If you’re planning to take a SIBO breath test, proper preparation is crucial for accurate results. Small Intestinal Bacterial Overgrowth (SIBO) is often linked to symptoms like bloating, gas, and IBS but if you don’t follow the test preparation guidelines carefully, you risk getting false positive or false negative results.

This guide will walk you through exactly how to prepare, what to eat (and not eat), and how to avoid the most common testing mistakes.

What Is a SIBO Breath Test

The SIBO test measures hydrogen and methane gases in your breath after consuming a sugar solution (typically lactulose or in some cases glucose). These gases are produced when bacteria ferment carbohydrates in the small intestine. Elevated levels indicate bacterial overgrowth a key indicator of small intestinal bacterial overgowth or Intestinal Methanogen overgrowth.

Why Proper Preparation Matters

If you eat the wrong foods before your test, or take certain medications or supplements, your results can be inaccurate potentially leading to misdiagnosis or mistreatment. For best accuracy, you must:

• Follow a specific low-fermentation diet the day before
• Fast for 12-hours before the test
• Avoid antibiotics, probiotics, and laxatives for two weeks.

What you should Eat before the SIBO Test

You’ll need to follow a low-fibre, low-fermentation diet the day before testing.

What to Eat before the SIBO Test:

• Plain white rice (boiled only)
• White bread or gluten-free white bread or white wraps – the more processed the better! Please ensure there is no fibre.
• Plain meats: chicken, turkey, beef, lamb, pork
• Fish- without batter or bread crumbs
• Eggs
• Tofu if you are vegan/vegetarian
• Black or green tea (without milk)
• Plain coffee (no milk, no sweeteners)
• Jam or jelly (no artificial sweeteners)

Tip: If in doubt, stick to meat, fish or eggs and plain white rice and avoid anything with fibre or fermentation potential.

You may find that you feel noticeably better when following this preparation diet, this is likely because reducing fibre intake temporarily minimises bacteria and fermentation in the gut leading to less bloating and gas.

However, some individuals may react to the lactulose solution used during the test and experience mild side effects such as diarrhoea, wind, or bloating for up to 24 hours.

Foods and Drinks to Avoid before the Hydrogen and Methane Breath test

These foods can lead to fermentation in the gut and must be strictly avoided the day before your test:

Avoid:

• Dairy (milk, cheese, yogurt, butter, ice cream)
• Wholegrains (wheat, oats, bran, multigrain, rye bread, wholemeal bread)
• Fruit (fresh or dried)
• Vegetables
• Legumes (lentils, peas, beans)
• Potatoes, pasta, brown rice
• Nuts and seeds
• Herbs, spices, and garlic/onions
• Sugary drinks, alcohol, or anything containing fructose or artificial sweeteners

How to prepare for the SIBO breath test- Fasting Guidelines

You must fast for 12 hours before your test typically overnight.

• Only drink water during this time
• No food, caffeine, tea, or supplements during the fast
• Aim to start the test first thing in the morning to make the fast easier

Medication and Supplements to avoid before the breath test

Some medications can interfere with your results.

• You should wait two weeks after completing a course of antibiotics or antimicrobial treatments (including herbal supplements such as Oregano, Berberine and Neem, Allimed before doing your SIBO test.
  
• Avoid probiotics such as symprove, Lactobacillus, Bifidobacteria for at least 1 week before testing.
  
• Refrain from taking laxatives or motility agents for 1 week beforehand.

Always consult your practitioner before stopping any prescription medications.

Also, do not perform the test:

• Within 4 weeks of a colonoscopy or bowel cleanse
• On the same day as any other breath test (e.g., lactose/fructose)

What Happens on Test Day?

Once you’ve fasted:

1. Collect your first breath sample (fasting baseline)
2. Drink the lactulose/glucose solution
3. Collect a new breath sample every 20 minutes for 3 hours
4. Label and return the test tubes as instructed

Avoid smoking, exercise, or sleeping during the test.

Common Mistakes That Can Ruin Your Results

Eating fruit or fibre the day before
Drinking milk or sugary drinks Not fasting for 12 hours
Taking probiotics or laxatives too soon
Missing sample times or forgetting labels

What Happens If You Eat the Wrong Foods Before a SIBO Test?

1. False Positive Results
   Bacteria in the small intestine ferment the sugars and fibres you have eaten, producing hydrogen and methane before the test even starts. This can artificially raise your baseline gas levels, making it look like you have SIBO when you may not.
   
   
2. Unclear Gas Patterns
   Eating fermentable foods can cause gas to be released at unusual times during the test window, making the results difficult to interpret. It becomes hard to tell whether the gas is from SIBO or just leftover fermentation from the diet.
   
   
3. Test Invalidity or Lab Rejection
   Some labs may flag your test as inconclusive or invalid if the results look inconsistent or if gas levels start too high at the 0-minute mark.
   
   
4. You May Have to Repeat the Test
   To get accurate readings, your practitioner might recommend repeating the test — which means delaying treatment and paying for another kit.
   

Summary Table: SIBO Test Prep at a Glance

Sample Meal Plan: Day Before Your SIBO Test

This sample plan follows the low-fermentation diet required the day before your SIBO breath test. It avoids fibre, dairy, sugar, fruit, vegetables, and wholegrains.

Remember you can eat up until 12 hours before your test then begin fasting overnight.

Breakfast

• 2 boiled eggs ot Tofu if vegetarian/vegan
• 1 slice of white toast (no butter) or gluten free white toast
• Black tea or black coffee (no milk, no sweeteners)

Mid-Morning Snack

• Plain white rice (small portion) or 1 slice of toast and chicken
• Herbal-free black tea or water

Lunch

• Grilled chicken breast or turkey
• Plain white rice (boiled)
• Water

Afternoon Snack

• Hard boiled egg
• Slice of gluten-free white bread
• Black tea or water

Dinner (Last Meal Before 12-Hour Fast)

• White fish or grilled lamb teaspoon of olive-oil, salt
• Plain boiled white rice
• Black coffee or water

No fruits, vegetables, dairy, spices, legumes, or wholegrains.
No chewing gum, alcohol, or sweeteners (natural or artificial).

Frequently Asked Questions About SIBO Test Preparation

1. What should I avoid the day before a SIBO test?

Avoid high-fibre foods (like wholegrains, vegetables, fruit), dairy, beans, spices, sweeteners, and anything that could ferment in the gut. Only eat plain proteins and white carbohydrates.

2. Can I drink coffee or tea before the test?

You may drink black coffee or black/green tea without milk or sweeteners up until 12 hours before the test. During the fasting period and test itself, only water is allowed.

3. Can I brush my teeth on the morning of the SIBO test?

Yes but do not swallow any toothpaste, and avoid using mouthwash that contains alcohol or xylitol.

4. What happens if I accidentally eat the wrong food?

It’s best to postpone the test by at least 48 hours and restart the pre-test prep. Eating fermentable food may produce inaccurate breath readings and invalidate the test.

5. How long do I need to fast before the test

You must fast for at least 12 hours before the test usually overnight. Only plain water is allowed during this period and throughout the test itself.

6. Can I exercise or smoke on the day of the test?

No. Avoid exercise, smoking, and vaping for at least 1 hour before and during the test. These can affect gas readings and test reliability.

7. What Happens If You Don’t Fast Before a SIBO Test?

Food still being digested can feed the bacteria in your small intestine and colon, causing gas to be produced before the test begins. This can lead to an elevated baseline reading, which may result in:

• A false positive diagnosis
• Or confusion about whether SIBO is actually present

The SIBO test is designed to measure when gas production starts after a sugar solution is ingested (usually lactulose or glucose). If you haven’t fasted:

• Bacterial activity might already be in progress
• The test can’t accurately distinguish small bowel fermentation from colonic fermentation

Active digestion (triggered by recent meals) changes gut motility and gas dynamics. This disrupts the normal flow of the sugar solution and gas production, leading to skewed test curves and potentially invalid results.

If results are unclear or not interpretable, the lab or your practitioner may ask you to repeat the test, delaying treatment and increasing costs.

8. Can I Take Lactulose if I’m Lactose Intolerant?

Yes most people with lactose intolerance can safely take lactulose for a SIBO breath test. Although lactulose is a synthetic sugar derived from lactose, it does not contain significant amounts of lactose and does not require the enzyme lactase to digest. Instead, lactulose passes through the small intestine undigested, where it reaches bacteria that ferment it into gas — exactly what the test is designed to detect.

If you are lactose intolerant, lactulose is unlikely to trigger your symptoms. However, if you have a true milk allergy (to milk proteins such as casein or whey), consult your practitioner before proceeding.

9. Should I Follow the Prep Diet for 24 or 48 Hours?

Most guidelines recommend a 24-hour prep diet before the SIBO breath test. However, for people with slower digestion — such as those with constipation, gastroparesis, or a history of opioid use a longer 48-hour prep may provide more accurate results. This gives time for fermentable residues to clear the small intestine fully, reducing the risk of false positives from colonic gas.

Your practitioner may advise a longer prep if you have:

• A history of constipation or slow gut motility
• Taken proton pump inhibitors (PPIs) or antibiotics recently
• Ongoing bloating despite a low FODMAP diet

10. Can I do the SIBO test if I have a virus?

No, it’s best to wait until you’ve fully recovered. Viral infections especially those affecting the gut can temporarily change how your digestive system functions, leading to inaccurate breath test results.

You should wait at least 3–5 days after your symptoms have resolved and ensure you’re not taking any medications that affect gut motility before testing.

11. Can I follow the prep diet if I’m vegetarian or vegan?

Yes, but it requires some careful planning. The standard prep diet typically includes animal protein because it’s non-fermentable and doesn’t feed gut bacteria. However, if you follow a vegetarian or vegan diet, you can still prepare effectively:

Be sure to read labels carefully and avoid sweeteners, gums, or thickeners like inulin, chicory root, or guar gum

12: Should I Extend the ‘prep’ diet if I am constipated or taking PPIs?

In some cases, yes.

You may benefit from a longer prep if:

• You have slow digestion or suffer from chronic constipation
• You’ve recently taken antibiotics or probiotics
• You’re on medications like proton pump inhibitors (PPIs), such as omeprazole
• You typically eat a high-fibre or plant-based diet

If any of these apply to you, speak with your practitioner. They may recommend extending the prep diet to 36–48 hours or adjusting medications and supplements temporarily to help ensure more reliable test results.

Author – Victoria Tyler BSc Hons MBANT

Victoria Tyler owns and runs two busy clinics : Nutrition and Vitality and the IBS and Gut Disorder Clinic.

Nutrition and Vitality, along with the IBS and Gut Disorder Clinic, were founded with the goal of helping patients alleviate IBS symptoms by uncovering and addressing the root causes of their digestive issues.

As a Registered Nutritional Therapist, Victoria holds a BSc (Hons) in Nutritional Therapy and has trained with the Institute of Functional Medicine. She is also accredited by BANT and CNHC.

Before transitioning into health, Victoria earned a degree in Economics and an MBA, working with corporations including Canon and Vodafone. However, her own health challenges led her to pursue a career in Nutritional Therapy.

With a passion for learning, Victoria is committed to staying at the forefront of Functional Medicine. She helps patients manage IBS and other digestive disorders, including SIBO, Candida, and IBD, by identifying and addressing their root causes.

Victoria strongly believes that every symptom has an underlying cause, and there is always a solution to every health condition. To learn more, or to see patient reviews, visit Victoria’s profile on Google.

DISCLAIMER: The information provided in this article is intended for general informational purposes only and should not be construed as medical advice, diagnosis, or treatment. The products and methods mentioned are not a substitute for professional medical advice from a trained healthcare specialist. Always seek the guidance of your doctor or other qualified health professional with any questions you may have regarding your health or a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. Use of the information and products discussed is at your own risk.