Microbiome Diet ~ 1. Dietary Fiber

What are the most important nutrients for supporting your gut microbiome? Which foods contain dietary fiber, and which do not? And is it really good for your health?

This is Part 1 of the Microbiome Diet series. Read Part 2: Microbiome Diet 2 – Animal-Based Foods.

What Is Dietary Fiber and Why Is It So Important?

You have probably heard of dietary fiber before—or rather, foods that are said to be rich in it.

There is no single exact definition of “dietary fiber,” but it can be described as complex carbohydrates that our bodies cannot fully digest on their own. They pass through the digestive tract undigested until they reach the colon, where they are broken down by the gut microbiome.

In a process called fermentation, beneficial gut bacteria digest these fibers. Fiber is their preferred food source. During this process, many beneficial compounds called short-chain fatty acids (SCFAs) are produced. SCFAs are essential for maintaining gut function and have anti-inflammatory effects.

Different Types of Fiber

“Dietary fiber” is not a single substance but a broad term covering a large and diverse group of complex carbohydrates. This group can be divided into two categories: soluble fiber, which dissolves in water, and insoluble fiber, which does not.

Soluble fibers are the preferred food of microbes in the colon and strongly support the production of SCFAs. Some soluble fibers form a gel-like substance in the gut, which helps slow the absorption of sugar and cholesterol into the bloodstream, directly reducing the risk of diseases such as diabetes.

Insoluble fibers add bulk to stool by attracting water into the intestines, helping to maintain regular bowel movements.

Depending on your diet, your gut microbiome receives different types and amounts of fiber. This, in turn, shapes which microbes thrive in the gut and how many beneficial SCFAs are produced.

How to Ensure You Get Enough and a Wide Variety of Fiber

1. Vegetables

A diet rich in plant-based foods provides many different types of dietary fiber, which supports a diverse gut microbiome.

We recommend eating at least three servings of 100 g of vegetables per day. Try to vary between coarse vegetables such as cabbage and root vegetables; dark green vegetables like spinach, broccoli, and kale; and colorful vegetables such as carrots, tomatoes, and red peppers. The more variety, the better.

2. Fruit

Fruit is another excellent source of fiber. It also contains many polyphenols that are not found in vegetables and are beneficial for the microbiome.

An intake of at least three 100 g servings per day is considered optimal—one serving with each main meal. When possible, prioritize grapes, citrus fruits, and berries such as blueberries, blackberries, strawberries, raspberries, and blackcurrants.

3. Whole Grains

There is evidence that consuming whole grains increases bacterial diversity in the gut, which is considered beneficial.

Eat at least two servings of whole grains daily, corresponding to about 75 g. Always choose whole-grain products when eating bread, pasta, rice, crackers, and breakfast cereals. This is also important when buying sandwiches or takeaway meals.

4. Legumes

Legumes (beans, lentils, chickpeas, etc.) are an important part of any diet. They are an excellent source of fiber and folate (vitamin B9) and are rich in protein and iron. They are also low in fat and sodium and contain no gluten or cholesterol. In addition, legumes have a low glycemic index, meaning they cause a smaller rise in blood sugar.

We recommend at least three to four servings per week. This corresponds to about 130 g per serving for vegetarians and about 50 g per serving for non-vegetarians.

5. Nuts and Seeds

Like legumes, nuts and seeds have many benefits: they are rich in fiber and contain healthy fats and protein. Walnuts, in particular, have been shown to have a positive effect on the gut microbiome. Tree nuts and peanuts also have this effect, though to a lesser extent.

We recommend three servings—about 30 g or three tablespoons—per day to boost your gut microbiome. You get the best effect when eating whole nuts, as 30 g of nuts provides about 1–3 g of fiber.

Conclusion

Dietary fiber is the key to a healthy microbiome. It is found in vegetables, fruits, whole grains, legumes, nuts, and seeds.

Consuming a wide variety of fibers from these food sources strongly supports a diverse microbial community in the gut, which is closely linked to better health. For this reason, fiber-rich foods form the foundation of a true “microbiome diet.”