SIBO: a common digestive disorder that is often missed

What is SIBO?

Small intestine bacterial overgrowth (SIBO) is an overgrowth of the bacterial population in the small intestine that often goes undiagnosed. Typically, the types of bacteria found in SIBO are foreign to the small intestine environment. Symptoms of this disorder can range from mild to severe, depending on how much dysfunction has developed (1).

Digestion & how SIBO develops

The digestive tract, also known as the gastrointestinal (GI) tract, digests food by breaking it down and absorbing nutrients. The mouth, pharynx, esophagus, stomach, and intestines are all part of the GI tract. Smooth muscles in these regions move food through while enzymes break the food down. The GI tract also includes the salivary glands, liver, pancreas, and gallbladder, which supply digestive juices. These gastric juices and acids are highly acidic and are produced to break food down in the stomach. Signals from the autonomic nervous system and hormones stimulate this gastric acid production (2). Hydrochloric acid, also a stomach juice, helps in the breakdown process and kills pathogens. Digestive enzymes split proteins, carbohydrates, and fats to aid in assimilation (3).

After food is digested in the stomach, it passes through the small intestine where nutrients are absorbed. The leftover remnants are pushed into the large intestine, where they are turned into feces as water extraction takes place. Under normal conditions, the large intestine is the primary carrier of bacteria and microbes.

When this process isn’t functioning correctly, bacteria can build up in the small intestine. Some causes of dysfunction include decreased digestive acid and juices, decreased speed of processing food through the small intestine, or damage in the intestines. Whatever the cause, dysfunction can lead to symptoms such as abdominal pain, gas and bloating, and diarrhea or constipation. As harmful bacteria overpopulate the gut, they release toxins which damage the mucosa — the membrane that lines and lubricates the GI tract to protect the body from invading pathogens. A damaged mucosa can lead to malabsorption, and can eventually cause nutrient deficiencies and inflammation (4). Harmful bacteria also inhibit gastric acid secretion by the parietal cells in the stomach; these secretions are needed to keep the bacteria count under control (5).

SIBO is a severe form of dysbiosis or “leaky gut.” Dysbiosis occurs when there is a disruption of the epithelial layer that makes up the mucosa. This layer functions as a barrier and is responsible for secretion as well as absorption. When dysbiosis occurs, this layer becomes permeable, allowing bacteria, chemicals, and toxins to leak out into the bloodstream. This causes inflammation and can contribute to autoimmune issues and neurodegenerative disorders.

A number of factors contribute to dysbiosis. Poor diet, stress, synthetic hormones, acid blockers, infections, and antibiotics — which prevent the GI tract from producing the right amount of nutrients and hormones — are some of the most common. These factors can influence the flora (microorganisms) of the gut, creating an unhealthy environment where SIBO can take over.

Types of SIBO

• Methane dominant (SIBO-C). Also known as constipation SIBO. When methanogens (methane-producing bacteria), such as Methanobrevibacter smithii, overpopulate the small intestine, they thrive on the hydrogen in your gut and produce methane, which slows transit in the intestines (6).
• Hydrogen dominant (SIBO-D). Hydrogen-producing bacteria overgrowth produces an excessive amount of hydrogen gas. This form is known for the symptoms of diarrhea and bloating.
• Mixed. A combination of methane and hydrogen dominant SIBO.
• Hydrogen sulfide dominant (SIBO-HS). This type occurs when Desulfovibrio spp. bacteria produce an abundance of hydrogen sulfide gas, which is toxic to the body.
• Small intestine fungal overgrowth (SIFO). Similar to SIBO, SIFO occurs when there is an overgrowth of fungus, with similar symptoms. SIFO and SIBO can be seen together, as 20% of SIBO patients are also diagnosed with SIFO (2). Since there is no test for SIFO, finding fungus in stool along with SIBO symptoms can be a good indicator that the patient has SIFO.

Common deficiencies associated with SIBO

The GI tract has the largest microbial population in the entire body, housing 500 different types of bacteria. In a healthy digestive system, these bacteria aid in the digestion of food. However, when there are too many bacteria or microbes in the GI tract, they compete for the body’s nutrients, stealing vitamins and preventing homeostasis. Some of the deficiencies associated with overgrowth:

• Inability to absorb fat-soluble vitamins. Bile in the stomach enables the extraction of fats from food. SIBO interferes with bile acids, making it difficult to absorb fat-soluble vitamins such as Vitamin A, D, E, K, and essential fatty acids (4).
• Exocrine Pancreatic Insufficiency (EPI). With EPI, the pancreas doesn’t produce enough digestive enzymes (7), which can lead to malabsorption. In 30–40% of cases of chronic pancreatitis (EPI), SIBO worsens the condition (1), affecting the gut flora and aiding in malabsorption.
• B12 deficiency. Commonly seen side effects include neurological issues, pain, and anemia (a deficiency of red blood cells).
• Iron deficiency. With a deficiency of iron, the body becomes anemic, causing fatigue.
• Neurological (the vagus nerve). The vagus nerve connects the brainstem to the abdomen and plays a big role in digestion, stimulating the stomach, liver, pancreas, and intestines. It controls smooth-muscle contractions, epithelial secretion, and intestinal permeability. When the vagus nerve isn’t functioning properly — often due to deficiencies caused by stress and anxiety — it can slow digestive processes and SIBO can occur, leading to other inflammatory issues like heartburn, IBS, and inflammatory bowel disease (8).

Root issues with SIBO

Many seemingly healthy patients are diagnosed with SIBO, but it is most commonly found in older individuals, women, and those who are overweight or obese. SIBO also thrives in those who are immunocompromised, as well as those with other GI disorders and food intolerances. Many other factors contribute, including the overuse of antibiotics or certain medications, surgery, and some of the underlying conditions listed here:

• Hypothyroidism. SIBO is seen in more than half of patients diagnosed with hypothyroidism (9). Hypothyroidism slows the body’s processes, including digestion. An underactive thyroid doesn’t make enough HCL (hydrochloric acid), preventing proper digestion and leading to malabsorption. Hypothyroid patients also experience frequent constipation, which makes the GI tract a breeding ground for SIBO since the intestines don’t remove waste and push out bacteria fast enough (10).
• Mold illness. Exposure to mold can create leaky gut. Mold and mycotoxins harm microflora and can cause inflammation and digestive dysfunction.
• Migrating Motor Complex (MMC) dysfunction. The MMC’s job is to move undigested food through the small intestine in three phases of contractions that clean out the stomach and intestines during fasting. Motilin, produced by the small intestine, is the main hormone regulator of the MMC (12). When the MMC is dysfunctional, SIBO can interfere with contractions and hormone production by slowing motility.
• Birth control. Hormonal birth control causes motility issues in digestion through its impact on the gallbladder.
• Antibiotics. These wipe out both helpful and harmful gut bacteria, disrupting the microbiome and allowing the gut to become a breeding ground for drug-resistant bacteria — making SIBO difficult to treat once it invades.
• Acid blockers (PPIs and antacids). Stomach acid helps keep bacteria in check. Reducing it long-term with proton pump inhibitors or antacids removes one of the body’s natural controls on bacterial overgrowth.
• Stress. Chronic stress slows digestion and reduces digestive secretions, creating conditions where bacteria can overgrow.
• Food poisoning. A bout of food poisoning can damage the migrating motor complex through an autoimmune response, and is a leading contributor to post-infectious SIBO and IBS.
• H. pylori. This infection can lower stomach acid and disrupt the upper digestive environment, contributing to overgrowth downstream.
• Endometriosis. The inflammation and adhesions associated with endometriosis can impair gut motility, one reason SIBO and “endo belly” so often overlap.
• Abdominal surgery. Surgery can create adhesions or alter anatomy in ways that slow transit through the small intestine.

Conditions associated with SIBO

Because SIBO both results from and contributes to digestive dysfunction, it overlaps with a number of other conditions. It is closely tied to irritable bowel syndrome (IBS), and is frequently seen alongside inflammatory bowel disease, food intolerances, and other GI disorders. Given how many systems the gut touches, addressing SIBO often means looking beyond the gut itself — to the thyroid, the nervous system, prior infections, and environmental exposures — to find what allowed the overgrowth in the first place.

SIBO is common, frequently missed, and very much treatable once it’s identified and its underlying drivers are addressed. If your digestive symptoms have lingered without a clear explanation, it’s a possibility worth investigating.