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New Research on the Cause of Small Intestinal Bacterial Overgrowth (SIBO)

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Small Intestinal Bacterial Overgrowth (SIBO) has been gaining acceptance as an underlying cause of gastro-intestinal symptoms like irritable bowel syndrome. This post goes into the misconceptions around SIBO, how the movement of food plays a part in SIBO, as well as the immune system, the vagus nerve, and food poisoning.

What is SIBO?

SIBO was first discovered in 1939 but rifaximin was not approved for its treatment until 2015. A landmark study published in 2000 by Pimentel et all at Cedars-Sinai Medical Center put SIBO on the map. It identified SIBO as being present in 78% of patients with irritable bowel syndrome (IBS) and found that treatment with antibiotics improved symptoms.

There is a misconception that SIBO is an infection, when in actuality SIBO is the presence of excess colonic bacteria in the small intestine. In contrast to the large intestine, the concentration of the bacteria in the small intestine rarely exceeds 1,000 organisms/ml. This is because gastric acid secretion and intestinal motility limit the overgrowth of bacteria in the small intestine. When these protective mechanisms against excessive bacterial growth fail, small intestinal bacterial overgrowth (SIBO) can result.

When the Body’s Protective Measures Fail

Upon ingestion of food, gastric acids and bile destroy and prevent bacteria from passing through the intestines. Conditions that cause achlorhydria, or lack of adequate stomach acid, are associated with SIBO. Achlorhydria can be caused by overuse of proton pump inhibitors or any other medication that suppresses the secretion of stomach acid and stress.

Also, proteolytic enzymes digest and degrade bacteria in the intestines. Chronic pancreatic insufficiency is associated with SIBO. Bile is responsible for cleansing the bowel so gall bladder dysfunction can play a role. Migrating motor complexes are responsible for peristalsis (constriction and relaxation of the muscles of the intestine) and cleansing the small intestine, i.e., proper gastric motility. Disorders of abnormal gastrointestinal motility, include irritable bowel syndrome, eating often during the day, narcotic use, post-radiation enteropathy, hypothyroidism, diabetes, mellitus, and scleroderma.

How Movement of Food Plays a Part in SIBO

A well-functioning ileocecal valve and forward motility or proper peristalsis prevents backward movement of colonic bacteria into the small intestine. Disorders that disrupt the forward movement of food through the intestinal tract include small intestinal diverticulosis, bowel strictures, post-operative adhesions, gastric bypasses with blind intestinal loops, and ileocecal resection.

The Immune System and the Vagus Nerve

Our immune system also plays a role in the movement of food. Immunoglobin A (IgA) which is abundant in the gastrointestinal tract, also prevents bacterial proliferation and maintains intestinal immunity. IgA deficiency is associated with an increased risk of SIBO. Other causes are long standing dysbiosis in the colon, and damage to the vagus nerve. Damage to the vagus nerve can result after injuries or surgery in the abdomen or spinal cord, viral infections, or diabetes.

Can Food Poisoning Cause SIBO?

It has been established through several studies that food poisoning or acute gastro-enteritis can be the cause of almost 60% of SIBO and/or IBS cases. Bacteria that cause food poisoning such a salmonella, campylobacter, shigella, e. coli, and possibly viruses and giardia, secrete a toxin called CdtB, which can lead to auto-immunity in the intestinal tract. The immune system while trying to eradicate the CdTB attaches to a similar looking molecule called vinculin. Vinculin is a protein that is responsible for movement in the small intestine. When high, the anti-CdTB and anti-vinculin antibodies reduced the number of cells making up the motility motor complex (MMC). The MMC sweeps the intestines every 90 minutes. When this complex is immobilized, food can remain in the small intestine longer than it should and SIBO can result.

One study found that the anti-viculin antibody is elevated in almost 38% of cases of scleroderma, an auto-immune condition that results in hardening and tightening of the skin. In these patients, anti-vinculin antibodies are associated with higher levels of GI symptoms.

SIBO as a Gastro-intestinal Condition

SIBO is now recognized as a diagnosable gastro-intestinal condition. It can be associated with diarrhea, constipation, abdominal bloating and gas, and even symptoms outside the intestinal tract due to its histamine response. Symptoms may include fatigue, brain fog, and an unusual amount of food sensitivities. Some additional signs may be restless leg syndrome, chronically low ferritin levels, and elevated total bilirubin levels without other causes. New treatments are being developed based on these new findings including antibiotics, herbal remedies, motility activators, and diet.


If you think you may have SIBO, contact us to schedule an appointment with Dr. Denise Clark: 303-884-7557.