Small Intestinal Bacterial Overgrowth (SIBO)

What is small intestinal bacterial overgrowth (SIBO)?

Small intestinal bacterial overgrowth (SIBO) is an overgrowth of nonpathogenic bacteria in the small intestine ^[2]. Normally, there is only a small amount of bacteria in the small intestine (they are mostly in the large intestine), but they may overgrow under certain conditions. The main symptom is abdominal bloating.

SIBO Causes and Risk Factors

A. Disruption of natural barriers, which allows bacteria to colonize the small intestine ^[1,2,3,4]:

• Reduced or absent secretion of gastric acid (hypo- or achlorhydria) due to gastritis caused by Helicobacter pylori, autoimmune gastritis in pernicious anemia, resection of the vagal nerve (vagotomy), use of proton-pump inhibitors
• Gastrointestinal surgery: partial stomach removal, gastric bypass (bariatric surgery), removal of the last part of the small intestine including the removal of the ileo-cecal valve that prevents bacteria to enter from large to small intestine, extensive resection of the small intestine, mainly in infants, resulting in short bowel syndrome (SBS).
• Immune deficiency due to HIV/AIDS, IgA deficiency, severe malnutrition, liver cirrhosis ^[5], chronic lymphocytic leukemia ^[6], common variable immunodeficiency and severe malnutrition
• Abnormal connection (fistula) between the small and large intestine, for example, in Crohn’s disease ^[7]

B. Impaired digestion or absorption of carbohydrates:

• Celiac disease ^[1,2]
• Crohn’s disease ^[1,7]
• Short bowel syndrome ^[1]
• Chronic pancreatitis ^[1,8]
• Possibly: fructose malabsorption, lactose intolerance

C. Decreased motility of the small intestine ^[1,2]:

• Neurological disorders: diabetic autonomic neuropathy with delayed gastric emptying (gastroparesis), myotonic dystrophy, Parkinson disease, Chagas enteropathy
• Infiltration of the small intestinal wall: scleroderma, amyloidosis
• Hypothyroidism

D. Blind pouches in the small intestine ^[1,2]:

• Diverticles in the small intestine
• Blind loop syndrome
• Megacolon in Chagas disease

E. Obstruction of the small intestine ^[1,4]:

• Tumors
• Pseudotumors, polyps, strictures and adhesions in Crohn’s disease
• Radiation enteritis

F. The following conditions have been associated with SIBO:

• Old age (associated with malnutrition, decreased mobility, increased prevalence of diabetes) ^[1,2,13,14]
• Chronic alcoholism ^[2]
• Irritable bowel syndrome ^[1,2]
• End stage renal disease ^[1,2]
• Acromegaly ^[10]
• Cystic fibrosis ^[11,12]
• Fibromyalgia ^[2,15]

SIBO Symptoms

Symptoms of SIBO may include ^[1,2]:

• Abdominal bloating (distension) and pain
• Excessive gas (flatulence)
• Stomach upset (dyspepsia)
• Chronic diarrhea
• Symptoms caused by nutrient malabsorption: weight loss, fatigue, anemia (see Chart 1)

Chart 1. Nutrient Malabsorption in SIBO
Nutrient	Consequence
Lactose	Bloating, diarrhea
Fructose	Bloating, diarrhea
Fats	Greasy stools – steathorrhea, weight loss
Proteins	Muscle wasting, swelling, weight loss
Vitamin A	Dry eyes, night blindness
Vitamin B12, folate	Macrocytic anemia, peripheral neuropathy
Iron	Microcytic anemia
Calcium	Muscle twitching (tetany)
Selenium	Dermatitis

Chart sources: ^[1,4]

SIBO Complications

A. Impaired digestion or absorption of nutrients:

• Impaired digestion of lactose (lactose intolerance) ^{[16,17,18,19]} and fructose malabsorption ^[2,18]
• Protein malabsorption and protein-losing enteropathy due to damage of the small intestinal lining resulting in muscle wasting ^[1,2,20]
• Fat malabsorption resulting in greasy stools (steatorrhea). Mechanism: bacteria may inactivate bile acids, which are necessary for the absorption of fats ^[2]. Severe SIBO with fat malabsorption may be associated with reduced absorption of fat-soluble vitamins:
  • Vitamin A: dry eyes, night blindness
  • Vitamin D: osteomalacy with bone fractures, decreased calcium absorption, resulting in increased oxalate absorption and increased risk of oxalate kidney stones and in muscle cramps (tetany) ^[1,4]
  • Vitamin E: Neuropathy
  • Vitamin K: Increased bleeding
• Impaired iron absorption resulting in microcytic anemia with weakness and paleness ^[1]
• Impaired vitamin B12 and folate absorption resulting in macrocytic anemia and neuropathy with paleness, tingling or numbness ^[1,4]
• Impaired selenium absorption resulting in dermatitis ^[1]

Other Possible Complications of SIBO

• Non-alcoholic fatty hepatitis ^[2]
• Rosacea. In several studies, acne rosacea was associated with SIBO; when SIBO was eradicated, rosacea was often healed at the same time ^[21].

In some individuals diagnosed with irritable bowel syndrome (IBS), antibiotics helped relieve symptoms, which suggests their symptoms were more likely caused by SIBO than IBS ^[22,23,24].

SIBO is a common cause of false positive results of breath tests for fructose malabsorption and lactose intolerance ^[18].

Diagnosis

SIBO can be diagnosed by the hydrogen, methane, bile acid or xylose breath test ^[25].

Treatment

Treatment of SIBO can includes ^[26]:

• Correction of the underlying disorder
• Correction of nutrient deficiencies, if necessary
• Oral antibiotics, such as ciprofloxacin, clindamycin, metronidazole, rifaximin or tetracyclin, depending on the circumstances (causing bacteria, the underlying problem, age); initial treatment may last for 1-2 weeks and, if necessary, may be prolonged or repeated at a later time.

There is INSUFFICIENT EVIDENCE about the effectiveness of probiotics in treatment of SIBO ^[2].