SIBO vs IBS: How to Tell the Difference

If you have bloating, abdominal pain, excessive wind or changes in your bowel habits, you might have been told you have irritable bowel syndrome (IBS). But could small intestinal bacterial overgrowth (SIBO) be responsible for your gut symptoms instead?

The difficulty is that IBS and SIBO can look and feel pretty similar, causing very uncomfortable digestive symptoms, and trouble digesting food.

Understanding SIBO can help you work out what investigations might be useful and what steps you can take to improve your digestion.

What is SIBO and how does it differ from IBS?

What is IBS?

Irritable bowel syndrome (IBS) is a disorder of the gut-brain connection, characterised by recurring abdominal pain and changes in bowel habits, including constipation, diarrhoea or a mixture of both.

It involves changes in how the gut functions, including gut sensitivity, movement of food through the digestive system and communication between the gut and brain.

IBS is usually diagnosed using the Rome IV criteria, alongside appropriate investigations to rule out other conditions. Symptoms need to have been present over a period of time rather than being a short-lived episode of constipation or diarrhoea.

What is SIBO?

Small intestinal bacterial overgrowth (SIBO) happens when there are too many microbes living in the small intestine.

Although we naturally have some microbes in this part of the digestive system, an overgrowth can cause excessive fermentation when food arrives from the stomach. The microbes produce gas, which can lead to bloating, abdominal discomfort and changes in bowel movements.

Normally, stomach acid, bile and the migrating motor complex (MMC) help keep small intestine bacteria under control. The MMC is a wave of movement that sweeps through the small intestine between meals, clearing away food debris, dead cells and excess bacteria.

When these protective processes are disrupted, for example following food poisoning or through certain medications, the risk of SIBO can increase.

What symptoms do SIBO and IBS share?

There is a real overlap between IBS and SIBO symptoms which is why you can’t tell which condition you have simply by looking at how your digestion feels.

Bloating

Both conditions can cause a swollen, stretched or uncomfortable feeling in your abdomen. With SIBO, excess fermentation in the small intestine produces gas, which contributes to bloating. IBS can also involve changes in gut movement and sensitivity, and gas production in the large intestine.

Abdominal pain or discomfort

You might experience cramping, pressure, aching or sharp pains in both conditions, related to the amount of gas, or sensitivity.

Wind and gas

Excessive burping or passing wind can occur with either condition. Fermentation by gut microbes produces gas, and this can happen in both the small and large intestine. Constant daily burping can be one sign of SIBO.

Changes in bowel habits

Constipation, diarrhoea or alternating between the two can happen with IBS and SIBO. Changes in gut motility can influence how quickly food moves through your digestive system and how much water is absorbed from your stools.

Food sensitivities

You might notice that certain foods make your symptoms worse. Fermentable carbohydrates can increase gas production, particularly when there is bacterial overgrowth. People with IBS can also be sensitive to particular fermentable carbohydrates, which is why a low FODMAP diet can sometimes help manage symptoms.

In SIBO some people experience issues with fat digestion leading to greasy stools, steatorrhoea and weight loss.

What symptoms are more likely to point to SIBO than IBS?

Although symptoms overlap, there are some patterns and background factors that might make SIBO worth investigating.

Bloating straight after eating

If you regularly experience increasing abdominal distension and bloating as soon as you eat anything, this might indicate SIBO. The fermentation of foods within 30-60 mins after a meal might be down to excessive microbes in the small intestine. Bloating is a big issue for almost everyone I work with who has SIBO.

Symptoms that flare unrelated to stress or anxiety

IBS symptoms can be influenced by the gut-brain connection, including stress and emotional responses. In SIBO you could find your digestive issues are completely unrelated to how you feel, and even when you’re super relaxed the bloating is out of control.

Reactions to foods high in fermentable fibres

If foods containing fermentable carbohydrates, such as lentils, mushrooms or certain fruits, cause significant bloating and discomfort, fermentation could be playing a role. This can happen in both IBS and SIBO, but the symptoms could arrive sooner in SIBO.

Nutritional gaps without an obvious cause

SIBO can interfere with normal digestion and nutrient absorption, and hydrogen dominant SIBO has been linked to B12 deficiency and high methane with low iron levels. If you have unexplained nutritional deficiencies alongside ongoing digestive symptoms, or unintentional weight loss this is worth discussing with your healthcare professional.

What is the difference between hydrogen and methane SIBO symptoms?

SIBO breath tests measure gases produced by microbes in your digestive system. The three main gases measured are hydrogen, hydrogen sulphide and methane.

These gases are produced when microbes ferment carbohydrates. The gas enters your bloodstream and travels to your lungs, where they are breathed out. A breath test measures how much of each gas is present and how levels change after you drink a test solution, usually containing lactulose.

Hydrogen: Excess hydrogen gas in the small intestine is typically associated with loose stools, urgency, abdominal pain and bloating. Its presence on a breath test can help identify patterns of bacterial fermentation in the small intestine. However, the amount of hydrogen measured doesn’t tell you exactly how severe your symptoms will be.

Methane: Methane is particularly interesting because higher levels can be associated with slower gut movement. This can contribute to constipation, as food and waste move more slowly through the digestive system. When methane production is associated with constipation, the condition is often referred to as intestinal methanogen overgrowth (IMO).

Hydrogen sulphide: This third type of SIBO is harder to test for in the UK. Often associated with an eggy smell to the wind, Hydrogen sulphide gases can also be linked to abdominal pains, feeling very ill, diarrhoea and urgency, but also sometimes constipation. It’s also sometimes called Intestinal Sulphide Overproduction as the gas can also be made in the large intestine, and unlike methane or hydrogen we do make some hydrogen sulphide naturally in the body.

It’s important to remember that symptoms alone cannot tell you which gas is present. A breath test, interpreted alongside your symptoms and wider health history, can help build a more complete picture.

How is SIBO confirmed if it cannot be seen on a scan?

Unlike some digestive conditions, SIBO cannot usually be identified through a standard scan or endoscopy. Instead, the most commonly used investigation is a breath test. Tests can be done at home, or in a hospital setting.

You’ll drink a sugary solution, usually containing lactulose or glucose, and then breathe into collection bags or phials at regular intervals, typically every 15–20 minutes. The test measures levels of hydrogen and methane in your breath over two to three hours.

Breath testing isn’t foolproof, and false positive and negative results can occur. It’s also important to interpret the results with someone who really understands SIBO alongside your symptoms and health history, rather than looking at the numbers in isolation.

Can someone have both SIBO and IBS at the same time?

Yes. It’s possible to have both IBS and SIBO, which can make understanding your digestive symptoms more complicated.

Research suggests that around a third of people with IBS may also have SIBO, although estimates vary considerably between studies. This means that some people diagnosed with IBS may have bacterial overgrowth contributing to their symptoms.

However, having IBS doesn’t automatically mean you have SIBO, and I wouldn’t recommend rushing into testing as soon as you’re diagnosed with IBS.

If you’ve been managing IBS for some time but your symptoms aren’t improving despite trying different approaches, it may be worth discussing whether SIBO testing is appropriate.

It’s also important to remember that treating SIBO doesn’t necessarily resolve all your digestive symptom. You may still have underlying gut sensitivity or other dietary factors associated with IBS that need attention. Read more about why you might not feel better after SIBO treatment.

What should you do if you are not sure whether you have SIBO or IBS?

If you’re struggling to work out what’s behind your digestive symptoms, try not to get caught up in diagnosing yourself through online symptom lists. IBS and SIBO have so much overlap that it’s difficult to distinguish between them without looking at your individual circumstances, and where relevant testing.

Start by speaking to your GP about your symptoms and whether further investigations, including SIBO breath testing, might be appropriate.

At Goodness Me Nutrition, I work with people who have IBS and SIBO. I take a thorough health history and look carefully at your current diet, eating habits, symptoms and wider health before making recommendations for further testing.

If you’re wondering whether I can help with your digestive issues please book a discovery call to tell me about what you need help with and then I can explain more about how I work.

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