Most people assume fiber is a single, simple nutrient, but it’s a little more nuanced than that. In reality, “fiber” is an umbrella term for several types of indigestible carbohydrates that behave differently in the body and deliver entirely different health benefits.
The three main types to understand are soluble fiber, insoluble fiber, resistant starch, and resistant dextrin—and the amount and mix you eat of them influences far more than just digestion.
In this article, learn more about what fiber actually is, how each type works, where to find them in food, and why getting the right balance can support everything from gut function to metabolic health.
What Fiber Actually Is (and Why Your Body Can’t Break It Down)
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Takeaway: Even though you don’t break fiber down, it drives key processes that shape digestion, inflammation, and metabolism.
Fiber is a group of nondigestible carbohydrates that play a powerful role in blood sugar control, cholesterol levels, microbiome diversity, inflammation, and overall metabolic health.
Essentially, fiber is the component of plant foods that our bodies cannot digest. Unlike sugar and starches, which are broken down by enzymes in the gastrointestinal tract, fiber passes through largely intact.
But just because we don’t digest fiber doesn’t mean it isn’t used.
When fiber reaches the colon, it becomes fuel for your gut microbiome—the trillions of bacteria that help regulate digestion, immunity, inflammation, metabolism, and even aspects of mood.
Many types of fiber are fermented by these microbes, producing short-chain fatty acids (SCFAs) like butyrate, acetate, and propionate. These compounds help strengthen the gut lining, regulate inflammation, and support metabolic health.
Not all fibers act the same, though. Their structure determines whether they dissolve in water, whether gut bacteria can ferment them, and what benefits they ultimately deliver.
That’s why understanding the four major types—soluble fiber, insoluble fiber, resistant starch, and resistant dextrin—can help you build a diet that supports digestion, blood sugar, cholesterol, and overall gut and metabolic health.
Soluble Fiber: The Gel-Former
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Takeaway: Soluble fiber works like a gel that balances blood sugar, lowers unhealthy cholesterol, and feeds beneficial gut bacteria.
As the name suggests, soluble fiber is soluble in water—it dissolves in the intestines and forms a thick, gel-like substance. This viscosity is what sets soluble fiber apart and gives it several important health benefits.
First, soluble fiber slows gastric emptying, the rate at which food moves through the stomach and small intestine. By delaying digestion, it helps keep blood sugar steadier after meals and prevents sharp glucose spikes.
Soluble fiber also binds to bile acids, which are made from cholesterol, and helps carry them out of the body. Because the liver must use circulating cholesterol to make new bile acids, this process can lead to lower LDL cholesterol over time.
Finally, soluble fiber is highly fermentable, meaning gut bacteria readily break it down to produce short-chain fatty acids (SCFAs).
These SCFAs help regulate inflammation, strengthen the gut barrier, and play an important role in appetite control and metabolic health.
Common sources of soluble fiber include:
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Oats and oat bran
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Beans and lentils
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Chia seeds and flaxseeds
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Apples
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Avocados
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Citrus fruits
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Berries
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Barley
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Psyllium husk
Insoluble Fiber: The Bulking Agent
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Takeaway: Insoluble fiber keeps digestion moving by adding bulk to stool, preventing constipation, and supporting long-term gut health.
Conversely, insoluble fiber does not dissolve in water. Instead, it adds physical bulk to stool and helps move things along in the digestive tract.
This makes insoluble fiber especially important for regularity and overall digestive comfort.
Because it increases stool volume and speeds up transit time, insoluble fiber helps prevent constipation and supports a consistent digestive rhythm.
It’s also less fermentable than soluble fiber, meaning gut bacteria don’t break it down as readily. While it produces fewer short-chain fatty acids, its mechanical role in digestion is essential.
Insoluble fiber also helps maintain gut health by reducing pressure in the colon, which may lower the risk of issues like diverticulitis and hemorrhoids.
And by keeping things moving, it supports a balanced gut environment and helps remove waste products more efficiently.
Foods that are rich in insoluble fiber include:
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Whole wheat and whole grains
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Wheat bran
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Nuts and seeds
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Carrots, celery, and green beans
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Cauliflower and broccoli
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Kale and spinach
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Fruits and vegetables with edible skins, such as apples, pears, kiwis, and most berries
Resistant Starch: The Microbiome’s Favorite
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Takeaway: Resistant starch feeds your gut bacteria, boosts butyrate production, and supports stronger digestion and metabolic health.
Resistant starch is a unique type of starch that acts more like a fiber, “resisting” digestion in the small intestine and reaching the colon intact. There, it acts as a fuel source for the bacteria in the gut microbiome. Functionally, it behaves like soluble fiber but has some additional benefits.
There are several forms of resistant starch, but the two most common types found in everyday foods are:
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Naturally occurring types: Found in foods like green (less ripe) bananas, oats, barley, and legumes
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Retrograded starch: This type develops when starchy foods like potatoes or rice are cooked and then cooled.
Both types work the same way: resistant starch escapes digestion and becomes fermentation material for beneficial gut bacteria.
Because resistant starch is highly fermentable, it produces large amounts of short-chain fatty acids, particularly butyrate—the preferred energy source for colon cells.
Higher butyrate levels are linked to a stronger gut barrier, reduced inflammation, better insulin sensitivity, and healthier overall metabolism.
Resistant starch may also help balance post-meal blood sugar spikes, increase fullness, and contribute to a more diverse, resilient microbiome.
Common sources of resistant starch include:
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Cooked and cooled potatoes, rice, and pasta
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Oats and barley
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Beans and lentils
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Green bananas and plantains
Resistant Dextrin: Slow Fermentation, Fewer Symptoms
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Takeaway: Resistant dextrin is a soluble, fermentable fiber that supports blood sugar regulation and is easily digested.
Lastly, resistant dextrin is a fermentable, soluble fiber that completely dissolves in water but does not form a thick gel, unlike many other soluble fibers.
It’s produced when starches—most often wheat, corn, or tapioca—are heat-treated and enzymatically modified. This process changes their structure so they resist digestion in the small intestine and reach the colon intact.
While it behaves similarly to resistant starch, resistant dextrin differs in that it remains fully soluble and non-gelling. These properties make resistant dextrin easy to incorporate into foods and beverages—as well as easier on digestion for many people.
Like resistant starch, resistant dextrin is fermented by gut bacteria, producing SCFAs like butyrate. However, it tends to ferment more slowly, which is a benefit: gradual fermentation is associated with fewer digestive symptoms like bloating or gas.
Because of its neutral taste, high solubility, and stability, resistant dextrin is commonly added to fiber supplements, functional beverages, and fortified foods. Common sources of resistant dextrin include:
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Fiber supplements labeled as wheat dextrin, corn dextrin, or resistant dextrin
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Fortified cereals, bars, and beverages
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Some low-carb or “added fiber” products
How Much Fiber Do You Actually Need?
Most people fall far short of the amount of fiber needed for optimal health.
Current recommendations suggest aiming for about 25 grams of fiber per day for women and 38 grams per day for men, or roughly 14 grams of fiber per 1,000 calories eaten.
However, the average adult gets barely half the recommended amount.
Just as important as the amount of fiber is the balance of different types. Many people consume mostly insoluble fiber from wheat-based products but very little soluble fiber or resistant starch, which play key roles in blood sugar control, cholesterol management, and gut health.
Increasing your fiber intake doesn’t need to happen all at once. In fact, ramping up too quickly can lead to bloating or discomfort.
A good goal is to increase gradually—no more than about 5 grams per day—stay well hydrated, and prioritize variety through fruits, vegetables, whole grains, beans, legumes, nuts, seeds, and cooked-and-cooled starches.
Bottom Line: Fiber Isn’t One Size Fits All
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Fiber isn’t one thing—it’s a group of compounds that work differently in the body.
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Soluble fiber slows digestion, steadies blood sugar, and helps lower LDL cholesterol.
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Insoluble fiber adds bulk to stool and supports regular, comfortable digestion.
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Resistant starch feeds beneficial gut bacteria and boosts butyrate for better gut and metabolic health, while resistant dextrin is similar but easier to digest.
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Getting enough fiber matters, but getting a mix of all three types is also important.
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Eating a variety of fruits, vegetables, whole grains, legumes, nuts, seeds, and cooked-and-cooled starches helps support a healthy microbiome and long-term metabolic health.
Author: Cambria Glosz, RD
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References:
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