SIBO (Small Intestinal Bacterial Overgrowth) involves overgrowth of bacteria in the small intestine. The small intestine should be relatively sterile in comparison to the far more populated large intestine. But in cases of slow bowel transit (which can occur for a variety of reasons, most commonly hypothyroidism or low vagal tone), bacteria in the large intestine migrate upstream and colonize the small intestine.

Because of this, there has been much debate among functional medicine practitioners as to whether probiotics are a good idea (and if so, which ones), whether prebiotics (the food for the probiotics) are a good idea, or whether it’s best to avoid all of the above. The concern is, if SIBO is overgrowth of what are otherwise good bacteria, just in the wrong place, won’t probiotics (or prebiotics) make it worse?

…Not necessarily, it turns out.

Probiotics and SIBO

While I have had some SIBO patients who worsen on probiotics, they tend to be more the exception than the rule. In fact, I’ve found more often than not, the addition of probiotics to the therapy tends to prevent relapse. The studies are now backing this up.

There are some arguments out there that the bacteria in the probiotics need to be different from those bacteria involved in SIBO (so no bfidobacillus or lactobacillus); rather, they should be soil-based strains in the bacillus family. And indeed, this study shows that those are effective. But this study shows lactobacillus also works, and this study shows that lactobacillus + bifidobacillus worked better than the antibiotic metronidazole (which, granted, is not the main antibiotic recommended for SIBO, but it is sometimes prescribed for that purpose).

Overall it seems that probiotics—and even prebiotics—are beneficial for SIBO.

Prebiotics and SIBO

A few more studies to demonstrate that even prebiotics are beneficial: this study was conducted using bifidobacillus and inulin and showed significant improvement in SIBO symptoms with no adverse effects.

Likewise, this study shows that a bacillus probiotic with FOS (fructooligosaccharides) also showed significant improvement in SIBO symptoms.

Even children with SIBO respond positively to a combination of pre- and probiotics.

Low FODMAP and Prebiotics?

This does seem rather confusing, because the Low FODMAP diet, which restricts fermentable carbohydrates, is also effective for SIBO because it deprives the overgrown bacteria of a food supply. Yet prebiotics are fermentable carbohydrates. This is why traditionally, prebiotics have been restricted in cases of SIBO.

Bottom line is, we don’t know why prebiotics don’t seem to have an adverse effect in (most) SIBO cases, whereas fermentable carbohydrates in food do seem to worsen symptoms. Then again, no single patient seems to react to every high FODMAP food (and some SIBO patients do better on the Specific Carbohydrate Diet instead, which has a lot of similarities to the FODMAP diet, but isn’t the same.) So the diet piece of a SIBO protocol can be more of a trial-and-error process anyway.

The Upshot

It looks like both probiotics and prebiotics have a place in SIBO protocols—except for people who do worse with them (and there are a few of those, unfortunately! But they are relatively rare, in my experience.)