SIBO Symptoms: How to Recognise Small Intestinal Bacterial Overgrowth

SIBO — Small Intestinal Bacterial Overgrowth — is one of the most under-recognised digestive conditions. Its symptoms overlap significantly with IBS, making it easy to dismiss as “just a sensitive gut.” But SIBO has a measurable cause, a specific diagnostic test, and a targeted treatment protocol. Understanding the symptom pattern is the first step toward getting the right investigation.

What is SIBO and why does it cause symptoms?

The small intestine normally contains very few bacteria — fewer than 10³ organisms per millilitre, compared to 10¹² per millilitre in the colon. This low-bacterial environment is maintained by several defence mechanisms: gastric acid, bile, intestinal motility (especially the migrating motor complex between meals), and immune secretions.

When any of these mechanisms fail — due to low stomach acid, reduced gut motility, structural abnormalities, or immune dysfunction — bacteria from the colon can migrate upward and proliferate in the small intestine. These bacteria compete for nutrients, ferment carbohydrates before they can be absorbed, and produce gases (primarily hydrogen and methane) that cause the characteristic SIBO symptoms.

Core SIBO symptoms

Bloating and distension

The hallmark of SIBO. Bloating in SIBO tends to be progressive throughout the day — worse after meals and by evening — and often severe enough to make clothing uncomfortable. Unlike simple bloating after a heavy meal, SIBO-related distension is persistent and disproportionate to what was eaten. Many patients describe looking “six months pregnant” by end of day despite eating normally.

Excessive gas (flatulence and belching)

Bacterial fermentation of carbohydrates in the small intestine produces hydrogen and methane gas in far greater quantities than normal. Excessive flatulence — particularly odorous or immediately after eating — and belching (from gas escaping upward into the oesophagus) are consistent features.

Altered bowel habits

The bowel pattern in SIBO depends on which gas is predominant:

• Hydrogen-dominant SIBO: more commonly associated with diarrhoea, urgency, and loose stools
• Methane-dominant SIBO (IMO): more commonly associated with constipation, hard stools, slow transit
• Mixed-type: alternating diarrhoea and constipation — a pattern frequently misdiagnosed as IBS

Abdominal pain and cramping

Diffuse abdominal discomfort or cramping, often without a clearly localised point of pain. Symptoms frequently worsen after eating — particularly after fibre-rich or fermentable foods — and improve temporarily after bowel movements or passing gas.

Fatigue and brain fog

A frequently overlooked symptom cluster. Bacterial overgrowth causes systemic low-grade inflammation and can interfere with the absorption of B vitamins (particularly B12), which are essential for neurological function and energy metabolism. Many SIBO patients report profound fatigue, difficulty concentrating, and mental sluggishness — often dismissed as anxiety or burnout before the gut diagnosis is made.

“The brain fog in SIBO is real — it’s not imagined, and it’s not separate from the digestive issue. When you’re not absorbing B12, when your gut is inflamed and your microbiome is disrupted, your cognitive function suffers. Treating the overgrowth often dramatically improves the mental clarity patients had given up on.” — Taissa Castello, nutritionist (CRN-4 25106120)

Nutritional deficiencies caused by SIBO

• Vitamin B12 deficiency: bacteria consume B12 before it can be absorbed; causes fatigue, anaemia, and neurological symptoms
• Iron deficiency: chronic intestinal inflammation reduces iron absorption
• Fat-soluble vitamins (A, D, E, K): bacterial overgrowth disrupts bile acid metabolism, impairing fat absorption and the vitamins that depend on it
• Protein malabsorption: in severe or long-standing SIBO, protein digestion may also be compromised, causing muscle loss and poor wound healing

SIBO vs IBS: how to tell them apart

SIBO and IBS share symptoms so significantly that 30–80% of IBS patients are estimated to have SIBO. The key differences:

• SIBO has a measurable cause: bacterial overgrowth confirmed by breath test. IBS is a functional diagnosis made after excluding other causes.
• SIBO has a targeted treatment: specific antibiotic protocols (rifaximin ± neomycin) that reduce or eliminate the overgrowth. IBS treatment is symptom-focused.
• SIBO is associated with nutritional deficiencies: blood tests showing B12, iron, or vitamin D deficiency alongside gut symptoms warrant SIBO investigation.
• SIBO-specific trigger pattern: symptoms often worsen dramatically with high-FODMAP foods — onion, garlic, legumes, fructose — and improve with fasting (because there is no substrate for fermentation).

The breath test: diagnosing SIBO

The gold-standard diagnostic test for SIBO is the hydrogen and methane breath test. You consume a sugar solution (glucose or lactulose), then blow into collection tubes at regular intervals over 2–3 hours. The test measures the hydrogen and methane exhaled — gas produced exclusively by gut bacteria, not by human cells.

A rise in hydrogen >20 ppm above baseline within 90 minutes of the glucose challenge, or methane levels >10 ppm at any point, indicates bacterial overgrowth. Proper preparation (a low-FODMAP diet for 24 hours prior, 12-hour fast, no antibiotics for 4 weeks) is essential for test accuracy.

For the complete SIBO management guide including treatment and relapse prevention, read: SIBO: Complete Guide for International Patients.

Frequently asked questions

How is SIBO different from normal bloating?

Normal bloating after a heavy meal or a gassy food is transient — it resolves within a few hours. SIBO bloating is progressive, beginning early in the day after the first meal and worsening throughout the day. It is also disproportionate to what was eaten: a small, simple meal produces significant distension. If you consistently look pregnant by evening regardless of what you eat, SIBO should be on your differential.

Can SIBO cause anxiety and mood changes?

Yes. The gut-brain axis connects the enteric nervous system to the central nervous system. Bacterial overgrowth in SIBO disrupts this communication, can compromise B-vitamin absorption (essential for neurotransmitter synthesis), and maintains a state of low-grade systemic inflammation. Anxiety, low mood, and cognitive difficulties are reported by a significant proportion of SIBO patients and often improve with successful treatment.

What makes SIBO symptoms worse?

Symptoms typically worsen with: high-FODMAP foods (onion, garlic, legumes, apples, honey), fibre-rich foods (because fibre is fermentable), large meal volumes, and eating too frequently (which prevents the migrating motor complex — the gut’s between-meal cleaning sweep — from clearing bacteria). Symptoms often improve after fasting or with a low-fermentation diet.

Do I need antibiotics to treat SIBO?

Antibiotic therapy (rifaximin for hydrogen-dominant SIBO, with neomycin or metronidazole added for methane-dominant IMO) is the primary treatment and must be prescribed by a gastroenterologist. Nutritional support — including FODMAP diet guidance during treatment and microbiome restoration afterward — is an essential part of management, but it does not replace antibiotic therapy.

Read also: SIBO: Complete Guide | FODMAP Diet: Complete Guide | The Gut-Brain Connection | Autoimmune Disease and Nutrition

Medical disclaimer: This article is for educational and informational purposes only. It does not replace a medical or nutritional consultation. Taissa Castello is a registered nutritionist (CRN-4 25106120) — this content does not constitute medical advice. Always consult your physician and a qualified nutritionist before making health decisions.

Taissa Castello Fonseca

Nutricionista Clínica • CRN-4 25106120

Especializada em doença celíaca, SIBO, doenças autoimunes e saúde da mulher. Celíaca há 9 anos. Atende 100% online para todo o Brasil.

Sobre a Taissa Agendar consulta @taissacastello.nutri