It is not for no reason that increasing fibre in your diet is recommended. Contained in fruits, legumes, veggies and whole grains, this carbohydrate helps keep you healthy, with its virtues backed up by scientific evidence: a diet rich in fibre reduces your risk of developing heart disease, diabetes and arthritis, among other things. Also, and more importantly, it seems to decrease all-cause mortality rate. The reason why fibre leads to all these benefits was previously unclear, but scientists have found new evidence that the gut microbiota play a key role in facilitating the health gains from a veggie- and fruit-rich diet.
In fact, it doesn’t look like fibre is beneficial per se for our bodies. We cannot break down the fibre (made by a long chain of carbohydrates) into sugar molecules, so it passes through the entire human digestive tract undigested until it arrives in the colon, where our gut microbiota chow it down.
Two recent studies published in the scientific journal Cell Host and Microbe have shed some light on the role gut microbiota play in providing benefits from a high fibre diet. In both studies, scientists switched mice from a high-fibre diet to a low-fibre diet and observed that when the animals ate a low-fibre diet, the size and diversity of their gut microbiota diminished. Moreover, the lack of fibre caused shrunken intestines and provoked an immune reaction, inflammation.
After these scientists had demonstrated the effects of a lack of fibre in the diet on gut microbiota and on mouse health in general, they wanted to know whether they could reverse the symptoms and restore a healthy gut microbiota. To do so, they introduced inulin, a prebiotic non fermentable fibre, to the diet of the mice. And yes, they saw some improvements, some restoration of the gut microbial community, but it was only partial.
“Diets that lack fibre alter the bacterial composition and bacterial metabolism, which in turn causes defects to the inner mucus layer and allows bacteria to come close [encroach], something that triggers inflammation and ultimately metabolic disease,” says Gunnar C Hansson, professor at the University of Gothenburg and a coauthor of the study. “It is not enough just to add fibre to your diet; it also depends on which bacteria you carry.”
This is because different dietary fibres feed different groups of microbes. Species of bacteria are specialised in thriving on a type of fibre, even making specific enzymes to break down that fibre; that is why it is so important to follow a diverse high fibre diet, in order to have a diverse, balanced and rich microbial community. For instance, some bacteria die if they do not get fibre, and other species depending on them also die. There are even some types of bacteria that, not having fibre to eat, will start feeding on the mucus layer, which is a protection our gut has against bacteria leaking into the bloodstream.
There are even more gut reasons to be sure you are taking care of your gut microbiota by eating your fibre! After they have obtained all the energy they need from dietary fibre, they get rid of the fragments as waste. This waste, which includes short-chain fatty acids, is absorbed by our intestine and used as fuel.
Zou J, Chassaing B, Singh V et al. Fibre-Mediated Nourishment of Gut Microbiota Protects against Diet-Induced Obesity by Restoring IL-22-Mediated Colonic Health. Cell Host and Microbe, 2017. DOI: 10.1016/j.chom.2017.11.003
Schroeder B, Birchenough, G, Stahlman M et al. Bifidobacterium or fibre protect against diet-induced deterioration of the inner colonic mucus layer. Cell Host and Microbe, 2017. DOI: 10.1016/j.chom.2017.11.004
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