If you’re struggling with gut issues, histamine intolerance, and seemingly random food reactions, you may have already heard of Small Intestinal Bacterial Overgrowth (SIBO). Did you know there are 3 types of SIBO? Not all SIBO is the same. Each type presents with different symptoms, requires different treatment approaches, and can impact histamine metabolism differently.

If you’re dealing with histamine intolerance, understanding SIBO can be a game-changer in getting to the root of your symptoms. In this blog, we’ll break down the three types of SIBO, their symptoms and the link to histamine intolerance.

What Is SIBO?

SIBO occurs when bacteria that should primarily reside in the large intestine migrate and overgrow in the small intestine. This overgrowth can ferment carbohydrates in the small intestine, leading to bloating, gas, pain, and malabsorption of nutrients.

When SIBO is present, it can trigger inflammation, increase intestinal permeability (a.k.a. “leaky gut”), and worsen histamine intolerance by affecting gut function and mast cell activation. Let’s look at how each type of SIBO can uniquely impact histamine metabolism and overall gut health.

The 3 Types of SIBO & the Symptoms

There are 3 types of SIBO: Hydrogen, Methane, and Hydrogen Sulfide.

1. Hydrogen-Dominant SIBO (H2 SIBO)

What It Is:

Hydrogen SIBO occurs when bacteria in the small intestine produce excess hydrogen gas as they ferment carbohydrates. This is the most common form of SIBO.

Symptoms of Hydrogen SIBO:

• Chronic diarrhoea (often urgent, loose stools)

• Bloating (especially within 30-90 minutes after eating)

• Abdominal pain and cramping

• Nutrient malabsorption (especially iron and B12 deficiencies)

• Fatigue and brain fog

How Hydrogen SIBO Affects Histamine Intolerance:

• The overgrowth of hydrogen-producing bacteria can damage the small intestine lining, reducing DAO enzyme production, which is crucial for breaking down histamine.

• Increased inflammation in the gut can lead to higher histamine release and worsen symptoms like headaches, rashes, and flushing.

• Diarrhoea can further deplete the body’s nutrient stores, making it harder to manage histamine breakdown.

2. Methane-Dominant SIBO (IMO – Intestinal Methanogen Overgrowth)

What It Is:

Methane SIBO (often called Intestinal Methanogen Overgrowth or IMO) is caused by an overgrowth of methane-producing archaea in the small intestine.

Symptoms of Methane SIBO:

• Chronic constipation (often severe and slow transit)

• Excessive bloating and distension

• Gas with a strong, foul odour

• Belching and reflux

• Weight gain and difficulty losing weight

• Brain fog and fatigue

How Methane SIBO Affects Histamine Intolerance:

• Methane-producing microbes slow gut motility, which increases histamine retention and worsens symptoms.

• Constipation means toxins and histamine-stimulating compounds stay in the gut longer, exacerbating histamine reactions.

• These microbes can also increase inflammation and trigger mast cell activation, leading to more histamine-related symptoms.

3. Hydrogen Sulfide SIBO

What It Is:

Hydrogen sulfide SIBO is the least common type and occurs when bacteria produce excess hydrogen sulfide gas, which has unique effects on the gut and nervous system.

Symptoms of Hydrogen Sulfide SIBO:

• Alternating diarrhoea and constipation

• Rotten egg-smelling gas

• Severe bloating and abdominal pain

• Sensitivity to high-sulfur foods (like eggs, cruciferous vegetables, and garlic)

• Fatigue and dizziness

• Nerve-related symptoms like tingling and burning sensations

How Hydrogen Sulfide SIBO Affects Histamine Intolerance:

• Hydrogen sulfide-producing bacteria can deplete butyrate, a compound that helps regulate inflammation and gut barrier function, worsening histamine issues.

• Increased hydrogen sulfide can directly irritate mast cells, triggering histamine release.

• Many high-histamine foods (like fermented foods) can worsen symptoms for those with hydrogen sulfide overgrowth.

What Causes SIBO?

Your body usually knows how to keep intestinal bacteria in check. However, SIBO is able to thrive in the following situations:

• Low stomach acid

• Long term stress (this lowers stomach acid)

• Chronic constipation

• Crohn’s disease

• Diets high in sugar and/or refined carbohydrates

• Diets high in red meat or saturated fats

• Smoking

• Antibiotic overuse

• Use of medications that disrupt gut bacteria, lower stomach acid level, or contain opiates (e.g. NSAIDS, Proton pump inhibitors, codeine or morphine)

• Structural abnormalities in the digestive tract, such as scarring from surgery

• An impaired nervous system due to trauma or chronic illness.

SIBO, Histamine, and the Gut Connection

If you have both SIBO and histamine intolerance, your gut microbiome is likely dysregulated, making it harder to process and eliminate histamine effectively. Some key takeaways include:

• Addressing SIBO is essential to improving histamine metabolism and reducing symptoms.

• Gut motility is crucial—poor movement of food and waste through the intestines allows bacteria to overgrow and histamine to build up.

• Dietary strategies should be personalised—some people with histamine intolerance find relief with a low-FODMAP diet, while others need to focus on low-sulfur or low-fermentation approaches.

• Healing the gut lining with nutrients can help restore proper histamine breakdown.

Is SIBO causing your histamine issues? Read more in this blog.

If you’re experiencing chronic gut issues, histamine intolerance, and food sensitivities, it’s worth investigating whether SIBO is playing a role. The 3 types of sibo – hydrogen, methane, and hydrogen sulfide – each have distinct symptoms and treatment approaches. Addressing the right type of SIBO, supporting gut motility, and healing the gut lining can significantly improve both digestion and histamine intolerance.

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