Does SIBO cause weight gain?

The short answer is yes, SIBO may cause weight gain, and ironically nutritional deficiencies. This combination of issues happens as a result of the constipation type of SIBO, which is also often associated with methane type SIBO or intestinal methangen overgrowth (IMO). While there are several names for this type of SIBO, the names are all referring to the same process.


SIBO with constipation

In constipation, because of the slow movement of food through the intestines (which is the direct effect of methane on the intestines) more calories are absorbed simply because the food is slowly moving versus traveling through the intestines at a normal speed. The small intestine is only doing what it normally does, which is to absorb what is in it. Again, when motility is slowed by the methane gas, the small intestine absorbs as it normally would, but for longer periods of time and hence more calories are taken up. The bottom line is weight gain.


SIBO and nutritional deficiencies

In SIBO with constipation, which again is caused by methane gas made by methanogens (a type of normal bacteria-like organism that is part of the microbiome), a hydrogen fuel source is needed for the methanogens to make methane. So in SIBO with constipation, whether pure constipation or mixed constipation with diarrhea, the primary issue began with the normal large intestinal bacteria taking up residency in the small intestine. Those normal large intestinal bacteria who are now living in the small intestine are now able to directly compete with your body for the nutrients from the food you have taken in. So it is a double whammy: more calories absorbed with less nutrients absorbed! The most common nutritional deficiencies in such cases are iron and vitamin B12.


Does SIBO always cause weight gain?

No. In the diarrhea types of SIBO patients may lose weight and also have nutritional deficiencies. The reason why some people with the diarrhea type of SIBO lose weight is that the food passes too fast through the intestines and doesn’t allow for the proper absorption of calories or nutrients.

Some people with SIBO don’t see changes to their weight. Such persons may or may not have nutritional deficiencies.


What to do if you think you have SIBO?

If you think you may have SIBO, take the next step to enhance your health and well-being by working with a skilled functional medicine doctor.

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