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Soluble vs Insoluble Fiber - Which Is Best? (A Dietitian Explains)

If you’ve been diving into the world of fiber recently, and trying to figure out what is best for your gut and overall health, you’ve probably been wondering what is the difference between the 2 types of fiber you keep hearing about - soluble versus INsoluble fiber? And just as importantly: Which one helps with bowel movements? Which one helps support the good bacteria in your microbiome? And which one helps lower cholesterol? Well, in this article, we’re going to answer all those questions. Plus, we’ll even look at which foods and supplements contain these types of fibers. Let’s go!

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    What's the difference between soluble vs insoluble fiber?

    So when you ask most health professionals what the difference is between SOLUBLE and INsoluble fiber, they usually try to keep it really simple. They mainly talk about how the fiber acts in water and how it helps you poop!

    In fact, here’s a quick clip of a nurse explaining the difference between soluble and insoluble fiber to a puppet.

    Now, we don’t mind that answer for a kids show. In fact, kudos to The Friday Zone for discussing such an important topic to younger viewers. But for you Essential Stacks fans, who want to know how the different types of fiber can improve your health…it just won’t do.

    Instead, we need a more detailed understanding of how these fibers work in our bodies.

    A better explanation - soluble, viscous & fermented

    So to really understand this - being the difference between SOLUBLE and INsoluble fiber - the first thing you need to know is that there are actually 3 different types of soluble fiber and 2 different types of insoluble fiber.

     

    Soluble Fibers

    Insoluble Fibers

    Type

    Non-Viscous Fermented

    Viscous Fermented

    Viscous Poorly Fermented

    Non-Viscous Poorly Fermented

    Non-Viscous Fermented

    So apart from how they dissolve in water, soluble and insoluble fibers also differ in terms of:

    • “viscosity” and
    • “fermentability”

    Which I’ll quickly explain now.

    So if a fiber is viscous, it means that it can thicken and form a gel inside your GI tract. And if a fiber is fermentable it just means that it can be fermented…or in other words, “eaten”…by the bacteria in your gut.

    These differences are important, because it means these fibers can help you achieve different health goals, which we’ll talk about later in this article.

    But guess what - if that didn’t make a whole lot of sense to you, don’t worry, because all you really need to know for now is that there are 5 types, they are all somewhat different and they each deliver a unique mix of benefits.

    Examples of soluble and insoluble fiber foods & supplements

    And just before we look at those benefits, let me show you some examples of real foods and supplements that contain these types of soluble and insoluble fibers. As hopefully that will make it easier for you to picture them.

    Soluble fiber foods and supplements

    1. Examples of them would be inulin, which you can find in foods like chicory root, as well as vegetables like asparagus, onion and of course garlic.
    2. Another example, would be partially hydrolyzed guar gum, which you can find in fiber supplements like our very own Friendly Prebiotics.
    3. Another example of soluble fiber would be beta glucans, which you get in wheat based products like a loaf of bread.
    4. Perhaps one of my favorite examples would be the soluble fiber, pectin, which you can find in all sorts of fruits, especially in apples.
    5. And of course another great example, and probably the most well known soluble fiber, is psyllium husk. And you can of course find this in supplements like Metamucil.

    If we take a quick look at the table now, you can see which type of soluble fiber each of these examples fits in.

     

    Soluble Fibers

    Type

    Non-Viscous Fermented

    Viscous Fermented

    Viscous Poorly Fermented

    Examples

    Inulin, e.g. garlic

    PHGG, e.g. Friendly Prebiotics

    Beta-glucans, e.g. wheat

    Pectin, e.g. apples

    Psyllium, e.g. Metamucil 

    Insoluble fiber foods and supplements

    1. So cellulose is one of the best examples, and you’ll find this insoluble fiber in everything from legumes to fruit to vegetables, including everyone's favorite, broccoli!
    2. Another example of insoluble fiber would be resistant starch, which has become quite trendy over the last couple of years. You’ll find this in cooked & cooled potatoes, like the kind you might eat in a potato salad. As well as in fruits like green bananas.

    If we bring the table back up, you’ll see which type of insoluble fibers these belong to.

    Insoluble Fibers

    Non-Viscous Poorly Fermented

    Non-Viscous Fermented

    Cellulose, e.g. vegetables like broccoli

    Resistant starch, e.g. potatoes

    Soluble vs insoluble fiber in foods

    It’s important to point out here that most foods will contain a mix of soluble and insoluble fibers. And if you want to learn more about that, check out our article on soluble and insolible fiber foods.

    Now you have an idea of the types of foods and supplements that are packed with the various types of soluble and insoluble fiber, it's time to answer the big question: what benefits do these different types of fiber deliver?

    Which fiber works best for bowel movements?

    Well, given bowel movements are probably the #1 reason people try to get more fiber into their diet, let’s take a look at which soluble and insoluble fibers work best here.

     

    Soluble Fibers

    Insoluble Fibers

    Type

    Non-Viscous Fermented

    Viscous Fermented

    Viscous Poorly Fermented

    Non-Viscous Poorly Fermented

    Non-Viscous Fermented

    Examples

    Inulin, e.g. garlic

    PHGG, e.g. Friendly Prebiotics

    Beta-glucans, e.g. wheat

    Pectin, e.g. apples

    Psyllium, e.g. Metamucil 

    Cellulose, e.g. vegetables like broccoli

    Resistant starch, e.g. potatoes

    Promotes bowel movements

    Indirectly helps via stool volume, softness & microflora

    Indirectly helps via microflora

    Indirectly helps via stool bulking

    Yes

    Indirectly helps via microflora

    So as you can see they ALL help with bowel movements.

    Obviously, insoluble fibers such as cellulose, will help the most directly, as they stimulate gut motility (meaning movement through your GI tract), as well as adding bulk to your stool (meaning they make it bigger).

    But the other fibers play an equally important role in making bowel movements pleasant.

    For example, soluble fibers like partially hydrolyzed guar gum (PHGG), can help attract more water into your stool, allowing it to soften. And when your stool has more moisture, it will move along smoothly and with less discomfort.

    So hopefully you can see now, there is no one fiber doing all the work here. Instead, they really work like a team to promote healthy bowel movements.

    Which fiber feeds your good bacteria best?

    Now, the next big question: which of these fibers help most with supporting the good bacteria in our gut? In other words, which ones are fermentable by the bacteria in your gut?

    And just before we look at the results it’s interesting to see online that most people, including reputable health websites, seem to suggest only soluble fiber can help. But are they right?

    Let’s take a look at what our research team found by actually diving into the studies.

     

    Soluble Fibers

    Insoluble Fibers

    Type

    Non-Viscous Fermented

    Viscous Fermented

    Viscous Poorly Fermented

    Non-Viscous Poorly Fermented

    Non-Viscous Fermented

    Examples

    Inulin, e.g. garlic

    PHGG, e.g. Friendly Prebiotics

    Beta-glucans, e.g. wheat

    Pectin, e.g. apples

    Psyllium, e.g. Metamucil 

    Cellulose, e.g. vegetables like broccoli

    Resistant starch, e.g. potatoes

    Promotes bowel movements

    Indirectly helps via stool volume, softness & microflora

    Indirectly helps via microflora

    Indirectly helps via stool bulking

    Yes

    Indirectly helps via microflora

    Supports good bacteria 

    Yes

    Yes

    Not much

    Not much

    Yes

    So this is interesting!

    As you can see here 2 of the soluble fibers do help, but 1 doesn’t. Meanwhile, 1 of the insoluble fiber types actually does work for feeding and nourishing the good bacteria in your gut, also known as probiotics.

    We were kinda shocked to see so few people discussing this online.

    Either way, once again, what we’re seeing here are the benefits of consuming both soluble and insoluble fiber.

    During our research we found a fascinating stat that I just wanted to share with you quickly: it turns out that 70% of the fiber we consume can be eaten or ‘fermented’ by the bacteria in our gut. Which means the majority of fiber is fermentable.

    And for these fibers that selectively feed the probiotics, we actually call them prebiotics. Or prebiotic fiber if you prefer. A term you’ve probably heard before.

    Obviously these types of fiber are crucial for our gut health, as well as overall health. And that’s mainly because they help the good bacteria in our microbiome to grow stronger, as well as help our bodies produce beneficial short chain fatty acids (SCFAs) like butyrate.

    And if you want to geek out with us and find out why all of this matters for your health, check out our article on the benefits of probiotics.

    Which fiber helps most with lowering cholesterol?

    Finally, which fibers help most with cholesterol?

     

    Soluble Fibers

    Insoluble Fibers

    Type

    Non-Viscous Fermented

    Viscous Fermented

    Viscous Poorly Fermented

    Non-Viscous Poorly Fermented

    Non-Viscous Fermented

    Examples

    Inulin, e.g. garlic

    PHGG, e.g. Friendly Prebiotics

    Beta-glucans, e.g. wheat

    Pectin, e.g. apples

    Psyllium, e.g. Metamucil 

    Cellulose, e.g. vegetables like broccoli

    Resistant starch, e.g. potatoes

    Promotes bowel movements

    Indirectly helps via stool volume, softness & microflora

    Indirectly helps via microflora

    Indirectly helps via stool bulking

    Yes

    Indirectly helps via microflora

    Supports good bacteria 

    Yes

    Yes

    Not much

    Not much

    Yes

    Lowers cholesterol

    Indirectly helps

    Yes

    Yes

    Not much

    Indirectly helps

    So as we can see most of the fibers help in some manner, but it is our 2 soluble fibers there that are helping the most in a direct way.

    If we look at the research, we can see that this is due to their “Viscosity”. And just to recap…“viscous” just means that the fiber can thicken and form a gel when ingested in your body.

    And this gel works to move cholesterol along and reduce absorption.

    Interestingly, two types of fibers - being Beta glucan and Psyllium - do this so well, that the FDA has actually approved manufacturers of these products to make the claim that they “lower cholesterol”.

    Which fiber foods are best?

    So now you know the difference between soluble and insoluble fiber, I’m sure you’re interested in getting more of both these fibers in your diet. So the big question becomes…which foods pack the most fiber content?

    Well to make all of this super easy, check out our free Top 100 Fiber Foods Checklist, where you’ll finally be able to see at a glance, exactly which foods deliver the most fiber, including how much is soluble versus insoluble!

    Evidence Based

    An evidence hierarchy is followed to ensure conclusions are formed off of the most up-to-date and well-designed studies available. We aim to reference studies conducted within the past five years when possible.

    • Systematic review or meta-analysis of randomized controlled trials
    • Randomized controlled trials
    • Controlled trials without randomization
    • Case-control (retrospective) and cohort (prospective) studies
    • A systematic review of descriptive, qualitative, or mixed-method studies
    • A single descriptive, qualitative, or mixed-method study
    • Studies without controls, case reports, and case series
    • Animal research
    • In vitro research

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    References

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