We all know that fiber is good for us – and our digestive systems. But do we really understand why? I’m here to explain fiber in all its glory.
What IS fiber?
Fiber is actually a carbohydrate! Dietary fiber consists of long strands of carbohydrates that are indigestible in our gut. Fiber greatly contributes to how other foods are digested and absorbed. Fiber’s purpose is to help food maintain its structure and strength. It is designed to be difficult to break down in order to protect food (like a peel or shell).
What does fiber DO?
The human body does not have the proper enzymes to fully digest fiber; it does not provide us with any calories or nutrients. Fiber does 3 main things for us:
- It slows down the rate of digestion and absorption of food in our intestines
- It forms the bulk of our fecal matter
- It keeps waste moving down the intestinal tract after everything has been digested and absorbed
What types of fiber are there?
There are two types of fiber that are equally important in our diets: soluble and insoluble. Soluble fiber dissolves in water, acting like a sponge. It forms a gel-like substance that slows things down in our GI tract, making us feel fuller for longer and preventing spikes in our blood sugar. Insoluble fiber is the opposite; it forms the bulk of our stool and speeds things up in the GI tract. Because of its indigestibility, insoluble fiber can ferment in the large intestine. This fermentation is a good thing. Fiber is a bioactive compound that allows the bacteria that live in our colons to thrive. Healthy and happy gut bacteria aid digestion, absorption, vitamin activation, protection against harmful bacteria, immunity and weight maintenance.
What are net carbs and total carbs?
As mentioned earlier, fiber is an indigestible carbohydrate. But on the nutrition label, it is counted in the total carbohydrates. Total carbohydrate includes all of the carbohydrate in a food (including fiber, starches, and sugars). Why include fiber into our carbohydrate count if it isn’t digested?
Example: A slice of white bread contains 13 grams of total carbohydate, with 0 grams of fiber. A slice of wheat bread contains 16 grams of total carbohydrate with 5 grams of fiber. Because the wheat bread has 5 grams of carbohydrate that is not absorbed, we subtract it from the total carbohydrate count (16 – 5 = 11 grams). So, in essence, we are absorbing less carbohydrate from the wheat bread than from the white bread.
This is what we call net carbs. This system gives us a better representation of how much our bodies are actually absorbing.
Can I eat TOO MUCH fiber?
Too much of each type of fiber can lead to uncomfortable side effects. Too much insoluble fiber can make us gassy and can cause diarrhea. Too much soluble fiber (or not enough insoluble fiber) can cause constipation. It is important to balance our intake of each type of fiber.
What foods are high in fiber?
Some foods high in insoluble fiber are: whole wheat, wheat bran, nuts, seeds, brown rice, zucchini, celery, fruit skins, leafy vegetables, skin of root vegetables and corn.
Foods high in soluble fiber are: oat bran, beans, lentils, peas, psyllium husks, avocados, nuts, flax and chia seeds, and bananas.
Fun fact: fiber can lower your cholesterol!
This one is a little confusing. Fiber, specifically soluble fiber, reduces the bile that is reabsorbed in the GI tract. Bile is a substance produced by the liver and excreted by the gallbladder that helps us absorb dietary fats. Bile is made in the liver using cholesterol. It is stored in the gallbladder and released when there is fat in the GI tract to be absorbed. Dietary fiber prevents the re-absorption of bile in the GI tract (we can re-use bile so we don’t have to keep making it), causing it to be excreted in our fecal matter. The liver is forced to use the cholesterol in our blood to produce more bile, thus decreasing our overall blood cholesterol.
What about juice and smoothies… do they count as fiber?
Juice has been stripped of its fiber; it is one of those beverages that seems healthy, but is actually very high in sugar with little fiber to slow down its absorption. (I like to call it “vitamin soda”)
Smoothies, on the other hand, still contain some fiber. However, the blades destroy the integrity of most of the insoluble fiber – which prevents it from doing its job. When making a smoothie, it is okay to use some whole fruit (with the peel) but mostly vegetables to limit the sugar. Adding some seeds (like flax) will increase the fiber content as well. And don’t use juice!
Now I think we can all agree... Fiber is a Superhero!
Shannon Burke is the Institute for Responsible Nutrition's Online Education Manager and a soon-to-be Registered Dietitian. She recently graduated from UC Davis with honors, one of the nation's top universities for nutrition research.