Digestive Health
August 24, 2022

Fiber 101: The Basics and Not-So Basics of Fiber

Hands holding baby purple eggplants at the farmer's market
min read
Key Takeaways
  • Fiber is a non-digestible carbohydrate
  • Two types of fiber: dietary fiber and functional fiber.
  • Soluble fiber is known to absorb water
  • Insoluble fiber can't be dissolved in water
  • A viscous fiber will move more slowly through the digestive tract
  • Fermentability - the speed by which fiber is broken down by the bacteria

Dietitians are always touting the benefits of fiber. Fiber helps to regulate your blood sugar and energy levels, and it keeps you feeling full and satiated for a longer amount of time. It can reduce your risk for cardiovascular disease, lowers cholesterol levels, and shows protection against colon and breast cancer. 

As gut health dietitians, it’s safe to say we’re a bit obsessed with fiber - we totally geek out over it. Why, you ask? Fiber plays SUCH a huge role in your gastrointestinal health - it can help you go to the bathroom in many ways, it feeds your microbiome, which leads to a strong immune system and helps maintain the integrity of your gut lining. We often recommend fibrous foods and supplements to our clients to help them achieve their “poop goals.” 

But it isn’t a blanket recommendation for everyone - and this is where the science nerd in us comes out. We look at the various properties that different fibers have to determine which is the best option for your specific needs. How do we decide what to recommend to who? Read on to find out!

Fiber Basics

What is Fiber?

Fiber is a non-digestible carbohydrate that’s only found in plant foods. People lack the digestive enzyme to break fiber down, and it passes through our gastrointestinal tract relatively intact. There are two types of fiber: dietary fiber and functional fiber.

Dietary fiber is found in fruits, vegetables, grains, nuts, beans, legumes, and seeds. Some of the more specific names of dietary fibers include cellulose, hemicellulose, lignins, gums, pectin, fructans, beta-glucans, and resistant starches. 

Functional fibers are made from isolated, extracted, or manufactured non-digestible carbohydrates. These are added to foods or supplements to increase their fiber content and occasionally enhance texture and palatability. Supplement examples include products like Metamucil, Benefiber, and FiberCon. Chicory root, inulin, pectin, gums, and resistant starches are examples of functional fibers added to foods to increase their fiber content, texture, and palatability. 

Fiber Not-So Basics

Fiber can be further classified based on its molecular size, solubility, its viscosity, and its fermentability. This is where we get to flex our science brains -after all, we are science-geeks at heart!

Soluble Fiber

Solubility refers to the ability of a substance to dissolve in water. Soluble fiber is known to absorb water like a sponge. It forms a gel-like texture that keeps poop soft and formed. It moves more slowly through the digestive tract. Dietary sources of soluble fiber include oats, grains, and the insides of fruits and veggies. Functional sources (aka those used for supplements or as food additives) include psyllium, methylcellulose, and wheat dextrin. Soluble fiber can be helpful for constipation by increasing the water in stool and diarrhea by helping it to form. 

Insoluble Fiber

Insoluble fiber is what we traditionally think of as roughage. Insoluble fiber can't be dissolved in water; you can see it when you poop. It adds bulk to stool and speeds up the digestive transit. Dietary insoluble fiber is the bran, skins, stalks, and seeds of foods. Functional sources include wheat bran, calcium polycarbophil, and ground flaxseed. Insoluble fiber is potentially helpful for constipation and slow transit. 


The viscosity of a fiber describes its ability to thicken and form a gel when mixed with a liquid like water. Viscous fibers include psyllium, pectins, gums, and beta-glucans. Examples of non-viscous fibers include some forms of cellulose, some hemicelluloses, inulin, and lignins (seeds, stems, and the bran layer of grains, i.e., wheat bran). A viscous fiber will move more slowly through the digestive tract, which can aid in energy balance, helps optimize nutrient absorption, and allow time for stool to bulk up.


Fermentation feeds and provides energy to our gut bacteria. Fermentability describes the speed by which fiber is broken down by the bacteria in our large intestine as well as how much of the fiber molecules they are consuming. The gut bacteria then release by-products such as beneficial short-chain fatty acids and gas into the body. Examples of fermentable fibers include beta-glucans (oats and barley), guar gum, pectin (citrus, nuts, root veggies, jams), xanthan gum, and resistant starch (green bananas and legumes). 

The rate at which a fiber is fermented depends on its chain length. Some fibers have long chains. They are highly fermentable, but the speed at which the process occurs is slower and the gas produced is at a more controlled rate. There are others known as FODMAPS (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols), which are short-chain fibers. These are fermented at a quick rate and may cause discomfort and gas, especially in people with Irritable Bowel Syndrome or other Disorder of the Gut-Brain Interaction (formally known as Functional Gastrointestinal Disorders). 

What Does All This Science Around Fiber Mean For You? 

All of the science behind fiber can get confusing. It’s hard to know what and how much (check out our blog to learn more about this topic) to eat for your specific goals. But don’t worry, we got you covered! We can help you determine which sources of fiber have the properties that are best for YOUR digestive system.

Whether you suffer from IBS, chronic constipation, SIBO, diarrhea, or if you’re just looking to have better energy throughout the day, we can help reduce your bloating and get you pooping!‍

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