We all have heard that fiber is essential for our gut microbiota and therefore for our overall health, but does that mean that every time we eat a high-fiber snack we are actually feeding our gut bacteria the fiber it needs?

Interviewed during the Gut Microbiota for Health World Summit held in Miami last March, Eric Martens, a researcher at the University of Michigan Medical School (USA) where he directs a lab on gut microbiota and nutrition, starts by explaining why reading labels and making them out is sometimes complicated for most people. That is because there are, in fact, three categories of fiber on food labels. The ‘total carbohydrates’, which is everything, from simple sugar to starch and fiber; and sugar and fiber as two independent sub-groups.

“So if you have 50g of carbohydrates, you might see 20g of sugar and 5g of fiber, but the other 25g is the starch that they hide on the food label,” says Martens. That way of labelling foods sometimes makes things complicated for consumers who want to stick to a healthy diet. Why is fiber said to be so healthy? How can we know what kind of fibers we need to eat and where to find them?

According to Martens, “of all the different major nutrient groups that we eat, fiber is the one component of our diet that directly feeds our gut microbiota.” When we eat protein, for instance, we digest it and absorb it in our small intestine. The same thing happens with fats and most sugars. In the case of non-digestible fibers, we do not have the enzymes needed to break them down and digest them. Only gut bacteria can do that. They digest fibers and produce short chain fatty acids, whose beneficial effects on health are well documented, as we already explained in this blog. By eating fiber, then, we are ensuring those trillions of microbes are well-fed so they can help us stay in good health.

We can get fibers from a range of different sources: grains, nuts, vegetables, fruit. “Depending on how you cook them and what you eat and how much of it you eat, it will affect your gut microbiota differently,” explains Martens.

At present there is no scientific evidence to say whether one type of fiber is more or less beneficial than another type, but it has been shown that “eating many different diverse fiber sources is probably better than eating one thing. If you got all your fiber from just oats, it might not be as good as eating it from oats and other wholegrains and nuts and vegetables and fruits. That’s because you’re feeding more diversity and eating more diverse molecules from different sources, and those probably translate to more balanced, richer and diverse gut microbiota.”

Feeding well your gut microbiota is also key for your physiology. We have in our gut a kind of defensive wall, the intestinal barrier, that keeps our friendly bacteria where they have to be. This intestinal barrier is covered by a mucus layer. When gut microbiota is not properly fed, they forage on the mucus layer and there is the risk potentially harmful bacteria spread all over the body causing infection or disease.

So, what should we eat to increase fiber intake and be healthier? The answer is quite simple: “Wholegrains, fruits, vegetables,” says Martens. We should also make sure we include prebiotics, which are foods like asparagus or artichoke that are rich in fermentable sugars that feed precisely gut bacteria; probiotics, which are bacteria proven to be beneficial for health that can be found in some fermentable foods, like yogurts. And finally, do not forget to have synbiotics, which are a combination of both pre and probiotics (for instance, a bowl of yogurt with some sliced banana).

Cristina Sáez
Cristina Sáez
Cristina Saez is a freelance science journalist. She works for several media, for instance the Spanish newspaper La Vanguardia, where she coordinates the science section, Big Vang; as well as research centres and scientific societies. She has been awarded for her journalistic work, among others, with the Boehringer Ingelheim Award in Medical Journalism 2015. Follow Cristina on Twitter @saez_cristina