SIBO: when your gut bloats after almost everything

You eat a light lunch. By mid-afternoon your stomach looks six months pregnant. Some days it's diarrhoea. Other days nothing moves at all. You've cut out gluten, then dairy, then onions. It helps a little. Then it doesn't.

If that sounds familiar, you've probably come across the word SIBO. Here is what it is, what the latest research says, and why clearing it is only half the job.

What SIBO is

SIBO stands for small intestinal bacterial overgrowth.

Most of your gut bacteria live in the large intestine. The small intestine is meant to have far fewer. Its job is to absorb nutrients, and it has ways of keeping bacteria in check: stomach acid, bile, digestive enzymes, and a wave of muscle movement between meals that sweeps the small intestine clean.

When those defences weaken, bacteria that belong further down can settle higher up. They ferment food before you've had the chance to absorb it. That produces gas, and the gas produces symptoms.

SIBO is not an infection. These are ordinary gut bacteria in the wrong place, or in the wrong numbers.

Three types of gas, three patterns

Doctors now describe SIBO by the gas the microbes make.

Hydrogen. The classic type. Often linked to bloating and loose stools.
Methane. Made by archaea, not bacteria, so it now has its own name: intestinal methanogen overgrowth (IMO). Methane slows the gut down, so constipation is common.
Hydrogen sulphide. The newest type to be described. It can only be measured with a few specialised breath tests.

The type matters, because it changes what tends to help.

How it can feel

Bloating that builds through the day
Wind, belching, rumbling
Diarrhoea, constipation, or both
Pain or cramping in the stomach
Tiredness and brain fog

Many people with irritable bowel syndrome (IBS) have SIBO. Not everyone with IBS does. More than thirty conditions can cause IBS-type symptoms, so it's worth ruling out the others before settling on SIBO.

What can lead to SIBO

It's rarely random. These are the most common starting points.

A stomach bug or food poisoning. Some bacterial toxins can trigger an immune reaction that affects the nerves which move the gut.
Slow movement through the gut, for example with an underactive thyroid, diabetes, or endometriosis.
Less stomach acid, for example after long-term use of acid-lowering medicines (PPIs).
Previous gut surgery or adhesions.
Low levels of bile or digestive enzymes.

Finding the cause matters as much as treating the overgrowth. It's the main reason SIBO comes back.

Why bloating is more common than it used to be

A review published this month looked at why bloating has become one of the most common gut complaints of our time.

Diet and stress play a part, but they don't explain everything. The authors describe a gut that has become more sensitive. The gut lining lets more through, low-grade inflammation is more common, and the connection between the gut and the brain is under strain.

They also raise a newer question: whether everyday exposures such as air pollution, microplastics and hormone-disrupting chemicals make the gut less resilient. That's still a hypothesis. There's very little direct evidence in people yet. It is one reason we look at the whole picture, not only the bacteria.

How SIBO is tested

Breath test. This is the most common test and can often be done at home.

You follow a simple diet the day before.
You drink a sugar solution, usually lactulose or glucose.
You breathe into tubes every 15–20 minutes for two to three hours.

Bacteria ferment the sugar and release hydrogen and methane, which show up in your breath.

A breath test isn't perfect. The result depends on which sugar is used, how fast your gut moves, and how carefully you prepared. It can miss SIBO, and it can suggest SIBO when there isn't any. It should always be read alongside your symptoms and history.

Blood tests can give clues. Low vitamin B12, low iron, or folate that is high without supplements can all point towards bacteria using nutrients before you do. Low vitamin D and other fat-soluble vitamins can suggest fat isn't being absorbed well.

Antibody tests (anti-CdtB and anti-vinculin) may help show whether a past stomach bug is the likely trigger.

Treatment: what the newest review found

A systematic review published in September 2026 brought together 39 studies of antibiotic treatment for SIBO and methane overgrowth.

No single antibiotic was best for everyone.

Rifaximin was the most studied. It gave the best balance of symptom relief and few side effects.
Metronidazole normalised the breath test most often in the one study that compared three antibiotics directly. It helped pain and bloating less, and caused more side effects.
For methane overgrowth, rifaximin combined with neomycin worked best. In one study, 85% of people felt better with the combination, compared with 56% on rifaximin alone.

Feeling better and a normal breath test don't always go together. In 11 of the 27 studies that measured both, the two didn't match. Most often people felt better even though their breath test wasn't normal yet. The authors conclude that success should be judged on how you feel, how well you tolerate treatment and whether it comes back, not on the breath test alone.

Beyond antibiotics

The same review also looked at what else has been studied.

Partially hydrolysed guar gum (PHGG), a gentle soluble fibre, taken with rifaximin raised breath-test normalisation from 62% to 85%.
Herbal formulas. In one study, herbal formulas did as well as rifaximin. More recently, berberine was found to be no worse than rifaximin in an early trial. Both results need confirming in larger studies.
Saccharomyces boulardii, a yeast probiotic, improved results in one small study when added to antibiotics.
An elemental diet, a liquid diet of pre-digested nutrients for two to three weeks, reduced overgrowth in a 2025 trial. It is demanding, and is usually kept for people who haven't improved in other ways.

Diet matters too. A lower-fermentation diet, similar to low FODMAP, can ease symptoms while you treat the cause. It works best as a short-term tool. Kept up for months, it can starve the bacteria you want in the large intestine.

Always talk to your doctor before starting antibiotics, herbs or a restrictive diet.

Why SIBO comes back: look after the garden

Think of your gut as a garden. If a plant keeps getting sick, you don't just pull it up and plant a new one. You ask why. Is it the soil, the water, the light?

SIBO works the same way. Antibiotics or herbs can clear the overgrowth, but if the conditions that let it grow are still there, it often comes back. In one study, 44% of people had SIBO again within nine months of successful treatment. Being older, long-term use of acid-lowering medicine and having had your appendix removed all raised the risk.

So treatment has two parts. First clear the overgrowth, then fix the environment that let it grow. Three things matter most.

1. How your gut moves

Between meals, a wave of muscle contractions sweeps through the small intestine roughly every one and a half to two hours. It is the gut's cleaning wave, and it carries bacteria and leftovers down to the large intestine. It only runs when you're not eating, so constant snacking keeps interrupting it.

Leave three to four hours between meals, and a longer break overnight.
Treat what slows the gut down, such as an underactive thyroid or high blood sugar.
After treatment, some doctors prescribe a low dose of a medicine that stimulates gut movement, to lower the risk of relapse.

2. Stomach acid

Stomach acid is your first filter. It kills many of the bacteria that arrive with food before they reach the small intestine. With less acid, more of them get through. Long-term acid-lowering medicine is the most common reason, but a long-standing H. pylori infection and ageing can also lower acid.

If you've taken acid-lowering medicine for a long time, ask your doctor whether you still need it. Don't stop on your own, as heartburn often comes back hard at first.
Eat slowly and chew well, so digestion gets a proper start.
If you have ongoing stomach problems, ask about a test for H. pylori.

3. Your nervous system

Digestion is run by your nervous system, largely through the vagus nerve. Under stress, the body switches to fight-or-flight and puts digestion on hold. Less acid and fewer enzymes are released, and the gut moves differently. Long-term stress can keep the gut in that state.

Eat sitting down, without a screen, rather than on the run.
Take a few slow breaths before a meal to help your body shift into rest-and-digest mode.
Protect your sleep, and find regular ways to switch off, such as walks, yoga or breathing exercises.

And then

Reintroduce a wide variety of foods once symptoms settle, so the bacteria in your large intestine get fed again.
Recheck nutrients such as B12, iron and vitamin D.

How Wellth can help

At Wellth we look at the whole picture: your symptoms, your history, your blood markers and how you live.

We can help you understand a breath test you've already done, check the nutrients SIBO often affects, and build a plan that addresses the cause, not only the symptoms.

If you've been bloated for months and nobody has found out why, start with your numbers. Then we'll work out the next step together.

We're here to help. If you'd like to talk about your gut, your symptoms or your results, get in touch with us.

Sources

Koller A, Săsăran M, Lupu A, Mărginean C. Antibiotic therapy for small intestinal bacterial overgrowth: a systematic review of comparative efficacy, clinical–test discordance, and pediatric evidence gaps. Frontiers in Pharmacology, 25 September 2026. PMC13601715.
Piccioni A, Di Bari F, Candelli M, et al. Abdominal Bloating in the Modern Era: Pathophysiological Mechanisms and Environmental Contributors. Biomolecules 2026;16(9):1359. doi:10.3390/biom16091359
Pimentel M, Saad RJ, Long MD, Rao SSC. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. Am J Gastroenterol 2020;115:165–178.
Rezaie A, et al. Effect, tolerability, and safety of exclusive palatable elemental diet in patients with intestinal microbial overgrowth. Clin Gastroenterol Hepatol 2025;23:2306–2317.
Lauritano EC, Gabrielli M, Scarpellini E, et al. Small intestinal bacterial overgrowth recurrence after antibiotic therapy. Am J Gastroenterol 2008;103:2031–2035.