What to Eat Before a SIBO Test and What to Avoid

The day before a SIBO breath test, you should eat only simple, low-residue foods that leave minimal undigested material in your gut: plain white rice, baked or broiled chicken or fish, eggs, and white bread are the standard options. You need to avoid anything high in fiber, fermentable carbohydrates, or complex sugars, because leftover residue from those foods can feed gut bacteria overnight and produce gas that throws off your baseline readings. After that restricted day, you fast for at least 12 hours before the test, drinking only plain water. Getting the prep right matters more than most people realize, and the details go beyond just skipping breakfast.

How the Test Works and Why Food Matters

A SIBO breath test measures hydrogen and methane in your exhaled breath after you drink a sugar solution, typically glucose or lactulose. The idea is straightforward: human cells cannot produce hydrogen or methane gas. If those gases show up in your breath after you swallow the test sugar, they came from bacteria fermenting that sugar somewhere in your digestive tract.¹ Bacteria in the small intestine (where they are not supposed to be in large numbers) ferment the sugar quickly, producing a measurable rise in gas that shows up earlier and higher than gas from normal colon bacteria.

The problem is that leftover food in your gut can also feed bacteria and produce gas before the test even begins. If you eat a bean-heavy dinner the night before, bacteria will be fermenting that fiber all night. By morning, your baseline hydrogen level could already be elevated, making it impossible to distinguish test-related gas from leftover food-related gas. The entire diagnostic logic depends on starting from a clean, low baseline and watching for a rise. Anything that muddies the baseline muddies the result.

What to Eat the Day Before

Most testing facilities provide a list of approved foods for the 24 hours before your test. The common thread is simplicity: foods that are absorbed high in the digestive tract, leave little residue, and contain minimal fermentable material. The goal is a quiet gut by the time you start fasting that evening.

Safe choices generally include:

Plain white rice: one of the most universally recommended prep foods because it is almost completely absorbed in the upper small intestine and leaves very little for bacteria to work with.
Baked or broiled chicken, turkey, or fish: plain, unseasoned or lightly salted. Avoid marinades, breading, or sauces that contain sugar, garlic, or onion.
Eggs: scrambled, boiled, or poached, cooked in a small amount of butter or oil.
White bread: plain, without added fiber or whole grains. Some clinics also allow plain saltine crackers.
Clear broth: chicken or beef broth without vegetables, noodles, or added seasonings beyond salt.
Plain water, black coffee, or plain tea: no milk, cream, sweeteners, or flavored varieties.

The pattern is obvious once you see it: everything is bland, white or light-colored, low in fiber, and free of complex sugars. You are not trying to eat well or balanced on this day. You are trying to eat boring on purpose, so your gut has as little to ferment as possible by morning.

What to Avoid and Why Each Item Causes Problems

The restricted list is longer than the approved list, and each category has a specific reason behind it.

High-fiber foods: whole grains, brown rice, oats, bran, and anything made with whole wheat. Fiber passes through the small intestine undigested and feeds bacteria in the colon, which raises your baseline gas levels.
Beans, lentils, and legumes: these are among the most fermentable foods you can eat. Even a small serving can generate substantial gas production overnight.
Vegetables, especially cruciferous ones: broccoli, cauliflower, cabbage, Brussels sprouts, onions, garlic, asparagus, and artichokes all contain complex carbohydrates that bacteria readily ferment. Most prep guidelines exclude all vegetables entirely for the day before, not just the gassy ones.
Fruits: contain fructose and sorbitol, both of which are fermentable sugars. Even low-sugar fruits can contribute enough substrate to shift baseline readings.
Dairy products: milk, yogurt, ice cream, and soft cheeses contain lactose. If you have any degree of lactose malabsorption (which is common and often undiagnosed), undigested lactose will feed bacteria and produce hydrogen. Hard aged cheeses and butter are sometimes permitted because their lactose content is negligible.
Sugar, honey, corn syrup, and artificial sweeteners: sugar alcohols like sorbitol, xylitol, and mannitol are especially problematic because they are poorly absorbed and highly fermentable. Regular sugar in large amounts can also be an issue.
Alcohol: can alter gut motility and affect bacterial metabolism. Most guidelines say no alcohol for at least 24 hours before the test.
Chewing gum and candy: many contain sugar alcohols, and the act of chewing gum can cause you to swallow air, which may affect gas measurements.
Sauces, condiments, and processed foods: ketchup, barbecue sauce, salad dressings, and many processed foods contain hidden sugars, high-fructose corn syrup, or fiber additives that can generate fermentation.

The general rule is: if you are unsure about a specific food, skip it. One day of eating plain rice and chicken is not going to harm you, and the cost of an inaccurate test (repeating it, potentially getting misdiagnosed, or missing a real diagnosis) is much higher than the cost of a boring dinner.

The Overnight Fast and the Morning of the Test

After your last restricted-diet meal, you begin fasting. Most guidelines call for a minimum of 12 hours with nothing but water before the test. Some clinics ask for 8 to 12 hours; follow whatever your specific provider instructs, but 12 hours is the most common recommendation. This fasting period allows your gut to clear residual food and gives bacterial gas production time to settle down to baseline.

On the morning of the test, drink only small sips of plain water. No coffee, no tea, no juice, no flavored water, no supplements, no medications unless your doctor specifically told you to take them. Brushing your teeth is fine, but avoid mouthwash that contains alcohol or sweeteners, since bacteria in your mouth can also ferment sugars and contribute to early breath hydrogen readings. Some clinics ask you to rinse your mouth with an antiseptic mouthwash right before the test to reduce oral bacterial fermentation, but that is their step to administer, not yours to do at home with a random mouthwash.

Avoid vigorous exercise the morning of the test. A hard workout increases your breathing rate and can alter how gases exchange in your lungs, potentially affecting measurements. A gentle walk to the clinic is fine, but skip your morning run or gym session.²

Medications and Supplements That Can Interfere

Diet is only one part of preparation. Several medications and supplements can alter breath test results, and some need to be stopped well in advance.

Antibiotics are the biggest concern. The North American Consensus on hydrogen and methane breath testing recommends avoiding antibiotics for at least four weeks before the test.³ Antibiotics suppress bacterial populations indiscriminately, so if you have just finished a course, the bacteria in your small intestine may be temporarily reduced regardless of whether you actually have SIBO. Testing too soon after antibiotics can produce a false negative, meaning the test says you are fine when you are not.

Probiotics and prebiotics should also be stopped before the test, typically for at least a week and ideally longer. Probiotics introduce live bacteria, and prebiotics feed them. Both can change the composition and activity of your gut microbiome enough to alter gas production patterns.²

Prokinetics (medications that speed up gut motility) and laxatives can also interfere. These change how quickly food and test substrates move through your digestive tract, which affects when and where bacteria encounter the sugar solution. Since the timing of gas production is a key part of interpreting the test, anything that alters transit speed can shift the results. Most guidelines recommend stopping laxatives and prokinetics at least a week before testing.

Proton pump inhibitors (PPIs) present a more nuanced situation. Long-term PPI use has been associated with higher rates of positive breath tests in some studies, suggesting that stomach acid suppression may itself promote bacterial overgrowth.⁴ Whether to stop a PPI before testing depends on why you are taking it and what your doctor wants to learn. If the question is “do I have SIBO,” and you take a PPI daily, your doctor might want to test you while on it, since that reflects your real-world gut environment. If the question is “did the PPI cause my SIBO,” stopping it first might make more sense. This is a conversation to have with your gastroenterologist, not a decision to make on your own.

Smoking Before the Test

Do not smoke on the morning of the test, and ideally not the evening before either. Smoking introduces carbon monoxide and other gases into your lungs, and it can also affect gut motility. Consensus guidelines list smoking among the factors that can alter baseline gas concentrations.² This applies to cigarettes, vapes, and cannabis. If you smoke regularly and cannot abstain for the full prep period, at minimum do not smoke the morning of the test before or during the breath collection.

What Happens If You Do Not Follow the Prep

If you eat something off the approved list or break your fast, your test may still go ahead, but the results become harder to trust. The most common consequence of poor preparation is an elevated baseline hydrogen reading. When your first breath sample (before you even drink the test solution) already shows high hydrogen, the lab cannot reliably distinguish the rise caused by SIBO from the rise caused by last night’s dinner. Some clinics will cancel the test outright if your baseline is above a certain threshold, which means you wasted a trip and have to reschedule and repeat the prep.

Even if the test proceeds, a messy baseline can produce a false positive (the test says you have SIBO when you do not) or a false negative (elevated baseline masks a real but modest rise in gas). The prep diet exists specifically to reduce these avoidable errors.² Breath testing already has recognized limitations in terms of sensitivity and specificity, and the variability of how quickly food passes through different people’s guts adds another layer of uncertainty.⁵ Sloppy prep makes an imperfect test even less reliable.

If you realize you accidentally ate something you should not have, call your clinic before going in. They may advise you to come anyway, or they may save everyone time by rescheduling. Honesty matters here, because if the clinician does not know your prep was imperfect, they will interpret your results at face value, and that could lead to unnecessary antibiotic treatment or a missed diagnosis.

Common Mistakes People Make During Prep

Beyond the obvious errors of eating the wrong foods or forgetting to fast, several subtler mistakes trip people up.

Flavored water and herbal tea are frequent culprits. Many flavored waters contain sucralose, stevia, or fruit extracts. Herbal teas often contain ingredients like chicory root (a prebiotic) or licorice that can affect digestion. Stick to plain water and, if your clinic permits it, plain black coffee or unflavored black or green tea during the prep day only (not during the fasting window).

Cooking oils and butter in normal amounts are generally fine, but heavy sauces, gravies, or fried foods should be avoided. The issue is not the fat itself but the other ingredients that come along for the ride: flour-based roux, sugar in sauces, onion and garlic in virtually everything. When in doubt, keep it plain.

Some people assume that because they are fasting, they can take their morning supplements as long as they do not eat food. Many supplements contain prebiotics, fiber, or fermentable ingredients as fillers. Vitamin gummies are essentially candy. Fiber supplements are obviously off the table. Even capsule-form supplements may contain inulin or other prebiotic fibers as inactive ingredients. Unless your doctor says to take a specific supplement the morning of the test, skip them all.

Timing is another common stumble. If your test is at 8 a.m. and you need a 12-hour fast, your last food should be at 8 p.m. the night before at the latest. People who eat dinner late or snack after dinner sometimes end up with an insufficient fasting window. Set an alarm if you need to, and plan your restricted-diet dinner early enough to give yourself the full 12 hours.

When Your Clinic’s Instructions Differ From General Advice

Different clinics and different gastroenterologists sometimes give slightly different prep instructions. One might say a 12-hour fast; another says 8 hours. One might allow plain potatoes on the prep day; another might restrict all starchy foods except white rice and white bread. Some clinics provide a printed prep sheet; others rely on verbal instructions or a generic handout.

Always follow your specific clinic’s instructions over general advice, including what you read here. Clinics sometimes adjust their protocols based on the substrate they use (glucose and lactulose have somewhat different prep requirements), the breath-testing device they use, or the patient population they typically serve. If your clinic says something that contradicts a general guideline, ask them why. There may be a good reason, or it may be a chance to catch an outdated instruction. Either way, the conversation helps.

If your clinic did not give you any prep instructions at all, that is a red flag worth addressing. The consensus view across gastroenterology guidelines is that pretest preparation is essential, and that failing to control for diet, medications, smoking, and exercise can alter baseline gas levels enough to compromise the test.² A clinic that skips prep instructions entirely may not be following current best practices for breath testing, and you may want to ask about their protocol before showing up.

Glucose Versus Lactulose and Whether Prep Differs

The two sugars used in SIBO breath testing behave differently once you drink them. Glucose is absorbed in the upper small intestine, so it only detects bacteria living in the first stretch of your small bowel. Lactulose is not absorbed at all and travels the entire length of the small intestine before reaching the colon, so it can theoretically detect overgrowth further down. Each has trade-offs in terms of how accurately it identifies SIBO, and researchers continue to debate which is preferable.

From a prep standpoint, the diet restrictions are essentially the same for both substrates. You are controlling the same variable (baseline fermentation) regardless of which sugar the test uses. The 12-hour fast, the low-residue diet, and the medication and supplement restrictions apply equally. The only practical difference is that lactulose will eventually reach your colon and be fermented by normal bacteria there, producing a late rise in gas that is expected and does not indicate SIBO. Your clinician handles that interpretive distinction; your job is the same either way.