SIBO (Small Intestinal Bacterial Overgrowth) is a condition in which bacteria that normally exist in small amounts in the small intestine or are present in the large intestine begin to proliferate in the small intestine. SIBO disrupts the normal digestion of food and absorption of nutrients.
Symptoms of SIBO include bloating, abdominal pain, diarrhea, or constipation. However, it is not just discomfort—SIBO can significantly impact quality of life, causing chronic fatigue, skin problems, weight loss, and vitamin and mineral deficiencies. The cause of SIBO is often other underlying diseases or issues, and SIBO is a dysfunction in the normal intestinal function resulting from this condition.
Functions of the Small and Large Intestines
As mentioned earlier, the problem with SIBO lies in the imbalance of the microbiota and the digestive system’s functions. To understand why this condition causes problems, it is important to recognize the differences between the small and large intestines and their functions:
Small Intestine
- The primary site for digestion and nutrient absorption (proteins, fats, carbohydrates, vitamins, minerals).
- Normally contains a relatively small number of bacteria—ranging from 10³ to 10⁶ bacteria per milliliter of content. This is kept in check by active peristalsis, bile action, and enzymes during normal functioning.
- Peristalsis: Regular contractions of the intestine push contents toward the large intestine, preventing stagnation.
- Gastric acid: Destroys harmful microorganisms that enter with food.
- Bile acids and pancreatic enzymes: Inhibit bacterial growth by creating unfavourable conditions for proliferation.
- Ileocecal valve: Prevents the backflow of contents from the large intestine into the small intestine.
Large Intestine
- The primary site for the fermentation of food residues by bacteria.
- Contains a massive microbiota—approximately 10¹¹–10¹² bacteria per gram of content, which is millions of times more than in the small intestine.
- The microbiota of the large intestine plays a key role in the formation of short-chain fatty acids (SCFAs), immune system support, and the synthesis of vitamins (such as K2, and B vitamins).
Natural Defense Mechanisms Against SIBO
- Structural Defense:
- Pyloric and ileocecal valves: These valves help prevent bacteria from moving from the stomach into the small intestine, and from the large intestine into the small intestine. The pyloric valve controls the movement of food from the stomach into the small intestine, while the ileocecal valve limits the migration of bacteria from the large intestine into the small intestine.
- Chemical Defense:
- Hydrochloric acid: The low pH in the stomach plays a crucial role in killing most pathogenic bacteria that enter the body through food. A deficiency in stomach acid can promote bacterial growth and the development of SIBO.
- Bile and digestive enzymes: Bile aids in the breakdown of fats and bacteria, while digestive enzymes break down proteins and carbohydrates. Disruptions in bile or enzyme production can impair normal digestion and contribute to the development of SIBO.
- Motility Defense:
- Migrating Motor Complex (MMC): This cyclic process occurring between meals helps clear the small intestine of food remnants, bacteria, and other substances. Reduced MMC activity can lead to stagnation in the intestines and promote excessive bacterial growth.
These defence mechanisms work together to prevent the development of SIBO and maintain healthy gut microbiota. Disruptions in any of these mechanisms can increase the risk of SIBO.
Migrating Motor Complex (MMC)
The Migrating Motor Complex (MMC) is a key process in maintaining intestinal health, particularly in preventing SIBO. It consists of several phases:
- Phase 1 (Resting State): Minimal activity, where the intestine rests and there is little to no movement of contents (lasting 45-60 minutes).
- Phase 2 (Submaximal Activity): Increased peristalsis, though not strong enough to move large amounts of food or bacteria (lasting about 30 minutes).
- Phase 3 (Maximal Activity): Powerful contractions that push food remnants and bacteria out of the small intestine (lasting 5-10 minutes, repeating every 90-120 minutes).
- Phase 4 (Transitional): A shift from maximal activity to the resting state.
MMC occurs between meals and plays a crucial role in preventing excessive bacterial growth in the small intestine. Disruption of MMC, such as in conditions like diabetes, hypothyroidism, or chronic stress, can lead to bacterial stagnation and the development of SIBO.
Every 90 minutes, a peristaltic wave occurs in the small intestine, helping to clear it of food remnants and bacteria. However, this process can be negatively affected by stress, food poisoning, or other factors. For instance, food poisoning can cause prolonged damage to the motor complex.
How SIBO Affects the Body
When the protective mechanisms of the small intestine are disrupted, bacteria from the large intestine begin to proliferate in the small intestine. Here are the potential issues that can arise as a result:
- Gas Formation in the Small Intestine:
- Unlike the large intestine, the small intestine is not designed to handle large amounts of bacteria and gas. The trapped gas stretches the intestinal walls, causing pain, bloating, and a feeling of fullness.
- The production of gases such as hydrogen or methane can lead to additional symptoms, including constipation or diarrhea.
- Damage to the Intestinal Lining:
- “Leaky gut”: Inflammation caused by bacteria increases the permeability of the intestinal walls, allowing toxins, bacteria, and undigested particles to enter the bloodstream.
- A leaky gut can contribute to systemic inflammation and the development of autoimmune diseases.
- Damage to the microvilli reduces the activity of enzymes like lactase, leading to secondary lactose intolerance.
- Nutrient Deficiency:
- Vitamin B12: Bacteria in the small intestine can absorb vitamin B12, leading to deficiency, which affects the nervous system and blood production.
- Folate: Bacteria can consume folate, leading to a deficiency, which is crucial for cell metabolism and nervous system health.
- Fats (especially fat-soluble vitamins): Due to issues with fat digestion and absorption (such as bile flow or intestinal function disruptions), problems with the absorption of vitamins A, D, E, and K may occur.
- Minerals: Minerals like calcium, magnesium, and iron may be poorly absorbed, as the normal gut microbiota supports the processes for their absorption, which is disrupted in SIBO.
- Proteins: Proteins and amino acids may also be poorly absorbed because the microbiota plays a crucial role in digesting food.
- Systemic Effects:
- Toxic byproducts from bacterial metabolism (endotoxins) can enter the bloodstream, causing fatigue, headaches, skin rashes, and other symptoms.
- Microbial imbalance can exacerbate symptoms of irritable bowel syndrome (IBS) and affect emotional and mental health through the “gut-brain axis.”
- Immune System Disruptions:
- Chronic inflammation in the intestines overloads the immune system, increasing the risk of allergies, food intolerances, and autoimmune diseases.
Symptoms of SIBO
Here is a list of potential symptoms of SIBO:
| Symptom Category | Symptoms |
| Main Gastrointestinal Symptoms | Bloating (gas): belching, flatulence |
| Abdominal pain and cramps | |
| Constipation, diarrhea, or alternating between the two | |
| Additional Gastrointestinal Symptoms | Heartburn (reflux or GERD) |
| Nausea | |
| “Leaky Gut” Syndrome Symptoms | Food sensitivities to multiple foods |
| Headaches | |
| Joint pain | |
| Fatigue | |
| Brain fog | |
| Skin manifestations (eczema, rashes) | |
| Respiratory symptoms (asthma) | |
| Emotional symptoms (depression, anxiety, concentration issues) | |
| Brain-related symptoms (e.g., autism) | |
| Malabsorption Symptoms | Steatorrhea (fatty stools), deficiency of fat-soluble vitamins: A, D, E, K |
| Anemia (iron- or B12-deficiency) | |
| Weight loss, weight gain |
Main Causes of SIBO
Impaired Intestinal Motility
- Delayed Peristalsis: Slow movement of food and waste through the intestines, as seen in conditions like diabetes, hypothyroidism, or post-surgical complications.
- Nervous System Dysfunction: Chronic stress, depression, or diseases affecting the autonomic nervous system can disrupt intestinal motility.
- Valve Dysfunction: Issues with the ileocecal valve or Peyer’s patches can lead to the backflow of bacteria from the large intestine to the small intestine.
- Chronic Constipation: Can cause bacterial backflow from the large intestine into the small intestine.
- Food Poisoning: Can damage intestinal motor function, impairing the ability to clear excess bacteria.
Alterations in Gastric Acid and Digestive Enzymes
- Reduced Stomach Acid: Due to proton pump inhibitors, hypochlorhydria, or gastric mucosa atrophy, creating a favorable environment for bacterial overgrowth.
- Deficiency in Digestive Enzymes: Can disrupt the balance of gut microbiota and promote pathogenic bacteria growth.
Anatomical Changes
- Post-Surgical Alterations: Surgeries involving the stomach or intestines can impact normal intestinal flow.
- Intestinal Obstruction: Scars or adhesions can block the intestinal lumen, creating conditions for bacterial overgrowth.
Immune System Disorders
- Immunodeficiency: Conditions such as autoimmune diseases, chronic infections, or long-term immunosuppressant use can compromise the gut’s immune response.
- Impaired Gut Barrier Function: Inflammatory diseases like Crohn’s disease or ulcerative colitis can allow bacteria to penetrate the small intestine.
- Autoimmune Diseases: Disorders like rheumatoid arthritis can also influence intestinal motility.
Microbiota Imbalances
- Antibiotic Use: Disrupts gut microbiota balance, enabling pathogenic bacteria to overgrow.
- Excessive Sugar and Carbohydrate Intake: Promotes the growth of harmful bacteria associated with SIBO.
- Dysbiosis: An imbalance between beneficial and harmful bacteria in the gut can contribute to SIBO development.
- Post-Infection or Parasitic Damage: Altered gut flora following infections or parasitic diseases may lead to SIBO.
Hormonal Changes
- Hypothyroidism: Reduced thyroid hormone levels slow down intestinal motility, leading to food stasis and bacterial growth.
- Diabetes: Nerve damage and slower gut motility in diabetes can encourage pathogenic bacteria proliferation.
- Hormonal Fluctuations: Changes in sex hormones, such as during menopause, may affect gut function.
External Factors
- Chronic Stress: Affects gut motility and immune system function, contributing to SIBO development.
- High-Sugar, High-Carb Diet: Promotes harmful bacterial growth and disrupts gut microbiota balance.
- Low Fiber Intake and Poor Digestion: A lack of fiber can impair motility and disrupt the gut microbiome.
List of Conditions Associated with SIBO
This list includes conditions that may contribute to the development or exacerbation of SIBO. Some of these conditions affect intestinal motility, immune response, or permeability, which can promote excessive bacterial growth in the small intestine. People with these conditions may have an increased risk of developing SIBO, and the onset of SIBO can worsen the symptoms of the underlying condition.
| Acne Rosacea | Acromegaly | Age: Elderly |
| Anemia | Atrophic Gastritis | Autism |
| Chronic Fatigue Syndrome | Chronic Lymphocytic Leukemia (CLL) | Chronic Prostatitis |
| Celiac Disease | Cirrhosis | Crohn’s Disease |
| Cystic Fibrosis | Diabetes | Diverticulitis |
| Dyspepsia | Erosive Esophagitis | Fibromyalgia |
| Gallstones | Gastroparesis | GERD (Gastroesophageal Reflux Disease) |
| Hepatic Steatosis | H. pylori Infection | Hypochlorhydria |
| Hypothyroidism / Hashimoto’s Thyroiditis | Interstitial Cystitis | IBS (Irritable Bowel Syndrome) |
| Lactose Intolerance | Leaky Gut Syndrome | Lyme Disease |
| Medications: PPI, Opioids, NSAIDs | Minimal Hepatic Encephalopathy | Muscular Dystrophy (Myotonic Type 1) |
| Myelomeningocele | NAFLD/NASH (Non-alcoholic Fatty Liver Disease/Steatohepatitis) | Pancreatitis |
| Parkinson’s Disease | Parasites | Restless Leg Syndrome |
| Rheumatoid Arthritis | Scleroderma | Surgery: Post-Gastrectomy |
Conclusion
SIBO represents a significant disruption in the balance of the gut microbiota in the small intestine, leading to various symptoms, including digestive issues, nutrient deficiencies, and systemic inflammation. Understanding the differences between the functions of the small and large intestines helps explain the mechanisms behind SIBO and related problems such as gas production, mucosal inflammation, and vitamin deficiencies. Disorders of intestinal motility, as well as breakdowns in chemical and structural defences, are key factors that contribute to excessive bacterial growth.
The diagnosis and treatment of SIBO, along with similar conditions, will be explored in more detail in the second part, which will also address the impact of factors such as hormonal changes and stress on the development of this condition.