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The effect of colonoscopy preparation on the gut microbiome

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Colonoscopies are crucial tools for diagnosing bowel cancer, but the jury is out on whether the preparation process is harmful or helpful.

New Zealand has one of the highest bowel cancer rates in the world. Indeed, it’s our second-highest cause of cancer death. However, people who are diagnosed with bowel cancer and receive treatment at an early stage have about a 90% chance of recovery.

Colonoscopies are vital tools for clinicians in diagnosing some bowel cancers and many other bowel diseases. The procedure allows clinicians to see the entire colon surface, remove small lesions known as polyps (which may become cancerous) and perform biopsies as required.

Before a colonoscopy, patients complete a process to clean out the gut, enabling the clinician to have a clear view of the colon surface. The preparation includes dietary changes and drinking a laxative solution to empty the bowels. Some clinicians ask their patients to drink three litres of a solution before an appointment, and others use a stepped process of one glass of a laxative solution the day before and a second glass of the solution on the day of the appointment.

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“The effect of bowel preparation on the gut microbiome is pretty massive,” says University of Otago, Christchurch, gastroenterologist Richard Gearry.

A number of clinical trials have investigated the effect of bowel preparation on gut microflora. Italian researchers analysing the effect of drinking four litres of polyethylene glycol-based solution (a laxative that increases the amount of water in the intestinal tract to stimulate bowel movement) on the gut microbiota of 10 patients, immediately after a colonoscopy and again one month later, found both the quantity and types of bacteria in the gut were affected. The changes were still apparent one month later.

US researchers also found changes after bowel preparation in both the composition and diversity of bacteria in the gut. There are changes in diversity and there are changes in the proportions of different bacteria – increases and decreases – that can be construed as positive or negative.

But those researchers’ analyses of gut bacteria don’t give us the full picture, says Gearry.

“Often now we don’t just look at what bacteria are there, but actually what they’re doing,” he says. This includes looking at the gene expression from the bacteria.

But, even with all that information, Gearry says the science has outstripped our ability to interpret what is going on clinically.

“We used to say things like, ‘Oh, yes, there might be various diseases and things associated with changes in the microbiome.’ But now that we’ve got the science to look at what is in the microbiome, and we’ve got the maths to do the very complicated statistical analyses to look at changes that are occurring, we still don’t fully understand everything about what the changes mean from a clinical perspective.”

In other words, more research is needed to determine what, if any, effect bowel preparation has on our gut microbiome. In the meantime, certainly don’t count on bowel preparation to flush out harmful bacteria, and don’t worry about it harming your microbiome.

“If someone has got a really good indication for a colonoscopy, that should override any concerns about the microbiome.”

In terms of rebuilding your microbiome, no one really knows how to add the right mix of the right bacteria for each individual. The best solution available is to eat a diet rich in wholegrains, vegetables and fruits.

This article was first published in the March 16, 2019 issue of the New Zealand Listener.