What is SIBO?

What Exactly Is Small Intestine Bacterial Overgrowth?

Put simply, Small Intestine Bacterial Overgrowth (SIBO) is an overgrowth of normal bacteria in the small intestine. It is not pathogenic bacteria, but regular bacteria that have either entered via the top of the digestive system (mouth and nose for example) or have migrated up from the bottom of the digestive system; the large intestine.

Why is this problematic? The small intestine is not supposed to have large numbers of bacteria living in it because the purpose of the small intestine is to absorb the nutrients from our food, not have them broken down by bacteria. When the bacteria overgrow in the small intestine, the normal balance is disrupted and many people can experience painful symptoms.

SIBO is affecting millions of people around the world, but is still a relatively unknown condition and often goes undiagnosed for years, if not decades. A study published in the American Journal of Gastroenterology in 2003 showed that 84% of Irritable Bowel Syndrome (IBS) patients had SIBO. Leading SIBO researcher and gastroenterologist, Dr Mark Pimentel from Cedars Sinai in Los Angeles says ‘most of IBS is actually SIBO’.

Up to 20% of Americans and 1 in 5 Australians have Irritable Bowel Syndrome. IBS is the most common digestive complaint in the world. Many of them will have it because of SIBO.

Where Does It Live?

The Human Digestive System

The Human Digestive System

The human digestive system is one long, continuous tube, divided into sections, that starts at the mouth and ends at the anus. It supports the complex process of digesting food by turning it into nutrients that the body uses for energy, and ultimately survival. The old saying 'you are what you eat' is partly true, because not only are we what we eat, but also what we absorb.

Digestion occurs before we even take our first mouthful of food.  When we start thinking about eating, start cooking or watching our food being prepared, the brain sends signals to the digestive tract to get ready for food. Our salivary glands start producing saliva, which helps us break down our food. Upon the first mouthful of food, our saliva output increases and our teeth and tongue help us chew our food, which is the first step in breaking our food down for digestion. Saliva contains enzymes that help to break down our food, which is why chewing our food thoroughly is so important in aiding better digestion.

Once we swallow, our food passes down our esophagus by a series of muscular contractions, peristalsis, before arriving at the stomach. The lower esophageal sphincter opens to allow food into the stomach, then closes again to keep it there. When this sphincter is not working correctly, people can experience GERD or heartburn, from the feeling of food coming back up the esophagus.

The Stomach

Not only does the stomach act as a holding pouch for food, but it also releases more digestive enzymes and acids to further enhance the digestion of food, by turning it into a thick acidic paste called chyme.

As the chyme is ready to be processed, peristalsis waves push it towards the pyloric sphincter, the gateway between stomach and small intestine. It opens briefly, allowing a small amount to pass through and to begin the process of absorption in the small intestine. All of the food does not pass through at once, as this would flood the system and not be a manageable volume to maximise absorption from.

The pyloric sphincter acts as a one-way gate, preventing chyme from regurgitating back into the stomach from the small intestine.

The stomach is a major defence system to the bacteria and pathogenic organisms that we intake, with most of them being destroyed at this point. If you don't have sufficient enzymes or acids, bacteria can survive and move through to the small intestine.

The Small Intestine

The small intestine is the work-horse of the digestive system, as this is where most of our nutrients are absorbed. It is approximately 6 metres long (20 feet) and is divided into three sections: the duodenum, jejunum, and ileum. The duodenum is mostly responsible for continuing to breakdown the chyme, supported by enzymes from the pancreas and bile from the liver, while the jejunum and ileum are mainly responsible for absorption of nutrients into the bloodstream through the villi and microvilli.


What Causes SIBO - Digestive Tract Showing Villi And Microvilli

These finger-like structures in the small intestine help us absorb our nutrients.  Each villus (singular villi) has microvilli covering it, increasing the surface area of the small intestine. Any leftover chyme then moves through to the large intestine.

Because the role of the small intestine is to absorb nutrients from food, it does not need the assistance of bacteria to do this, and should be a relatively sterile environment. If we have large numbers of bacteria in the small intestine, they compete with us for our food and interfere with digestion and absorption. This is why people with SIBO can experience uncomfortable symptoms and experience deficiencies - their small intestine is not equipped to deal with bacteria here.

The process of moving matter through into the large intestine is called motility. The migrating motor complex (MMC), or sweeper wave, pushes leftover waste and other matter like bacteria along the small intestine, cleaning it up so that it is not sitting stagnant. This process is highly dependant on a network of nerves, muscles and hormones, and occurs in a fasting state (ie. not when you are eating) every 90 minutes and overnight and is commonly disrupted in SIBO patients. It is why intermittent fasting, no snacking and 4-5 hours between meals is commonly advised for SIBO patients.

The valve between the small intestine and the large intestine is called the ileocecal valve, and acts as a one-way system, preventing matter in the large intestine from re-entering the small intestine.

It takes 3-5 hours for chyme to completely empty from the stomach and small intestine and to move into the large intestine.

The Large Intestine

The large intestine is approximately 1.5 metres long (5 feet) and connects to the small intestine and rectum, and is divided into the cecum, the ascending colon, the transverse colon, the descending colon and the sigmoid colon.

It houses trillions of bacteria. The primary function of the large intestine to complete the absorption of nutrients and water, synthesize certain vitamins, form feces and eliminate them from the body.

The bacteria that live in the intestines are called the gut microbiome, and each person's microbiome is unique to them.

What Causes SIBO - The Human Intestinal Structure Medical Diagram

Chyme enters the caecum from the small intestine and slowly moves its way through the large intestine by a process of peristalsis, with water being absorbed from it so it changes from a liquid state to a solid state. If the matter moves through too quickly, not enough water is absorbed and can lead to dehydration and result in diarrhoea. If the matter moves through too slowly, too much liquid can be absorbed, making the feces too solid and causing constipation.

Feces is mostly comprised of undigested food particles, bacteria, old epithelial cells from the GI mucosa, inorganic salts, and water. When it reaches the rectum, it sends a signal to the brain to let it know it needs to be emptied.  We commonly know this sensation as needing to do a poop.  We can voluntarily control the urge to poop by holding on until we can reach a toilet.  The feces is then passed through the anus and out of the body.

"I know what it's like to live with SIBO and painful gut issues"

Rebecca Coomes

I have struggled with gut issues most of my life and suspect I had SIBO from early childhood. I know how isolating it can be to live with these painful and invisible issues. I know how frustrating it can be when it feels no one understands what you're going through. And I know how hard it can be to get the right kind of medical help. That's why I developed The Healthy Gut, so I could help people just like you. I don't want you to suffer a moment longer. The Healthy Gut has the tools and support you need to live well with SIBO and beyond.

Rebecca Coomes, SIBO patient, Founder of The Healthy Gut, host of The Healthy Gut Podcast and author of the SIBO Cookbooks

Here are two episodes on The Healthy Gut Podcast. The first one is an interview with leading SIBO Dr Pimentel about what SIBO actually is. The second one is Sharon Treadgold's real-life SIBO journey. My fans regularly tell me how much my podcasts have taught them about SIBO and gut health. I guarantee they'll help you too.

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