SIBO – when too many bacteria overgrow in the small intestine

Symptoms

The symptoms of SIBO vary widely and depend on which gases the bacteria produce. Many people experience a noticeable buildup of gas shortly after eating, along with visible bloating, abdominal pressure, or a feeling of fullness. Abdominal pain, cramps, diarrhea, or constipation are also common, depending on whether hydrogen, methane, or hydrogen sulfide is dominant.

In addition to digestive issues, symptoms such as fatigue, difficulty concentrating, skin breakouts, or nutrient deficiencies (for example, vitamin B12 or iron) often occur, since the bacterial imbalance in the small intestine can interfere with the absorption of vitamins and minerals.

Cause

SIBO can have many underlying causes. In many cases, impaired intestinal motility plays a key role, particularly when the so-called migrating motor complex (MMC), the mechanism that regularly cleanses the small intestine, does not function properly. A weakened ileocecal valve, which separates the small and large intestine, previous antibiotic treatments, chronic stress, gastrointestinal infections, low stomach acid, or conditions such as PCOS, endometriosis, and hypothyroidism can also disrupt the gut’s balance and promote the development of SIBO.

Hydrogen SIBO (H2)

In hydrogen-dominant SIBO, bacteria such as Escherichia coli and Streptococcus are mainly responsible for producing hydrogen gas. They ferment sugars commonly found in FODMAP-rich foods, especially fructose and galactose. When these types of carbohydrates are consumed in excess, gas levels rise, often resulting in bloating and diarrhea.

Common Symptoms:

• Feeling of fullness
• Severe bloating within 30min – 1h
• Frequent diarrhea
• Abdominal cramps that worsen after eating sugar (FODMAPs) or high-carbohydrate meals
• Brain fog

Methan SIBO (CH4)

Methane-dominant SIBO is primarily caused by the microorganism Methanobrevibacter smithii, which produces methane gas. These bacteria slow down intestinal motility, often leading to constipation. This type of SIBO is characterized by symptoms such as severe constipation, bloating, and a feeling of fullness, especially after consuming FODMAP-rich foods containing fructose and certain types of sugars.

Mixed Type

In some cases, a mixed type of SIBO can occur, where both hydrogen- and methane-producing bacteria are present in the small intestine. This makes diagnosis more challenging, as both gases can be produced simultaneously, leading to a combination of symptoms associated with both hydrogen- and methane-dominant SIBO. This mixed form may cause alternating symptoms, such as frequent diarrhea (typical of hydrogen SIBO) and constipation (typical of methane SIBO).

Common Symptoms:

• Bloating and a feeling of fullness
• Severe constipation or irregular bowel movements, often worsened by the intake of FODMAPs
• Slow digestion
• Abdominal cramps and discomfort
• In some cases, weight gain
• Brain fog

Hydrogen sulfide SIBO (H2S)

Bacteria such as Desulfovibrio, Bilophila, Proteus, and Salmonella produce hydrogen sulfide (H₂S) when they break down sulfur-containing compounds found in foods like onions and garlic. These bacteria can be particularly problematic, as hydrogen sulfide not only worsens bloating but can also damage the intestinal lining. People with hydrogen sulfide-dominant SIBO often report a foul smell in their gas along with other intense digestive symptoms.

Common Symptome:

• Bloating and strong, foul-smelling gas, especially after
• consuming sulfur-rich foods such as onions, garlic, and eggs
• Abdominal cramps and discomfort
• Irregular bowel movements (constipation or diarrhea)
• Frequent skin issues such as acne
• Fatigue and brain fog

Diagnosis

The most common diagnostic method for SIBO is the breath test, in which hydrogen and methane levels in the exhaled air are measured at regular intervals after the ingestion of glucose or lactulose. These gases are produced when misplaced bacteria in the small intestine ferment sugars. A rise in hydrogen indicates hydrogen-dominant SIBO, while elevated methane levels point to a methane-producing overgrowth.

One major limitation of standard breath tests, however, is that they do not detect hydrogen sulfide (H₂S), the third possible gas in SIBO. As a result, patients with sulfide-dominant SIBO often receive false-negative results. Currently, there is only one specialized laboratory in the United States capable of reliably measuring all three gases.

Therapy

Antibiotics

Rifaximin is considered the preferred antibiotic for treating SIBO, as it acts locally in the small intestine and is barely absorbed into the bloodstream. This minimizes the risk of systemic side effects and helps preserve the bacterial balance in the large intestine, which is often disrupted by other antibiotics. Rifaximin is a gentler yet highly effective option for treating SIBO without significantly disturbing the overall microbiome.

Herbal alternatives

Herbal remedies can be just as effective as antibiotics in treating SIBO. Among the best known are oregano oil, berberine, and allicin, which have antibacterial properties and can specifically inhibit the growth of pathogenic bacteria in the small intestine. A 2011 study published in the Journal of Clinical Gastroenterology found that oregano oil and berberine were as effective as rifaximin in treating SIBO, with the added benefit of offering broader antibacterial effects against a wider range of bacterial species. These herbal compounds tend to have a milder impact on the microbiome and represent a gentle alternative to antibiotics, particularly in recurrent cases of SIBO.

In sulfur-dominant SIBO, where hydrogen sulfide-producing bacteria prevail, bismuth compounds are sometimes added to treatment, as they bind hydrogen sulfide and can reduce its toxic effects on the intestinal lining.