SIBO & Fructose Breath Testing
Breath tests can help clarify whether symptoms are being driven by small intestinal bacterial overgrowth (SIBO) and/or fructose malabsorption. Both tests measure hydrogen (H₂) and methane (CH₄) gases in the breath, which are produced when bacteria ferment carbohydrates.
Test selection and interpretation depend on symptoms and history.
What is SIBO?
Small Intestinal Bacterial Overgrowth (SIBO) occurs when an excessive number of bacteria grow in the small intestine. Normally, bacteria are more abundant in the colon. When bacteria overpopulate the small intestine, they can interfere with digestion, nutrient absorption, and overall gut health.
SIBO is often associated with bloating, digestive discomfort, and food intolerances, and it is frequently linked to Irritable Bowel Syndrome (IBS).
Common SIBO symptoms
Symptoms overlap with IBS. If digestive issues are persistent, breath testing may help clarify whether bacterial overgrowth is a contributing factor.
Symptoms of SIBO (digestive + systemic)
✅ Digestive symptoms
- Persistent bloating (often worsening throughout the day)
- Excessive gas (flatulence)
- Abdominal discomfort or cramping
- Diarrhea, constipation, or alternating between both
- Frequent indigestion or heartburn
✅ Nutritional & systemic symptoms
- Unintentional weight loss or difficulty maintaining weight
- Vitamin/mineral deficiencies (commonly B12, iron, fat-soluble vitamins)
- Fatigue and brain fog
- Weak hair and nails (possible malabsorption)
- Skin issues (acne, rosacea, or eczema)
What causes SIBO?
SIBO can develop when the normal mechanisms that regulate bacterial growth in the small intestine are disrupted.
- Slow gut motility (reduced Migrating Motor Complex activity can allow bacteria to accumulate)
- Low stomach acid (including hypochlorhydria or long-term PPI use)
- Structural or surgical changes (scar tissue, adhesions, prior abdominal surgeries)
- Food poisoning or gut infections (post-infectious motility disruption)
- Underlying health conditions (IBS, celiac disease, Crohn’s, diabetes, hypothyroidism)
Identifying potential drivers is important for long-term gut health and reducing recurrence.
How is SIBO diagnosed? (breath testing)
SIBO is commonly assessed using a breath test that measures hydrogen (H₂) and methane (CH₄) gases produced when bacteria ferment carbohydrates.
How the SIBO breath test works
- Preparation: a special diet is followed for 24 hours before the test to minimize background gas levels.
- Test solution: you drink a glucose or lactulose solution.
- Breath collection: breath samples are collected over ~2–3 hours.
- Interpretation: elevated hydrogen and/or methane within an early window can suggest SIBO.
A healthcare provider can help determine which test is most appropriate based on symptoms and medical history.
Who is at risk for SIBO?
- People with IBS, celiac disease, or Crohn’s disease
- Individuals with chronic bloating or gas after eating
- Those who have taken long-term antibiotics or proton pump inhibitors (PPIs)
- People with a history of food poisoning or gut infections
- Individuals with diabetes, hypothyroidism, or autoimmune diseases
- Anyone with prior abdominal surgery, adhesions, or motility disorders
Glucose vs Lactulose SIBO breath testing
Both tests measure hydrogen and methane on the breath. The difference is how far the test sugar travels through the small intestine.
Glucose Breath Test
How it works: Glucose is absorbed quickly in the first part of the small intestine. If overgrowth is present in the upper small intestine, fermentation can produce hydrogen and/or methane that appears on the breath test.
Pros
- More specific: less likely to be falsely positive because glucose typically doesn’t reach the colon.
- Best for upper (proximal) SIBO: helpful when symptoms feel “upper GI.”
Cons
- May miss lower (distal) SIBO: glucose is absorbed before reaching farther segments.
- Less sensitive overall: can under-detect cases located farther down.
Lactulose Breath Test
How it works: Lactulose is not absorbed, so it travels through the entire small intestine before reaching the colon. Bacteria present anywhere in the small intestine can ferment it and produce hydrogen and/or methane gases.
Pros
- Detects upper and lower SIBO: useful for broader coverage.
- More sensitive: can capture cases located farther down the small intestine.
Cons
- Higher false-positive risk: fermentation in the colon can be mistaken for SIBO.
- Interpretation can be trickier: timing and transit time matter.
Which test should you choose?
- If symptoms feel more upper GI (early bloating, reflux, upper discomfort), the glucose test may be a better fit.
- If symptoms include bloating that worsens over time, constipation, or “full intestinal” discomfort, lactulose may be more informative.
- If you’re not sure, your clinician can help choose a test approach based on symptoms and history.
Fructose Breath Test (Fructose Malabsorption)
The fructose breath test assesses fructose malabsorption (how well fructose is absorbed), not SIBO directly. It measures hydrogen and methane in the breath after ingesting a fructose load.
If fructose is not absorbed well in the small intestine, it can reach bacteria and be fermented, producing gas and symptoms. SIBO can sometimes cause a false-positive fructose test because bacteria in the small intestine ferment fructose earlier than expected.
When to assess fructose
- Symptoms triggered by fruit, honey, agave, or high-fructose foods
- Bloating, gas, cramping, or diarrhea after smoothies or fruit-heavy meals
- IBS-type symptoms where FODMAP foods are strong triggers
- Persistent symptoms after SIBO treatment to assess whether fructose malabsorption remains a driver
What results can mean
- Positive: fructose is not being absorbed efficiently and is being fermented by bacteria.
- Possible reasons: primary fructose malabsorption OR secondary causes (including SIBO, inflammation, post-infectious changes, celiac disease, IBS).
- Negative: fructose absorption is likely adequate; symptoms may relate to other FODMAPs, motility, or different drivers.
How the fructose breath test works
- Preparation: a special diet is followed 24 hours before the test to reduce baseline gas.
- Test solution: you drink a measured fructose solution.
- Breath collection: samples are collected over ~2–3 hours.
- Interpretation: increased hydrogen and/or methane suggests fermentation of unabsorbed fructose.
In many cases, clinicians prefer to evaluate or address SIBO first, then consider fructose testing if symptoms remain specifically fructose-triggered.
Next steps: explore breath testing
If you experience persistent bloating, digestive discomfort, or unexplained nutrient deficiencies, breath testing may provide useful information. Test selection and interpretation are most meaningful when guided by your history and symptom pattern. The first step is to book an initial consultaiton with Dr. Lococo, ND. This can be booked online.
Prefer email? Contact us at info@lococowellnessclinic.com .
Educational content only. This page does not diagnose, treat, or claim to cure disease. Breath test selection and interpretation should be guided by a qualified clinician and considered alongside your medical history and symptoms.