the stools.

A bout of food poisoning has been found to be one of the common causes of hydrogen-dominant SIBO.

Methane-dominant SIBO

If your SIBO is methane-dominant (now classified as intestinal methanogen overgrowth or IMO) you will likely suffer from chronic constipation, bloating, burping and acid reflux directly after eating, and non-digestive symptoms such as headaches, fatigue and brain fog.

IMO is associated with ‘things coming back up’ so symptoms like acid reflux, burping and nausea are common. Studies have shown the methane gas that is released causes a backward motion in the small intestine. 

Woman with stomach pain sitting on bed

SIBO and weight gain

SIBO is more commonly associated with weight loss as it interferes with the body’s ability to absorb nutrients, however, there are large numbers of individuals with SIBO who struggle with weight gain. “Recent studies have indicated a link between SIBO and obesity. However, it is unclear which came first, and the likelihood is that obesity is the cause rather than vice versa,” says Victoria.

So, is SIBO a direct cause of weight gain? Victoria shares with us three reasons why you may be gaining weight with SIBO.

1. Slower transit time

Methane-producing SIBO has been shown to cause constipation and may cause a slower transit time, which means that food stays in the digestive tract longer than it should2. This slower transit time can lead to various symptoms such as nausea, abdominal pain and weight gain

2. Potential higher caloric intake

In addition, those with methane-producing SIBO may extract more calories from their food3. When you eat certain fibres known as glycans, you don’t typically absorb them. Still, in a high methane environment, there is the potential to process more calories from fibre-rich foods, increasing caloric intake, leading to weight gain.

3. Decrease metabolism 

Those with SIBO might be more prone to a decrease in metabolism – the bodily process of converting food and drink to energy – as the condition can affect leptin and insulin, which are hormones that impact your weight4

The bottom line

So, the bottom line is that there are strong cases for both why SIBO can cause weight gain, and why it may make you lose weight. As always, if you are struggling with your weight after a diagnosis with SIBO, it’s best to consult your doctor first, before making changes to your diet.

How can a nutrition professional help?

After following a treatment plan and confirming via breath test the lack of SIBO, nutritional therapy can support you to optimise your nutrition status – with SIBO you can often experience nutrient deficiencies due to the fast transit time in hydrogen-dominant SIBO and high levels of bacteria in both types of SIBO – repair the intestinal lining and manage a personalised diet that supports your weight management.

Your nutrition professional could help you identify and avoid foods that contribute to intestinal fermentation that aggravate SIBO, can help you manage your carbohydrate intake – the bacteria in SIBO feed off bacteria – and help identify foods that may contribute to weight gain, and offer healthy alternatives. They can also support lifestyle interventions to support whole-body health.

If you’re ready to connect with the support you need, you can reach out to Victoria Hamilton or use the advanced search tool to find a professional either in your area or online, who is trained to help with SIBO.


  1. Jung SE, Joo NS, Han KS, Kim KN. Obesity Is Inversely Related to Hydrogen-Producing Small Intestinal Bacterial Overgrowth in Non-Constipation Irritable Bowel Syndrome. J Korean Med Sci. 2017;32(6):948-953. doi:10.3346/jkms.2017.32.6.948
  2.  Bin Waqar S, Rehan A (May 28, 2019) Methane and Constipation-predominant Irritable Bowel Syndrome: Entwining Pillars of Emerging Neurogastroenterology. Cureus 11(5): e4764. doi:10.7759/cureus.4764
  3.  Turnbaugh PJ, Ley RE, Mahowald MA, Magrini V, Mardis ER, Gordon JI. An obesity-associated gut microbiome with increased capacity for energy harvest. Nature. 2006 Dec 21;444(7122):1027-31. doi: 10.1038/nature05414. PMID: 17183312.
  4. Ferolla SM, Armiliato GN, Couto CA, Ferrari TC. The role of intestinal bacteria overgrowth in obesity-related nonalcoholic fatty liver disease. Nutrients. 2014;6(12):5583-5599. Published 2014 Dec 3. doi:10.3390/nu6125583
Share this article with a friend
Written by Katie Hoare
Katie is a writer for Nutritionist Resource.
Written by Katie Hoare
Show comments

Find a nutritionist dealing with Gut health

All nutrition professionals are verified

All nutrition professionals are verified