What Is Small Intestinal Bacterial Overgrowth (SIBO)?

Medically reviewed by May 27, 2024| Written by

What Is Small Intestinal Bacterial Overgrowth (SIBO)?

Small Intestinal Bacterial Overgrowth, or SIBO, is a gastrointestinal problem caused by excess bacterial growth in the small intestine. The small intestine is a vital part of the digestive tract where95% of nutrient absorption occurs.

Normally, our digestive system consists of a balance of good and bad bacteria, which helps with efficient digestion. The body regulates the population of bacteria within the digestive tract throughseveral mechanisms, such as gastric acid release and efficient emptying of the small intestine to the large intestine.

When these protective mechanisms are compromised, bacteria find a suitable environment to overgrow in the small intestine. 

The problem arises when the harmful bacteria overgrow and disrupt normal digestion. These harmful bacteria in the small intestine ferment carbohydrates, releasing gas as a by-product and causing bloating.Bacterial interaction with bile salts alters the digestion and absorption of fats and, thus, fat-soluble vitamins. They also attack and consume essential proteins and vitamins in the diet. 

In the long term, inefficient digestion and malabsorption of nutrients lead to weight loss and other complications of malnutrition.

There is not much data on the prevalence of SIBO, but studies show that it is common among people suffering from digestive problems. For reference,research shows that one-third of people withIrritable Bowel Syndrome (IBS) have SIBO.

What is the Main Symptom of SIBO?

Symptoms of SIBO often correlate to the symptoms of other gastrointestinal problems. In SIBO, several changes in the normal digestion mechanism cause abdominal discomfort and affect regular bowel movements. 

Some common symptoms of SIBO are:

  • Bloating: Bacteria ferment the carbohydrates in the small intestine and release gas as a by-product. When there is an overgrowth of bacteria in the small intestine, it causes an excess of gas released from fermentation. 

The trapped gas causes a feeling of fullness and tightness in the abdomen, a condition called bloating.

Bacteria also compete with the body for nutrients, causing an imbalance that can trigger diarrhea. 

  • Constipation:The prevalence of gas-producing bacteria in the SIBO can lead toconstipation. The bacteria release gas from the fermentation of carbohydrates, which traps in the digestive tract and blocks regular gastric motility. It causes slower-than-usual bowel movements and other symptoms of constipation.
  • Abdominal Pain:Abdominal pain in SIBO is mainly due to excess gas build-up, which stretches the intestinal walls. Other conditions in SIBO, such as constipation and diarrhea, further worsen the pain.

Bacteria in the small intestine can also trigger abdominal pain by producing toxins and irritating the intestinal lining.

  • Weight Loss:Unintended weight loss in SIBO is due tocompromised digestion of macronutrients. Overgrowth of bacteria in the small intestine causes inefficient digestion of carbohydrates, proteins, and fats. 

Bacteria also feed on the essential nutrients in the diet, leading to malabsorption and weight loss. 

What are the Types of SIBO? 

Several types of bacteria can cause SIBO and require different treatment and diagnostic procedures. Based on the type of bacteria, there are three main types of SIBO:

1. Hydrogen Dominant SIBO

Hydrogen-dominant SIBO is a type of SIBO in whichhydrogen-producing bacteria excessively grow in the small intestine. These bacteria ferment the carbohydrates and produce hydrogen gas as a by-product, causing bloating. It is the most common type of SIBO and is diagnosed by excess hydrogen in theSIBO breath test.

Research shows that diarrhea is associated with hydrogen-dominant SIBO. Hydrogen-producing bacteria increase serotonin in the gut, stimulating gastric motility. 

Increased serotonin from overgrown bacteria can cause diarrhea, as the intestine contracts frequently and passes food too quickly.

2. Methane Dominant SIBO

Methane-dominant SIBO is a type of SIBO in which the population ofmethane-producing bacteria increases. These bacteria produce methane from hydrogen and carbon dioxide already present in the gut. It is the second most common type of SIBO, diagnosed when an SIBO breath test shows excess methane.

Studies show that methane-dominant SIBO is closely related to constipation, as this bacteriaslows the intestinal transit. Slow transit can also causeweight gain in methane-dominant SIBO, as a longer stay of food in the gut allows the absorption of more calories.

3. Hydrogen Sulfide Dominant SIBO

Hydrogen sulfide is a gas normally produced in the gut, and it has severalbenefits for gut health. However, when the bacteria that produce hydrogen sulfide overgrow in the small intestine, it is called hydrogen sulfide-dominant SIBO.

Symptoms of the excess of hydrogen sulfide in the gut include bloating, abdominal pain, diarrhea, and vomiting.Studies show that excess hydrogen sulfide can cause inflammatory bowel disease,irritable bowel syndrome, and colon cancer. 

What causes SIBO?

SIBO occurs when the body fails to regulate the normal count of bacteria in the gut. 

Though several factors contribute to SIBO, reduced gastric acid release and gastric motility are the primary causes. Structural abnormalities, weakened immune function, and certain medical conditions can also lead to SIBO.

1. Low Gastric Acid

Gastric juice containing gastric acid (HCL) and pepsin, is responsible for digestion and controls gut bacterial growth. Gastric juice with a pH lower than 3 can kill bacteria in the ingested foodwithin 15 minutes. However, a low acid release causes the pH to rise above 3 which isn’t effective against all bacteria and may result in bacterial overgrowth and therefore, SIBO.

Alow gastric acid release can result from gastric ulcers caused by H. pylori bacteria, medication, and aging. Adeficiency of nutrients including iron, zinc, and B vitamins can lead to low gastric acid. 

2. Reduced Gastric Motility

The small intestine keeps the food for a specific duration before passing it onto the large intestine. Normally, during this interval, the small intestine breaks down the food and absorbs the nutrients. 

But if gastric motility is compromised; the food and waste stay in the small intestine for longer periods. This promotes bacterial growth in the small intestine, causing the symptoms of SIBO.

Irregularities in thenerves and muscles controlling gut tissues can reduce gastric motility. Diabetes, hypothyroidism, Parkinson’s disease, and autoimmune diseases are somemedical conditions that can cause reduced gastric motility.

3. Structural Abnormalities

Structural changes in the small intestine can promote the build-up of bacterial populations. The structural abnormalities include the development of adhesions on the small intestine and the formation of pouches in the inside lining of the intestine, called small intestinal diverticulosis.

These abnormalities can result from surgeries associated with the gastrointestinal tract, such as theBillroth II procedure orRoux-en-Y anastomosis.

Structural abnormalities in the small intestine facilitate bacterial colonization by inhibiting the regular clearing of residual bacteria. Bacteria can accumulate and grow in the nooks and crannies, resulting from structural abnormalities, causing SIBO.

4. Compromised Immune Function

The immune system regulates the balance between good gut microbiota and harmful pathogens. When the immune function is compromised, it fails to prevent the overgrowth of harmful bacteria, which can cause SIBO.

The gut's immunity comprisesthree major components: intestinal microbiota, intestinal epithelial layer, and mucosal immune system.Study shows that problems with the mucosal immune system have a major role in the incidence of SIBO. The same study also reveals that hormonal and cellular immunity issues have no significant association with the development of SIBO.

5. Medications

Certain medications influence the body's biological process in a way that promotes bacterial overgrowth. The three most common medications that can cause SIBO are:

  • Proton Pump Inhibitors:Proton pump inhibitors (PPIs) are a class of drugs commonly used to treat acid reflux symptoms. These medicines work by reducing the gastric acid secretions. However, low levels of gastric acid are not enough to regulate bacterial overgrowth, thereby promoting SIBO.

Research shows that the incidence of SIBO is higher among people treated with proton pump inhibitors. 

  • Opioids:Opioids are commonly prescribed drugs for both acute and chronic pain. The gastrointestinal tract has a dense network of opioid receptors that respond to opioid drugs by several mechanisms.

Opioidsslow gastric emptying and thus help the bacteria feed and grow on the intestinal content. Astudy shows that a decrease in gastric motility by chronic use of opioids is the primary cause of gut microbial imbalance. 

  • Antibiotics:Antibiotics are the primary treatment option for SIBO. Ironically, they can also cause bacterial overgrowth. Certain antibiotics, especially broad-spectrum antibiotics, harm the beneficial gut bacteria used against pathogenic bacteria.

Loss of beneficial bacteria causes dysbiosis — gut microbial imbalance — and makes the gastric tract vulnerable to attack by pathogenic bacteria. This imbalance of gut microbiota by the dominance of pathogenic bacteria can lead to SIBO.

How Do You Cure Small Intestinal Bacterial Overgrowth (SIBO)?

The first step in treating SIBO is diagnosing the underlying cause and forming a treatment plan accordingly. For example, if the bacterial overgrowth is due to structural abnormalities in the gut, the doctors will try to fix the structural abnormality by surgery if possible.

The other common treatment options for SIBO are:

a) Antibiotics

Antibiotic therapy is the most common treatment option for all causes of SIBO. The prescription of antibiotics depends on the type of bacteria overgrown in the small intestine. Somecommon antibiotics for the treatment of SIBO include:

  • Rifaximin
  • Ciprofloxacin
  • Norfloxacin
  • Amoxicillin/clavulanate
  • Metronidazole + trimethoprim/sulfamethoxazole
  • Metronidazole + cephalexin

Antibiotic resistance is the most common complication of antibiotic therapy, and doctors try to prevent it by switching between types of antibiotics. The average treatment duration of antibiotic therapy for SIBO is7 to 10 days.

Some individuals with SIBO recover completely after antibiotic therapy. Others may experienceSIBO again after stopping the medicines. In such cases, individuals must take long-term antibiotics to regulate the bacterial count in the gut.

b) Probiotics

Probiotics help in efficient digestion and prevent the overgrowth of harmful bacteria in the gut. 

AStudy shows that probiotics can reduce hydrogen production by acting against the hydrogen-dominant SIBO. Anotherstudy suggests that probiotics have properties that improve gastric motility, which can help relieve the symptoms of SIBO.

However, treating SIBO with probiotics is controversial, as several studies indicate that using probiotics further worsens symptoms. For example, astudy shows that people taking probiotics gave higher levels of methane gas in SIBO breath tests, indicating increased severity of methane-dominant SIBO.

c) Nutritional Support

A well-planned diet can help manage the symptoms of SIBO. Such a diet essentially minimizes the risk of bacterial overgrowth and compensates for the nutritional deficiencies caused by SIBO. 

The diet must be low in fermentable products as the bacteria ferment those products in the gut, causing symptoms of SIBO. A diet low in fermentable products does not provide a suitable environment for the growth and development of bacteria in the gut,relieving the symptoms of SIBO.

Individuals with SIBO aredeficient in nutrients, including vitamins B12, calcium, iron, and other fat-soluble vitamins. Supplementation of the deficient nutrients is a key component of treating SIBO.

How Can I Prevent SIBO?

While you can't eliminate all the causes of SIBO, you can prevent it by following specific diet and lifestyle changes. These modifications are also helpful in preventing the recurrence of SIBO, which is not uncommon otherwise. 

1. Dietary Changes

Dietary changes to prevent SIBO should focus on planning a diet that fulfills all body requirements but doesn't facilitate the growth of bacteria in the gastrointestinal tract. This involves reducing fermentable products and shifting towards a lowFODMAP diet

FODMAPs are fermentable carbohydrates that your body can’t digest easily. Instead, bacteria ferment them in the small intestine, making the SIBO symptoms more severe.

With a low FODMAP diet, bacteria can't find a sustainable source to feed on and grow in the digestive tract. Add plant-based foods rich in probiotics to your diet, as they promote digestive functioning by facilitating good gut microbiota. 

2. Lifestyle Changes

Lifestyle changes include incorporating physical activities into your daily life, adopting healthy eating habits, and managing stress.Studies show that physical activity promotes gastric motility and helps the food pass through the intestine, preventing bacterial overgrowth. 

Moderate exercise also helpsrelieve the symptoms of SIBO, such as bloating and constipation. 

Follow a specific meal schedule and avoid snacking between meals. This will give the gastrointestinal system enough time to process and pass the food efficiently. Less accumulation of food in the small intestine reduces the risk of bacterial attack on the food contents and growth.

Avoid eating late at night or just before sleep becausesleep suppresses the activity of the digestive tract. Your body takes longer to clear the small intestine and pass the food to the large intestine, which can facilitate SIBO.Experts suggest the gap between your last meal and sleep must be 3 hours.

Stress management is another important aspect to consider in lifestyle modifications, as it can also cause SIBO or worsen existing symptoms.Research shows that stress releases several hormones that diminish the digestive system's activity and lower gastric motility, which can lead to SIBO.

What Foods to Avoid with SIBO?

If you are diagnosed with SIBO, you must know how diet can influence your condition and what foods to avoid. Certain foods promote the growth of bacteria in the gut and can worsen the symptoms of SIBO. Dietitians can help you plan a diet that fulfills your body's demands while starving the bacteria in the gastrointestinal tract.

Research shows that the best diet approach for digestive conditions is avoiding a diet high in FODMAPs. This means preventing fermentable carbohydrates, as thebacteria ferment them in the gut and use them as fuel for their growth.

Here are some foods high in FODMAPs that you should avoid with SIBO:

1. FODMAPs-Rich Fruits

Fruits have manyhealth benefits, but you must consume them wisely when diagnosed with SIBO. Sorbitol and fructose are the two prominent FODMAPs present in certain fruits, and consuming them can worsen the symptoms of SIBO. Fruits high in sorbitol and fructose include apples, mangoes, cherries, grapes,  apricots, blackberries, andothers.

Fruits are essential for a balanced diet, so add low-FODMAP fruits such as clementine, kiwi, oranges, papaya, and pineapples to your diet.

2. Certain Vegetables

Vegetables are significantlybeneficial for health, but several vegetables are high in FODMAPs, and you must avoid them in SIBO. Fructans and mannitol are the two prominent FODMAPs in vegetables. Vegetables high in FODMAP are asparagus, cauliflower, leeks, mushrooms, andothers.

Shift towards low FODMAP vegetables such as bean sprouts, cabbage, carrot, green bell pepper, spinach, and cooked tomatoes. 

3. FODMAPs-Rich Legumes and Pulses

Legumes and pulses are high in galactooligosaccharides (GOS) - a FODMAP. They cancause bloating and discomfort, worsening the symptoms of SIBO. Red kidney beans, split peas, chickpeas, lentils, and baked beans are the legumes and pulses high in GOS.

4. Dairy Products

Lactose is the main FODMAP present in some dairy foods. Buttermilk, milk, paneer, and yogurt are among the dairy products high in FODMAP. Instead, you can use low-FODMAP dairy foods such as processed cheese, ghee, lactose-free milk, and cream.

Adding plant-based milk such as soy, almond, and rice milk to your diet can also be a good choice in SIBO as they arelow in FODMAPs

5. Sweeteners

Sweeteners are high in fructose and sugar polyols like sorbitol and xylitol FODMAPs. Adding sweeteners to low-FODMAP foods can also increase your overall FODMAP intake and worsen the symptoms of SIBO. 

Note: You don't need to stick to a low-FODMAP diet forever. After recovering from SIBO, you can gradually increase the amount of FODMAPs in your diet. 


If you have symptoms like bloating, constipation, diarrhea, or abdominal pain, there is a probability that you have SIBO. The incidence of SIBO is often a cause of some underlying disease, and it is essential to address that to cure bacterial overgrowth.

Management of SIBO is important as otherwise, it can lead to severe complications of malnutrition. Along with eradicating bacterial overgrowth, supplementation is necessary to compensate for the loss of nutrients in SIBO. Appropriate diet and lifestyle changes can play a significant role in managing SIBO symptoms and preventing its reoccurrence.