Got IBS? Why It Might Really Be Small Intestinal Bacterial Overgrowth (SIBO)

Maria Cross
6 min readAug 8, 2023

What happens when bacteria colonise your small intestine, where they don’t belong


At about 20 feet (7 metres) long, the small intestine is really not that small at all. Or insignificant: it is the industrial hub of food digestion and absorption. With so much metabolic activity going on, the potential for mayhem is immense.

Irritable bowel syndrome is defined as a common disorder, with symptoms that include cramping, bloating, excessive gas, constipation and diarrhoea.

But the truth is that IBS is not so much a disorder as an umbrella term. It means: there’s something wrong with your gut, but we don’t know what. All your tests are negative, but you’ve still got all the symptoms. It’s a mystery.

Or is it SIBO?

Know your nemesis

Your colon is teeming with life, an invisible city of microorganisms that thrive on the remnants of your meals. But the small intestine is quite another matter. It has relatively few microbes; it’s not sterile, but it’s a pretty rural location, compared to the colon.

Food in the small intestine is broken down into particles small enough to pass through the gut lining and enter the bloodstream. Carbohydrates are reduced to simple sugars, such as glucose. Proteins become amino acids, fat is reduced to fatty acids and glycerol.

With SIBO, abnormally large colonies of microbes settle and breed in the intestines, where they don’t belong. The food supply is excellent, and the microbes feed themselves by fermenting the carbohydrates that are passing through, producing gasses in the process. They also steal your nutrients before they can be absorbed into the blood.

The symptoms you are most likely to experience with SIBO are:

  • Bloating
  • Pain
  • Constipation
  • Diarrhoea
  • Bad breath
  • Burping
  • Gurgling noises
  • Indigestion soon after eating
  • Feeling overly full after eating



Maria Cross

MSc. Registered nutritionist, specialising in gut and mental health. OUT NOW! My new book, How to Feed Your Brain. mariacrossnutrition @mariacross