Small Intestinal Bacterial Overgrowth (SIBO): What Is It?

Bacterial overgrowth in the small intestine causes a wide variety of gastrointestinal symptoms. Learn all about it.

Reviewed and approved by el médico Diego Pereira.

Written by Josberth Johan Benitez Colmenares

Last update: 19 March, 2024

Small intestinal bacterial overgrowth (SIBO) refers to the abnormal presence of bacteria in the small intestine. Unlike the large intestine, the small intestine doesn’t usually exceed 1000 organisms/mL. When it does so, a constellation of gastrointestinal symptoms ensues.

Until not long ago, the diagnosis of SIBO was controversial and questioned by the medical community. Even today, experts warn that the characteristics, symptoms, and treatment display a heterogeneous behavior. Despite the limitations around it, we’ll teach you everything you need to know about it in the following article.

Characteristics of small intestinal bacterial overgrowth (SIBO)

The proximal small intestine (duodenum) usually contains few bacteria in contrast to the large intestine. In part, this is due to the presence of stomach acid and peristaltic movements. Bacterial concentrations increase progressively throughout the small intestine, reaching an average of 38 billion in the large intestine.

According to specialists, the groups of bacteria under normal conditions in the small intestine are lactobacilli, enterococci, facultative anaerobes, and gram-positive anaerobes. Very rarely do they exceed 1000 organisms/mL, and when they do, there’s said to be small intestinal bacterial overgrowth.

SIBO isn’t always present when this proportion is exceeded, but only when it’s accompanied by health complications. Therefore, the characteristic symptoms are the following:

• Abdominal distension
• Flatulence
• Watery diarrhea
• Weight loss due to fat malabsorption
• Vitamin deficiencies
• Constipation
• Fatigue and weakness
• Nausea
• Malnutrition

The signs are very varied, as they don’t depend exclusively on the number of bacteria that make up the microbial flora; but rather the specific type of bacteria that manifests an overgrowth. Due to the function that the excess of certain pathogens fulfill, they’ll trigger different symptoms. It’s for this reason that the clinical manifestation of SIBO is highly variable.

Some experts have pointed to bacterial overpopulation in the gut with diseases such as nonalcoholic fatty liver disease (NAFLD), steatohepatitis (NASH), alcoholic liver disease, and cirrhosis. Despite this, most of the time, SIBO triggers only mild or moderate symptoms, and even a physical examination by doctors tends to be unrevealing.

The Causes of Small Intestinal Bacterial Overgrowth (SIBO)

The causes of small intestinal bacterial overgrowth are very complex. As the researchers warn, it develops when the homeostatic mechanisms that regulate local bacterial populations are disrupted. There are two processes that tend to trigger imbalances of this type, which are decreased gastric acid secretion and small intestinal dysmotility.

On the other hand, alterations in intestinal immune function and anatomical abnormalities of the digestive tract also explain many episodes. We’ll analyze the mechanisms through which a person can manifest small intestinal bacterial overgrowth through these pathways.

Decreased gastric acid secretion

Hypochlorhydria, or decreased gastric secretion of hydrochloric acid, is a risk factor for the development of SIBO. Gastric acid suppresses the growth of bacteria ingested through food, which makes it possible to control their population in the small intestine. Its decline may be a consequence of prolonged use of antacids, autoimmune diseases, and, paradoxically, H. pylori infection.

Small bowel dysmotility

It has been known for decades that intestinal motility disorders can cause SIBO. The movements of the digestive tract not only sweep the food but also the pathogens that nest in it. Therefore, chronic intestinal pseudo-obstruction, gastroparesis, colonic inertia, and others can all lead to SIBO.

Alterations in intestinal immune function

People with compromised immune systems often develop bacterial overgrowth. For example, those with acquired immunodeficiency syndrome, combined variable immunodeficiency, and IgA deficiency are prone to developing SIBO.

Anatomical abnormalities of the digestive tract

For example, small intestine diverticula, fistulas between the proximal and distal intestine, gastric resection, small intestine stenosis, ileocecal valve resection, and others. Abnormalities caused by surgical interventions in the intestinal tract may also be behind the overgrowth.

Other possible causes of bacterial overgrowth in the small intestine are metabolic disorders (diabetes and others), celiac disease, Crohn’s disease, malnutrition, kidney failure, and the intake of some medications. Natural aging can also lead to SIBO.

The treatment of Small Intestinal Bacterial Overgrowth (SIBO)

The treatment for SIBO addresses three axes, which are correcting the underlying cause, counteracting bacterial overgrowth, and providing nutritional support (when needed). Determining what’s causing the problem is essential, as the treatment roadmap is determined on this basis.

Correcting the underlying cause is done based on dietary, pharmacological, and surgical therapies. The choice varies according to the trigger, and the surgical alternatives are limited only to cases that can’t be addressed by the first two.

Addressing bacterial growth involves administering antibiotics. These reduce or eliminate bacterial overload, resulting in an improvement in symptoms such as inflammation, malabsorption, and diarrhea. There’s no preferred medication for this, so the choice is made based on the specialist’s criteria.

Lastly, the patient may require nutritional support to address malabsorption decompensation. This will address electrolyte imbalance, nutrient deficiency, malnutrition, and more. Evidence supports the use of probiotics to address some of the symptoms and supplement alternatives to control bacterial overgrowth.

SIBO is usually recurrent, so both the specialist and the patient must be attentive to the evolution after treatment. Assuming a healthy diet and other habits that promote intestinal health permanently can help prevent it. If you detect any of the symptoms outlined, don’t postpone a visit to a medical center.