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Soluble vs Insoluble fiber: The Difference Explained

You have heard the term “fiber” a million times now. You have heard it in school, on TV, radio, YouTube, etc. The term is used to describe many things in the wrong way. So, first and foremost, we’ve got to formulate a definition of the term “fiber”. We’ll start by talking about where you can find fiber because it’s easier, and then move on to soluble vs insoluble fiber.

Fiber can be found in most vegetables, fruits, whole grains, and legumes. Fiber is an essential nutrient and is usually an important component of any diet. So, what is soluble fiber, then? And what is insoluble fiber? We’ll go through all of these.

Soluble vs Insoluble Fiber

What is soluble fiber?

Soluble fiber is the type of dietary fiber that dissolves in water. You can find soluble fiber mainly in grains, legumes, seeds, nuts, and citrus fruits or carrots (1). Soluble fiber turns to gel and slows down digestion. This is the type of fiber you would find in the psyllium supplements you take from the pharmacy. The soluble fibers might be able to contribute to lowering the risk of heart diseases, but this still needs a little more research. Other types of food that are rich in soluble fiber are broccoli, winter squash, berries, artichokes, lentils, peas, etc (2). When you eat them and get your fair share of soluble fiber, then this fiber swells up in the water in your stomach until it turns into a thick gel which contributes to slowing down your digestion process. In the end, the gel is broken down by bacteria. This happens in your large intestine and provides you quite a small amount of calories.

It might seem like this soluble fiber is good for nothing. Actually, it has its fair share of “contributions” to our body’s well-being and we should not ignore it. First and foremost, soluble fiber helps control the levels of glucose in your blood and prevents rapid rises in blood glucose following a meal (3). Moreover, it can “interfere with the absorption of dietary fat and cholesterol” (4). Read more about the benefits of Soluble Fiber.

Summary: Soluble fiber dissolves in water. It is found in grains, legumes, seeds, nuts, citrus fruits, carrots, broccoli, winter squash, berries, artichokes, lentils, and peas. During digestion, it turns into a gel. The gel helps to absorb dietary cholesterol and fat, then excrete it from the system. The gel also helps to slow the amount of sugar released into the bloodstream, reducing blood sugar spikes.

What is insoluble fiber?

It’s quite easy. If soluble fiber dissolves in water, then insoluble fiber doesn’t (5). This is the kind of fiber that people suffering from chronic constipation should eat more. Insoluble fiber does not dissolve in water turning into a gel-like substance, but rather passes through our intestines mostly unchanged and helps move the other types of food through the digestive system and into our stool. It is the “bulk” kind of fiber. You can find insoluble fiber in wheat bran, vegetables, and whole grains (6). Read more about the benefits of Insoluble Fiber.

Summary: Insoluble fiber does not dissolve in water. It passes through our system unchanged, helping food move through the digestive tract and into our stool. It is often known as “bulk” fiber. This type of fiber helps people who experience constipation or other gastrointestinal issues. Wheat bran, vegetables, and whole grains contain insoluble fiber.

Soluble vs Insoluble Fiber: What’s the difference?

Fiber is a very important dietary constituent. Fiber is almost everywhere in the dietary products of today. Soluble and insoluble fiber are two faces of the same coin and they even co-exist in different kinds of foods. For example, the flesh of the apple contains soluble fiber, while the skin is rich in insoluble fiber. So, basically, soluble and insoluble fiber “live together and peacefully” in our world, in our food. The only kinds of food where you are sure to find differentiated soluble and insoluble fibers are in dietary products or nutrient bars that contain one or the other.

So, the first difference, when comparing soluble vs insoluble fiber, is that the first does dissolve in water and the latter does not. The first dissolves inside your stomach and forms some kind of gel-like substance helping your body in certain areas (7), while the other just adds up to stool and bulks “all things together” helping your body, in his own way (8). One thing they have in common is that they are both helpful to our body and to living a healthy life. While the soluble fiber decelerates digestion, insoluble fiber accelerates it and helps the formation of stool. Still, both “faces of the coin” are needed.

 

Summary: Both soluble and insoluble fiber co-exist in our food. They each have a unique benefit, soluble turning into a gel to lower cholesterol and manage blood sugar, and insoluble “bulking” food together and pushing it through the digestive tract. Soluble and insoluble fiber are both helpful to our body. They work together to decelerate digestion (soluble) and accelerate it (insoluble) to promote balance.

Let’s say you had a big meal rich in soluble fiber. This fiber dissolves inside your stomach and turns into a gel-like substance. This substance has very important absorption properties that help “clean up” our body from several substances which could have a negative impact on our health if they “grow up dangerously fast”. Let’s take, for example, carbohydrates, which enter your bloodstream and can cause dangerous spikes in your blood sugar levels, after eating. Soluble fiber blocks sugar molecules so that they’re not absorbed that fast in your blood which helps keep your glucose levels steady (9). This can be demonstrated better on an example like, say, drinking a glass of just orange juice. The sugar from that is metabolized almost immediately, which means your blood sugar climbs up in an instant. That is not happening if you eat a whole orange, because the fruit also contains soluble fiber and that makes the sugar assimilation more progressive, which is good for your body.

Summary: Soluble fiber dissolves in the stomach and turns into a gel. This gel helps to slow digestion and “clean up” substances that could have a negative impact. When eating or drinking carbohydrates, soluble fiber blocks sugar molecules and slowly releases sugar into the bloodstream. That’s why it’s best to eat whole fiber-rich fruit instead of drinking juice. The gel also absorbs excess dietary fat and cholesterol.

Insoluble fiber, on the other hand, won’t transform into anything in our stomach, but stay pretty much the same. Then it will start moving through the digestive system, taking with it pretty much everything and adding bulk to your stool. This means, your chances of constipation will lower considerably. Insoluble fiber can be beneficial for various digestive problems, like torpid and erratic bowel movements. People suffering from diverticulosis or other afflictions affecting the colon wall should add more insoluble fiber into their diet (10). Of course, first of all, you need to see a doctor, in these cases. Insoluble fiber can provoke a kind of “full feeling” in your stomach, after eating, which might last longer than usual. That is why fiber is recommended to people with weight management problems. They will get the feeling they’re full for longer and thus not eat many snacks after the main meal.  Dietary fiber, mainly soluble fibers, might have the ability to lower serum lipid levels. In 88% of the human studies reviewed, there has been recorded a significant reduction in “the level of serum total cholesterol by soluble fiber” (…)

“Of the studies measuring low-density lipoprotein cholesterol, 41 of 49 (84%) reported significant reductions. No significant changes were reported in 43 of the 57 (75%) studies that reported high-density lipoprotein cholesterol and/or in 50 of the 58 (86%) studies that measured triglyceride levels.” (11)

Summary: Insoluble fiber does not change into anything in the stomach. It starts moving through the digestive system, adding bulk to your stool and sweeping food through the digestive tract. It is beneficial for many digestive issues including diverticulosis, erratic bowel movements, or constipation. Insoluble fiber is also very filling, so is often recommended for weight management.

Is there a link between the quantity of fiber you eat and the reduced risk of a number of health problems that can appear along the way? This is still researched by scientists. So far, we cannot say for sure that a certain amount of insoluble of soluble fiber intake can reduce the risk of a certain disease by X%. We can, however, imply that that could be possible. One research study suggests there is a negative correlation between the amount of fiber you ingest and the chances of getting colorectal cancer (12). Still, this study cannot say if we’ve got to thank the soluble of insoluble fibers for that. We might as well get to thank them both, who knows? Medicine is a process and it won’t ever stop being a process of studying and discovering new things, links, correlations, and mainly cures or supplements that could make our lives easier and healthier. The first step, though, to a healthy life might be a positive attitude, lots of sport, and seeing a doctor when “something wrong” seems to bother you. The internet is not a doctor, but merely a huge book where you can get information, but you’ll never know which part of the information applies to you since you’re not a doctor and cannot “doctor yourself”.  We wanted to make this digression for you to remember about your doctor’s appointment anytime.

Summary: The link between quantity of fiber consumed and reduced risk of health problems is still being researched. A balanced diet rich in both fiber types, soluble and insoluble, is helpful for many reasons including a balanced gastrointestinal tract, healthy bowel movements, reduced cholesterol, balanced blood sugar, and weight management. Focus on eating fiber-dense foods for optimal health.

To conclude, soluble and insoluble fiber are two important components of our dietary regime. They should be part of our meals. As you could see, there are some differences between them, but, all in all, they’re both beneficial to our health, and studies are still being made, trying to uncover more of what nature has in store for us. And there you have it, Soluble vs Insoluble fiber explained in detail.

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Evidence Based

This article is based on scientific evidence, and written, fact-checked & medically reviewed by health experts.

Throughout this article, you'll find scientific references (clickable links to highly trusted peer-reviewed scientific papers, links denoted by the numbers in the parentheses (1, 2, 3)).