Small intestine bacterial overgrowth (SIBO) is where microbes overgrow in your small intestine. There are a complex set of risk factors for SIBO. If you’ve got SIBO it’s more than likely down to a combination of reasons, rarely just one cause.

Let’s take a look at how a breakdown in your normal digestive processes can result in bacteria colonising your small intestine. The main risks for SIBO are:

  1. Bacteria build up because it’s not being cleared from the small intestine properly (e.g. slow movement)
  2. Bacteria aren’t being killed by our defensive mechanisms (e.g. stomach acid). 

There are various ways these risk factors can develop.

Slow digestion can result in SIBO

If anything slows down the passage of food through your small intestine it can be a risk factor for an overgrowth of bacteria. Things that could slow down your small intestine include:

  • Partial obstruction – this could be because of adhesions which stick the small intestine to other nearby organs slowing the transit of food and bacteria. Other causes include strictures, which are a narrowing of the small intestinal passage, tumours, or anything else blocking the intestine. 
  • Small intestine diverticula – sometimes called blind loops, these are mini bulges in the small intestine which could collect pockets of food or bacteria. 
  • Abdominal surgery or injuries

The small intestine should be a one way corridor, with no movement back upwards. This helps to keep your intestines clear of bacteria. Sometimes the ileocecal valve, which is the link from the small intestine to the large intestine, can stop working as well as it should. This means there is an opportunity for bacteria to creep from the large intestine microbiome into the small intestine. 

close up of a white person's arms holding their stomach, wearing a grey vest. Yellow banner with the words Risk Factors for SIBO across middle

Your Migrating Motor Complex is a protection against SIBO

When we have finished the meal, we need to clean our kitchens. Did you know your gut has it’s own cleaner too? 

Let’s get to know your migrating motor complex (MMC). The MMC is a quick impulse that moves through the first two thirds of the small intestine whenever we haven’t eaten for around 90 minutes. It will stop for around 1.5-2 hours whenever we eat.

The MMC takes around 90 minutes to clean out the small intestine, and it helps remove old bits of food, bacteria and dead cells. If this cleaning mechanism isn’t working well, you can get an overgrowth of bacteria.

How the MMC becomes damaged

One of the most common reasons for damage to the MMC is from a prior episode of food poisoning. If you eat a dodgy kebab, or a seafood dish that was on the turn you need to flush the pathogens out of your digestive system.

You make antibodies to the invading pathogen that’s causing sickness and diarrhoea your body, but unfortunately the molecules can be very similar to those that control the MMC, which can leave you with damage to the cleaning process.

Read more about the Migrating Motor Complex and how it can become damaged.

This is is one of the reasons not eating too late at night is often helpful for people with digestive issues. When you have a 12-14 hour fast overnight your MMC can do its cleaning work multiple times during the night.  And the best part of that is you’re asleep for most of the time, so you shouldn’t notice not eating. 

Just a quick note – the speed of your food moving through your gut doesn’t change your MMC, so if your MMC is not working well, that doesn’t mean you’ll be constipated.

And in the same way the MMC doesn’t cause a bowel movement.  You can have a slow MMC, but have diarrhoea or a fast transit of food. 

Low stomach acid could be a risk factor for SIBO

One of the main purposes of your stomach acid is to kill off bacteria as they come in on your food. If you don’t have enough stomach acid you may be exposed to more pathogenic bacteria, which can move into the small intestine.

This could be worse if you have taken an acid suppressing medication for a long time (say longer than 6 months). Examples of this are a Proton Pump Inhibitor (PPI) or other Acid blocking medication such as a H2 (Histamine) blocker. It’s been shown that decreased stomach acid can lead to more bacteria in the stomach and the duodenum, the upper part of the small intestine. 

Taking probiotics may help reduce the effects of taking a PPI. In particular, certain probiotic strains called Lactobacillus reuteri 17938 and a mixed Lactobacillus may help prevent SIBO when taking a PPI.

You could also consider prokinetic supplements. Prokinetics can help keep the MMC moving if you have low stomach acid, and reduce the risk for SIBO. There are medicines which do this, but also herbal supplements such as ginger.

if you’re very stressed then you can produce less stomach acid. When you are in a state of fight or flight your body doesn’t prioritize digestion. Reduced blood flow to the digestive system means you could end up with reduced stomach acid.

white woman with mid length brown hair wearing light blue jeans and a light grey long sleeved t shirt sitting on grey sofa holding her stomach and looking down.

Poor bile and digestive enzymes are a risk factor for SIBO

We secrete digestive bile and enzymes along the small intestine which help breakdown food we eat into nutrients which can be absorbed. 

Digestive enzymes

Some of the digestive enzymes we produce when we eat have an antibacterial effect. If you don’t produce enough proteolytic enzymes it could lead to higher levels of pathogenic microbes growing in your small intestine. 

Low stomach acid can reduce digestive enzyme production. 

Bile and SIBO

Your liver releases bile when you eat, and one of it’s main purposes is to help absorb dietary fats. The other important function of bile is to help protect the small intestine from invading microbes

There is some evidence from mouse studies that closing the bile duct caused an greater than 10-fold increase in aerobic bacteria and a doubling of anaerobic bacteria in the small intestine. Other studies show that bile and unconjugated bile acids inhibit bacterial growth, so reduced bile may contribute to bacterial growth. 

Bile acid production could be reduced by liver damage, for example through high alcohol intake, or gallstones.

SIBO is a complicated condition, with multiple causes, and you need to better understand how to address these risk factors to help resolve your symptoms.  I recommend working with a nutritionist who has trained in SIBO for best results. 

Hi I'm Anna Mapson, registered Nutritional Therapist.

I help people with IBS and SIBO get control of unpredictable gut symptoms to find long term relief from painful and embarrassing IBS without restrictive dieting.

I can help you to:

  • understand your digestion better, so you recognise your triggers
  • eat a well balanced diet, with tasty meals that are simple to prepare
  • develop better digestion and more energy

Find more about my 3 month 1:1 Gut Reset programme