Small intestinal bacterial overgrowth

Small intestinal bacterial overgrowth (SIBO) is an excessive bacterial growth in the small intestine which in contrast to the large intestine in healthy individuals contains relatively small populations of bacteria.

Prevalence
The rate of a positive lactulose test is low in healthy adults (0 to 20%).

Risk factors
Risk factors include bowel resection, bariatric surgery, disordered motility, disorders of the immune system such as IgA deficiency, low stomach acid, the use of proton pump inhibitors and immunosuppressants, and recurrent antibiotic use.

Conditions that affect gut motility such as gastroparesis and celiac disease increase the risk of SIBO. Rates of SIBO are also higher in Crohn's disease, pancreatitis, and renal failure.

SIBO is more common among the elderly.

Diagnosis
SIBO can be diagnosed using a hydrogen breath test. The diagnosis of SIBO is controversial due to the species-dependent nature of breath tests and the lack of an agreed threshold for a positive test. It can also be diagnosed through bacterial culture, but this is rare as it requires intubation of the small intestine.

Pathophysiology
Bacteria commonly implicated in SIBO include Escherichia coli, Streptococcus, Lactobacillus, Bacteroides and Enterococcus. Higher levels of Enterococcus and Stretptococcus have been found in ME/CFS patients.

Health complications
The symptoms of SIBO can vary greatly depending on the severity and the species of bacterial populating the small intestine. Symptoms include bloating, abdominal distension, abdominal pain or discomfort, diarrhea, fatigue, and weakness.

It causes increased permeability of the small intestine. It can cause malabsorption of nutrients including iron and Vitamin B12, resulting in microcytic anemia or megaloblastic anemia.

Nutritional deficiencies
Vitamin B12 malabsorption may be caused by competitive uptake of B12 by bacteria in the small intestine. It can also cause excess folic acid due to synthesis by bacteria in the small bowel.

In severe cases, malabsorption of fat-soluble vitamins (A,D,E and K) due to the deconjugation of bile salts can cause neuropathies and immune dysfunction.

SIBO can also cause carbohydrate and protein malabsorption.

Comorbidities
Anecdotal reports suggest a high prevalence of SIBO among CFS patients. Several studies have shown that up to 84% of patients with irritable bowel syndrome have SIBO and that symptoms improve after treatment, while others fail to replicate these results.

One study found that a high number of fibromyalgia patients tested positive to a lactulose breath test, indicating SIBO, and that the degree of abnormality on the breath test correlated with the amount of pain reported.

Treatment
Standard treatment is a course of antibiotics. The best evidence for patients without constipation is for the use of rifaximin, an antibiotic that is stays in the intestine and is not absorbed in the body. Some SIBO researchers recommend adding Neomycin to rifaxamin for patients with constipation, as neomycin appears to help kill methane-producing bacteria. However, one study comparing two herbal formulations to standard treatment found herbs to be as or more effect as antibiotics for eradicating SIBO.

Experts also recommend the use of prokinetic drugs or herbs for those for whom dysmotility is an issue.

Probiotics may also be helpful. Lactobacillus casei has been found to improve breath hydrogen scores after six weeks of treatment. There is also evidence for VSL in the treatment of SIBO. However, some probiotics may exacerbate SIBO, in particular those containing D-Lactate producing strains.