What Does a High-Fiber Diet Actually Do?

You have probably been told to eat more fiber a hundred times, and probably never been told what it actually is or what it does on the way through. That is a shame, because the mechanism is the interesting part, and it is the part that helps you make sensible decisions in a supermarket.

What is fiber?

Dietary fiber is a non-digestible carbohydrate found in plant-based foods. Your small intestine has no enzyme capable of breaking it down, so unlike starch or sugar it is largely not absorbed — it travels the length of the digestive tract more or less intact. Fiber is mostly found in vegetables, fruits, whole grains and legumes. There is essentially none in meat, dairy, eggs or oil, which is the simplest explanation for why a diet built mainly on those runs short of it.

What are the different types of fiber?

Based on its behavior in water, fiber is split into two broad categories. Most plant foods contain both, in different proportions — the labels describe a spectrum rather than two separate substances.

• Soluble fiber: This type of fiber dissolves in water to form a gel-like substance in the digestive system. That gel is what slows the stomach emptying, and it slows the rate at which sugars and fats are absorbed further down. It also changes the way the gut handles dietary fat and cholesterol, because some of it binds to bile acids and leaves with them rather than being reabsorbed. Soluble fiber is also the type your gut bacteria are most able to ferment. Some good sources of soluble fiber are: Oats, Peas, Beans, Apples, Citrus fruits, Carrots, Barley, and Psyllium.

• Insoluble fiber: Insoluble fiber does not dissolve in water. It holds on to water instead, which adds bulk and softness to what passes through the large intestine — this is the property behind fiber’s long association with regularity. It ferments far less readily than the soluble kind, and it is the reason wheat bran behaves so differently from oats.

Some good sources of insoluble fiber are:

• Whole-wheat flour
• Wheat bran
• Nuts
• Beans
• Cauliflower
• Potatoes

What fiber does on the way through

Four things, roughly, and it is worth being precise about them because precision is what is usually missing:

• It adds bulk and holds water. Insoluble fiber increases the volume and water content of what moves through the colon, which is the most consistently documented physical effect fiber has and the reason it is associated with normal regularity.
• It slows things down at the top. The gel soluble fiber forms means the stomach empties more gradually and absorption further along is more gradual too. A practical consequence people notice is that high-fiber meals tend to feel more filling for longer than the same calories without it.
• It feeds your gut bacteria. This is where most current research is concentrated. Fiber that reaches the colon undigested is fermented by resident bacteria into short-chain fatty acids — butyrate, propionate and acetate. Butyrate in particular is a primary fuel for the cells lining the colon, which is a genuinely unusual arrangement: an organ partly running on the by-products of its own microbial population.
• It brings volume without calories. Fiber contributes bulk, water and chewing to a meal and very little energy, which is most of why high-fiber foods and high-calorie-density foods are rarely the same foods.

What the research is actually looking at

Fiber is one of the most-studied components of the diet, and the literature is worth describing accurately rather than summarizing into a promise.

Two umbrella reviews by the same author in the Journal of Chiropractic Medicine pooled the meta-analyses that already existed on fiber intake: one in 2017 on cardiovascular outcomes, and a 2018 umbrella review on type 2 diabetes. The broad survey of the field that most later writing leans on is a 2009 review in Nutrition Reviews, Health benefits of dietary fiber, which covers gastrointestinal, cardiovascular and metabolic outcomes together and is candid that the strength of the evidence varies a great deal from one to the next.

Note what sits underneath them. An umbrella review summarizes meta-analyses, which in turn summarize individual studies — and in nutrition, the studies at the bottom of that stack are largely observational: they follow populations and report what tracks with what. Layers of aggregation make a body of evidence easier to read; they do not turn an association into a cause. It matters here more than usual, because people who eat a great deal of fiber differ from people who eat very little in a long list of other ways at the same time — they tend to eat more vegetables and less ultra-processed food, and to differ in smoking, activity and income. Untangling fiber from the dietary pattern it arrives inside is the central difficulty of this field, and no single review settles it.

The reasonable reading is the boring one: fiber is a marker of, and a component of, a pattern of eating that is well worth having, and it is not a lever you pull on its own.

How much fiber, and how to increase it

National dietary guidelines generally put the adult target somewhere in the region of 25 to 30 grams a day, and survey after survey finds that most people in the US eat well under it. Two practical notes before the list: increase your intake gradually rather than all at once, since a sudden jump is what produces the bloating people blame fiber for, and drink enough water, since fiber works by holding on to it.

You can increase your daily fiber intake by eating a variety of fruits, vegetables, legumes and grains. Here are examples of foods you can eat to increase your fiber consumption:

• Pears: This delicious fruit is one of the best sources of fiber. A medium-sized pear contains around 5.5 grams of fiber.
• Apples: An apple is one of the healthiest fruits. To increase your fiber intake, eat one apple per day. A medium-sized apple contains around 4.4 grams of fiber.
• Lentils: We already know that lentils are packed with protein and many other important nutrients. But did you know that lentils are also a very good source of fiber? Just one cup of cooked lentils contains 13.1 grams of fiber. For a high fiber diet, you can try a lentil soup, spiced with cumin, coriander, turmeric and cinnamon for extra antioxidants.
• Moringa powder: Moringa leaf carries both soluble and insoluble fiber, and the powder is an easy way to add some to things that would not otherwise have much. There are multiple ways in which you can use moringa powder. You can add it to your soups, salads, juices and smoothies. You can also add it to cake, bread and brownies. It is served in teaspoons, so treat it as a top-up rather than as a main source — a cup of lentils will always do more for the total than a spoon of anything.
• Chickpeas: One cup of cooked chickpeas contains about 12.5 grams of fiber. There are so many ways you can use chickpeas to consume a high fiber diet. You can make hummus, chickpea salad or even a chickpea soup.

If you are managing an existing health condition or taking prescription medication, talk to a qualified healthcare professional before making a large change to your diet. This article is general nutrition information and not medical advice.

These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease.