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Are probiotics effective for treating small intestinal bacterial overgrowth?

Bacteria can cause trouble when they grow too much in places they shouldn’t, like the small intestine. Could probiotics prevent or reverse this process?

Study under review: Probiotics for Preventing and Treating Small Intestinal Bacterial Overgrowth: A Meta-Analysis and Systematic Review of Current Evidence.

Introduction

In one hour, a person’s mere presence in a room can increase the bacterial count in the air by 37 million[1]. So, we are literally surrounded and colonized by microorganisms, yet they prefer to live in some places more than others.

For example, the stomach and the upper regions of the small intestine contain less than 10,000 microorganisms/mL[2] of intestinal secretions. This makes sense since the small intestine, filled with bile and enzymes, is pretty inhospitable. Also, a lot of bacteria in the small intestine would lead to competition for the nutrients that are absorbed. The large intestine, on the other hand, can contain up to 10 trillion microbes per gram of intestinal content[3]. This is useful because these microbes break down indigestible fibers, produce shortchain fatty acids[4] that fuel the cells of the colon, and train immune system cells[5].

What happens when bacteria proliferate in regions they’re not supposed to be in? In a disorder known as “small intestinal bacterial overgrowth” (SIBO), the small intestines of affected individuals have higher concentrations of bacteria, relative to people without the disorder.

The excess bacteria seem to decrease[6] the amount of nutrients absorbed from food, damage the lining of the small intestine[7], and produce toxins[8] that can pass into the blood. Many of the symptoms of SIBO, such as nausea, abdominal pain, and altered bowel habits, are similar to irritable bowel syndrome (IBS), and some people with IBS may be likely to have SIBO[9].

But unlike IBS[10], SIBO can be diagnosed using biomarkers. One commonly used method is to administer certain carbohydrates to a patient and measure hydrogen breath levels[11] over a period of time, a process depicted in Figure 1. The gold standard for diagnosis is to take a sample[12] from the small intestine and measure the concentration of bacteria in that sample. However, this is somewhat invasive, so it’s not used as often.

Researchers believe that the factors that may lead to SIBO include gastrointestinal infections[13], disrupted motility of the small intestine[14], or low production of stomach acid[15]. Treatments typically involve the use of antibiotic drugs[6] to reduce bacterial counts.

Figure 1: The hydrogen breath test

Reference: Kerlin & Wong. Gastroenterology. 1998 Oct.

Unfortunately, these drugs may not be ideal for longterm use because of the possibility of producing antibiotic resistant bacteria[16] or disrupting the microbiome[17] in the gastrointestinal tract. In addition, antibiotics may not always work[18] for everyone.

Probiotics have been explored in several trials as an alternative to antibiotics to prevent SIBO occurrence and to treat SIBO[19]. However, the results have been mixed[20]. The study under review is the first meta-analysis to pool the results of several trials exploring the use of probiotics to prevent or treat SIBO.

A condition called small intestinal bacterial overgrowth (SIBO) is characterized by excess bacteria in the small intestine, which can damage the lining and cause symptoms similar to irritable bowel syndrome (IBS). SIBO is typically treated with antibiotic drugs. However, this treatment strategy is not without its problems. Probiotics have been explored in several trials to prevent and treat SIBO, and this is the first systematic review and meta-analysis to investigate their efficacy.

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