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Two Of The Biggest Fiber Myths, Debunked By A Gastroenterologist

Will Bulsiewicz, M.D., MSCI
Gastroenterologist By Will Bulsiewicz, M.D., MSCI
Dr. Will Bulsiewicz, M.D., MSCI is a gastroenterologist and internationally recognized gut health expert. He completed a bachelor’s degree from Vanderbilt University, a medical degree from Georgetown University, and a master's in clinical investigation from Northwestern University.
Bowl with healthy vegetabls and tofu. Ingredients: quinoa, lentils, dinosaur kale, smoked tofu, red cabbage, winter squash, radish, cress sprouts
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In nature, fiber is a part of the plant cellular structure. Plants have a total monopoly on this nutrient. So if you want it, there's only one way to naturally get it: from plants

From a nutritional perspective, fiber is a carb—it's what we would refer to as a complex carbohydrate. If you take multiple sugar molecules and link them together, you'd get fiber. That doesn't mean it behaves like sugar by any means. It doesn't.  

Digestion of refined sugar starts in the mouth, and in about 20 minutes, it's already been absorbed in the small intestine. Meanwhile, fiber remains unblemished as it passes through your mouth, stomach, and even 15 to 20 feet of small intestine, so by the time it reaches your colon, it's the same molecule that went in your mouth.  

Two of the biggest myths about fiber are that all fiber is the same and that it does nothing more than go in one end and shoot out the other. Let's dig deeper:

Myth 1: All fiber is the same, and all you have to do is count grams. 

You've been taught that all fiber is created equal—that whether it's in your breakfast cereal, the milky powder your grandma drinks, or in a granola bar, all forms of fiber are interchangeable. All you need to do is count the number of grams and you're good to go. What you've been told is wrong. 


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The source of the fiber you eat is critically important. The fiber in your cereal or breakfast biscuit is not the same as the fiber in your quinoa. This is conceptually similar to how the source of our fats and our protein determine the impact on our microbiome.

It's an oversimplification to reduce fiber to a number of grams and pretend that all grams are created equal. I have a much better way to source your fiber. We've been taught to count grams of fiber for two reasons. One, it's easy, and we like easy. And two, we have no clue how many types of fiber actually exist in nature.  

It's incredibly difficult to analyze the chemical structure of dietary fiber, and there are 400,000 plants on our planet, 300,000 of them being edible. So there must be hundreds of thousands—if not millions—of types of fiber in nature. But we haven't gotten around to figuring them all out yet. 

Given the complexities in analyzing dietary fiber, we've simplified it by saying there are two basic forms of dietary fiber: soluble and insoluble. You can tell which is which by submerging the fiber in water. If it dissolves, it's soluble. If it doesn't, it's insoluble. In both cases we are talking about huge categories of fiber and that most plants contain some mix of both.


Myth 2: Fiber just passes through us.

If you do a quick Google search on fiber, you'll find the general health benefits of fiber: It contributes to fantastic bowel movements by correcting diarrhea and constipation and increasing the weight and size of your bowel movement, lowers cholesterol, and controls blood sugar. 

These are all great things, and we should be celebrating these health benefits of fiber, for sure. But at the same time, we have been doing the undersell of the century here, folks.  

We've all been taught that fiber pretty much goes in the mouth and out your...well, you know. And along the way it sweeps some stuff out. While there may be some truth to these statements, we're being excessively simple about an incredibly complicated nutrient. So let's take a closer look. 

We humans lack the ability to process fiber by ourselves. Sure, we've got some enzymes called glycoside hydrolases that help us break down complex carbs, but we have only 17 of them—just 17. And none of them are designed for breaking down the larger molecules like fiber. In other words, we are literally incapable of processing fiber on our own.  

Now, if we lived encapsulated in a sterile bubble free from bacteria, we would never know the true power of fiber. But we get by with a little help from our friends. Because guess where you can find lots and lots of fiber and complex carbohydrate-processing enzymes? Yes, in our gut microbiota. 

Compared to the shockingly inadequate 17 that belong to us, our gut microbiota may contain upward of 60,000 of these helpful enzymes.

The fact that our microbiomes contain this insane number of digestive enzymes makes sense when you remember that there are 300,000 edible plants and potentially millions of types of fiber in our diet. 

By outsourcing fiber digestion to our microbes, we are taking advantage of their adaptability. Every single plant, every single type of fiber, requires a unique team of microbes working in concert to get the job done. It's demanding work, but what follows is magic.

From Fiber Fueled by Will Bulsiewicz, published by AVERY, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2020 by Will Bulsiewicz.


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