Soluble Fiber vs. Insoluble Fiber: What's The Difference & Do I Need Both?

Certified holistic nutrition consultant By Lindsay Boyers
Certified holistic nutrition consultant
Lindsay Boyers is a nutrition consultant specializing in elimination diets, gut health, and food sensitivities. Lindsay earned a degree in food & nutrition from Framingham State University, and she holds a Certificate in Holistic Nutrition Consulting from the American College of Healthcare Sciences.
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If you've read any nutrition-focused articles or browsed a grocery aisle in the last few years, you've probably come across some of the buzzy benefits of fiber (Fills you up! Aids digestion! Etc.!). But there's one thing about fiber that's not quite so advertised: the difference between soluble fiber and insoluble fiber. Seriously, what the heck is soluble fiber, and how do you make sure you're getting enough?

We talked to a handful of registered dietitians to get the lowdown on the different types of fiber—what they are and what they do—and compiled some of their best advice for making sure you get what you need every day.

Soluble vs. insoluble fiber.

Fiber, which is found exclusively in plants, is a type of carbohydrate that your body cannot fully digest. There are two main types of fiber—soluble (also called viscous) and insoluble (also called bulking). Your body handles each type of fiber differently, and, because of that, they play different roles in your health. 

"Soluble fiber dissolves into a gel-like substance that is important to help capture and remove toxins and cholesterol from the body," explains Amy Shapiro, M.S., R.D., CDN, and founder of Real Nutrition. Insoluble fiber is not digested by the body or absorbed into the bloodstream. Instead, it forms the "weight" and "bulk" of stools in the body, acting as a "broom."

Both forms can help you feel full by creating volume in the digestive tract that lets your brain know you've eaten, but each also has specific benefits that are tied to how your body handles them.

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Benefits of soluble fiber.

Soluble fiber dissolves in water and slows down digestion, controlling how and when certain nutrients, like carbohydrates, are absorbed in the body. This can help prevent big surges of glucose from entering the bloodstream at once and the resulting blood sugar spikes. Steady blood sugar can keep your hunger in check and prevent mood swings. It also translates to steadier levels of insulin and a smaller chance that you'll develop insulin resistance.

Soluble fiber also supports heart health and reduces your risk of heart disease by decreasing total and LDL cholesterol levels. It's also a great healthy weight-management tool. Soluble fiber blocks the absorption of fats and forms a viscous gel in your digestive tract that makes you feel full longer. 

But soluble fiber has some indirect effects too. Soluble fiber is an excellent source of food for the bacteria that live in your gut. When soluble fibers get to the colon, the bacteria feed on them, creating short-chain fatty acids, or SCFAs, like butyrate, acetate, and propionate. These SCFAs can provide energy, improve metabolism, regulate blood sugar levels, and fight off chronic inflammation.

Benefits of insoluble fiber.

Opposite of soluble fiber, insoluble fiber, which doesn't dissolve in water and passes through the digestive system intact, speeds up the rate of digestion. When insoluble fiber moves through the digestive tract, it pulls water into the stool, making it softer and easier to pass. This can help alleviate constipation and also improve overall digestive health and function.

Insoluble fiber may also help you maintain a healthy weight. In one study, participants who swapped their low-fiber breakfast cereal for a cereal high in insoluble fiber had decreased appetites and ate fewer calories throughout the day.

And since insoluble fiber doesn't get broken down by the body, it has no effect on your blood sugar levels, according to Shapiro.

Both types of fiber can also work together to help lower your risk of diabetes, metabolic syndrome, and certain types of cancer.

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Examples of foods with soluble fiber.

Many high-fiber foods contain a combination of both soluble and insoluble fiber. For example, one avocado contains about 9 grams of fiber70% of that is insoluble fiber and the remaining 30% is soluble fiber. That being said, some mostly soluble-fiber foods include:

  • Oatmeal
  • Blueberries
  • Brussels sprouts
  • Broccoli
  • Sweet Potatoes
  • Carrots
  • Turnips
  • Nuts
  • Beans
  • Lentils
  • Apples
  • Pears
  • Figs
  • Nectarines
  • Apricots
  • Flaxseeds
  • Barley
  • Psyllium husk

Examples of foods with insoluble fiber.

Some foods that are primarily sources of insoluble fiber are:

  • Avocados
  • Seeds
  • Fruit & veggie skins
  • Dark leafy greens, like spinach, kale, and arugula
  • Whole wheat bread
  • Amaranth
  • Brown rice
  • Wheat bran
  • Beans
  • Cauliflower
  • Potatoes
  • Green beans
  • Chia seeds
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How much soluble fiber do you need?

Low fiber intake can create immediate problems like constipation, weight gain, and sluggishness, but it can also create long-term health issues like an increased risk of heart disease. And since fiber is found in some of the most nutrient-dense foods, falling short on your fiber needs could mean that you're missing out on other important nutrients, too.

Yet, on average, Americans eat only 16 grams of fiber per day, much less than the recommended 25 daily grams for women and 38 grams for men. While there are no specific recommendations for exactly how much soluble and insoluble fiber you should eat, most health experts recommend including a variety of foods in your diet to make sure you're getting both types.

How to increase your fiber intake.

So, what's the best way to increase your daily fiber intake? "Add more plants to your diet," says Shapiro. "Try to incorporate plant foods at every meal (whole grains, nuts, seeds, fruits, and veggies). That can be oats or a smoothie at breakfast, salad at lunch, trail mix or fresh fruit for a snack, and cooked veggies at dinner."

To make sure you're getting adequate amounts of both types of fiber, Nichole Dandrea, M.S., RDN, registered dietitian and author of The Fiber Effect, takes it one step further: "Shoot for at least three colors, if not more, per meal," she says, "choosing from green, red, purple, yellow, orange, blue, tan." To translate that into food, think broccoli, tomato, cabbage, squash, carrots, blueberries, oatmeal, respectively. A meal example might look like a grain bowl with farro, broccoli, radish, carrots, and red onion.

In addition to eating more vegetables, you can also make the foods you're already eating more fiber-rich with a few small tweaks. 

"An easy, convenient way to increase fiber in one's diet is using ground flaxseeds and chia seeds," Carrie Kirkland, R.D., L.D., a bariatrics dietitian at Morrison Healthcare, tells mbg. "They are both versatile ingredients with minimal texture and taste. Two tablespoons of chia seeds have 8 grams of insoluble fiber, and 2 tablespoons of ground flaxseeds contain 4 grams of fiber. If you're struggling with dietary fiber intake, try adding them to your yogurt, smoothies, protein shakes, [or] cottage cheese."

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Should you take fiber supplements?

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"Ideally, we want to get [fiber] from foods," says Shapiro. "If you have a hard time meeting that through whole grains, nuts, seeds, fruit, and veggies, then you can add a supplement."

If you do decide to go for a fiber supplement, a high-quality greens powder with a fiber blend is a good option.

Kirkland warns against flavored fiber powders that have a lot of added sugar, which is something that you don't want. Because of this, it's important to read your supplement labels and make sure you're only getting the good stuff.

Avoid any fiber supplements with the following words, which are all fancy names for sugar, on their ingredient list:

  • Agave nectar
  • Brown rice syrup
  • Brown sugar
  • Cane sugar
  • Coconut sugar
  • Corn syrup (high-fructose corn syrup)
  • Dextrose
  • Evaporated cane juice
  • Fructose
  • Fruit juice concentrate
  • Glucose
  • Maltodextrin
  • Sucrose
  • Sugar
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Increase your fiber intake slowly.

Whether you aim to include more whole plant foods in your diet or you decide to go for a supplement, it's important to make sure you're slowly increasing your intake gradually over the course of a few weeks, especially if your diet is fairly low in soluble fiber now. This gives the bacteria in your gut time to adjust to the new food source.

Eating too much soluble fiber too fast can create excess gas and lead to uncomfortable symptoms, like bloating, stomach pain, and diarrhea

"Try increasing fiber intake by 5 grams each week, and make sure to drink plenty of water," says Dandrea. "If increasing fiber intake with beans, then be sure to soak them overnight first to reduce gas-forming compounds. Cooking them with a pressure cooker and with a piece of kombu seaweed can also help to reduce the gas-forming compounds."

The bottom line.

Both soluble and insoluble fiber are important components of a healthy diet. Soluble fiber helps balance blood sugar levels and keep you full, while insoluble fiber can alleviate constipation and keep you regular. Yet, even though fiber has so many health benefits, most people aren't getting close to enough. 

The best way to make sure you're meeting your needs is by including a variety of plant-based foods, like colorful veggies, nuts, and seeds, in your diet. If your diet is really low in fiber now, make sure you're increasing your intake gradually, over the course of several weeks, so you don't experience any uncomfortable digestive symptoms as your body adjusts.

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