Looking at how eating plant fibre affects your gut

15th July 2020

Hannah HarrisFibre is an important part of a healthy diet, but we don’t fully understand how it is handled by the body.

In this blog, Dr Hannah Harris from the Quadram Institute talks about recently published collaborative work that looks to address this.

A better understanding of how different sources of fibre react in the body and interact with the microbiome will help in the development of treatments for gut conditions.

Different foods can affect how your guts feel and this is especially true for dietary fibre. Some foods can make you feel bloated, other foods are used when you are constipated and to ‘keep you regular’. Less is known about how and/or why these foods work.

Fibre is a mix of different carbohydrates that the human body cannot digest. These fibres reach the colon intact, where they are a food source for the millions of bacteria within your gut. There are many different types of fibre, which behave differently in the gut, and the body. One of these key differences is how they interact with water, which is found throughout the body and the gastrointestinal tract.

Plantago ovata form


Prickly Pear 5half


Wheat close-up


The Quadram Institute along with partners at the University of Nottingham and King’s College London have tested psyllium, nopal, and wheat bran, three sources of dietary fibre which are used as laxatives. Psyllium is from the Plantago ovata plant found in Asia and produces a gel with contact with water. Nopal (extracted from prickly pear cactus) is consumed in Mexico, and produces a gel with water, although not as much as psyllium. Wheat bran does not produce a gel and is commonly consumed as a stool bulker.

How these fibres alleviate constipation is not fully understood.

To understand how these different fibres affect the gut we combined laboratory experiments at the Quadram Insitute with an advanced medical imaging technique called Magnetic Resonance Imaging (MRI) at the University of Nottingham.

At the Quadram Institute we used a laboratory model of the human colon to measure if each of the fibres would produce a different amount of gas. We did this by using a sensor to test how much gas was produced using stool samples from healthy individuals. These stools contain millions of bacteria which can ferment the food that humans cannot digest, producing gas and other beneficial particles. Stools were mixed with nutrients to keep the bacteria active and mixed with each of the fibres and measured at 37oC (body temperature).

In parallel, at the University of Nottingham, healthy adults were given a drink containing one of the three fibres and the amount of water in their intestines was viewed by MRI. MRI can safely look at different parts of your body by looking at the water contained in the different tissues and organs. Unlike x-rays that can be harmful in excess, MRI is not harmful so individuals can be repeatedly imaged throughout a day.

Each of the test fibres increased water in the small intestine. But once in the colon, the fibres behaved differently from each other, suggesting they function as laxatives in different ways.

Psyllium increased the amount of water in the colon, as well as the size of the colon. However, psyllium led to the production of very little gas suggesting that the bacteria were unable to utilise it as a nutrient source.

Nopal, unlike psyllium did not increase the amount of water in the colon. By the end of the experiment it did result in an increased colon size. Nopal also resulted in the production of more gas than psyllium. As this was less than from wheat bran it indicated that nopal is used by the colonic bacteria, but to a lesser extent than wheat bran.

Wheat bran also did not increase the size or the amount of water in the colon. Unlike psyllium it resulted in the production of a lot of gas, showing that it was easily broken down by the colonic bacteria.

These results suggest that the fibres work in different ways. Psyllium takes on water within the small intestine and holds it throughout the colon. As Psyllium is not easily broken down by the bacteria in the colon, it is able to retain a lot of water. Nopal holds some of the water from the small intestine through the colon, but is also utilised by the bacteria in the colon so its structure breaks down as it passes through the colon. Wheat bran is unable to hold the water from the small intestine, and the bacteria can easily break down the fibre so it doesn’t contribute to colonic water holding.

We have shown that these different laxatives work differently because of how they interact with water. This information is important as it provides tools for the development of treatments for people with conditions such as IBS as well as other gastrointestinal issues.

MRI images of the colon after consuming psyllium or sugar. Arrows point to the colon.
L: The colon after eating psyllium. The colon visibly larger as psyllium is holding water
R: The colon after eating sugar. Sugar does not reach the colon, so the size of the colon is smaller with little water or fibre within it. Adapted from original image provided Giles Major (University of Nottingham). Major G, et al, 2018 Neurogastroenterol Motil. doi:10.1111/nmo.13400

This research was funded by Medical Research Council (MRC UK) and CONACYT (Mexico) as part of a Newton Fund and BBSRC Institute Strategic Programme Food Innovation and Health.

Reference: Gunn, D., Murthy, R., Major, G., Wilkinson-Smith, V., Hoad, C., Marciani, L., …Spiller, R. Contrasting effects of viscous and particulate fiber on colonic fermentation in vitro and in vivo, and their impact on intestinal water studied by magnetic resonance imaging in a randomized trial. American Journal of Clinical Nutrition, DOI: 10.1093/ajcn/nqaa173


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