Small intestinal bacteria overgrowth (SIBO) is a common but under-diagnosed condition. Historically viewed as a controversial diagnosis due to its wide-ranging clinical presentations and overlap with other conditions, it has recently gained support as a viable explanation for gastrointestinal problems (1). Although antibiotics remain the most common treatment method for SIBO, functional medicine's holistic approach of addressing the root cause and restoring balance to the gastrointestinal tract through individualized approaches like diet modification and root cause identification is gaining popularity.
What is SIBO?
SIBO is a gastrointestinal disorder characterized by an excess and imbalance of bacteria in the small intestine (1). Unlike the large intestine, the small intestine normally contains few bacteria due to the rapid movement of food and the presence of bile (2). However, if the passage of food and waste products in the small intestine slows, bacterial overgrowth may occur, producing toxins and gas, interfering with the absorption of nutrients, and causing chronic bloating, constipation, diarrhea, and other symptoms (2).
Risk factors and causes of SIBO
It is unknown how prevalent SIBO is because it is thought to be under-diagnosed or attributed to an underlying disease (3). Its presence is extremely common in several predisposing conditions (3). In fact, research would suggest that up to 25% of patients with Crohn's disease also have SIBO (3), and one-fifth of those with celiac disease do as well. Other autoimmune diseases, chronic stress, vagal nerve dysfunction, gastroparesis, diabetes, and other illnesses that slow food movement through the small intestine can also cause SIBO (2).
Notably, SIBO is present in over half of patients with irritable bowel syndrome (IBS) and is associated with symptom exacerbation of this condition (4). However, some trials have reported a significant improvement in IBS after treatment of SIBO (3).
SIBO may also be caused by abdominal surgery complications or structural problems in or around the small intestine (2).
SIBO may be asymptomatic or, in extreme cases, cause malabsorption. Other symptoms may include (2, 3):
• Loss of appetite
• Abdominal pain
• Uncomfortable feeling of fullness after eating
• Chronic diarrhea or constipation
• Rapid, unintentional weight loss
It is important to note that the American College of Gastroenterology (ACG) states that no single symptom can be attributed explicitly to SIBO as symptoms often masquerade as other diagnoses (5). The ACG explains that this is due to the varied presentation of patients with SIBO and the number of underlying risk factors that can lead to its development (5). For example, in a patient with chronic pancreatitis, it is hard to determine if diarrhea is caused by an exocrine insufficiency or coexisting SIBO (5). Therefore, the ACG recommends that the symptoms, risk factors, and any history of attempted treatments for underlying conditions all be considered (5).
The potential consequence of untreated SIBO:
If SIBO is left untreated, a variety of medical consequences may occur. The excess bacteria break down bile salts – an essential component needed to digest fats – resulting in the poor absorption of fats, carbohydrates, and proteins. Due to the incomplete absorption of fats, the body cannot fully absorb fat-soluble vitamins A, D, E, and K, leading to vitamin deficiency. The bacteria also use vitamin B-12, which is needed for nervous system functioning and blood cell and DNA production. Vitamin B-12 deficiency can cause fatigue and tingling and numbness in the hands and feet. The continued presence of SIBO may limit calcium absorption, eventually leading to osteoporosis and kidney stones (2).
Diagnosing of SIBO
The diagnostic gold standard for SIBO diagnosis is a small intestine aspirate and fluid culture (1, 2). However, this test, which requires the insertion of a tube down the throat, remains unpopular due to its invasive nature and high cost (1, 2). As a result, glucose and lactose breath testing are more commonly used diagnostic tools. These breath tests, which measure the amount of hydrogen, methane, or hydrogen sulfide gas exhaled by patients after drinking a sugar-based water mixture, are non-invasive and inexpensive (1, 2). However, these tests are not without flaws and limitations. Glucose or lactulose breath testing has been found to produce false negatives in patients with distal SIBO and false positives in patients with a fast gut transit (1).
New techniques are being investigated to overcome the limitations of the currently available diagnostic tools. Metagenomics — the analysis of genetic material — can reconstruct bacterial genomes and study the diversity of the gut microbiome. It can also identify microbial pathways and antibiotic resistance genes. Although it is still in its early stages, recent studies show promise for its use as a diagnostic method (1).
Conventional medicine treatment of SIBO
Treating the underlying cause, nutritional support, and treating the overgrowth are the three pillars of SIBO treatment (6).
Antibiotic therapy remains the most common treatment, and a short course of antibiotics can significantly reduce the number of abnormal bacteria and reverse the mucosal inflammation caused by the bacterial overgrowth (2, 6). Rifaximin is the preferred antibiotic to treat SIBO as it has a reduced toxicity profile, is used in IBS treatment, and has been more effective in eradicating the bacteria than other antibiotics (1). It is worth noting that rifaximin is expensive - $1247 for a short course - and not routinely covered by many commercial health plans in the United States (4). It is also not without its risks, including developing antibiotic-resistant organisms and infections like Clostridium difficile colitis (1).
Additionally, the return of excess bacteria is likely if the underlying disease is not treated. One trial found the average symptom improvement duration was only 22 days, which translates to a need for at least 12 courses of antibiotics a year to provide consistent symptom relief (6).
Antibiotics are expensive, may cause side effects or complications - like resistance - and may require multiple courses (6). Although they are a proven method of treatment relief, 40% of patients with SIBO-like symptoms may continue to feel sick despite antibiotic treatment (1). Patients who do not find success with antibiotics should be evaluated for other diagnoses like disaccharide deficiency, food intolerance, and motility disorders or consider other treatment methods (1).