Does small intestinal bacterial overgrowth (SIBO) lead to severe nutrient malabsorption?

While nutrient malabsorption can happen as a result of SIBO, it's impossible to give levels or numbers because there’s no real data on it.

It’s likely dependent on the severity of the SIBO. 

Some people with SIBO have undigested food in their stool: that’s going to be a relatively severe case of nutrient malabsorption compared to someone with a mild case who is still producing adequate amounts of digestive enzymes from the pancreas.

Nutrient deficiencies like vitamin B12, iron and zinc are a common consequence of having SIBO.

It’s also a vicious circle: research has found that part of treating SIBO has to involve restoring nutrient status, because optimal nutrient status is—ironically— important for a healthy microbiome.