What’s the Difference Between Soluble Fiber and Insoluble Fiber, Exactly?

Everyone needs fiber — possible more fiber than you might think. While it's best known for promoting regular bowel movements, fiber can also help control blood-sugar levels, and certain types of fiber can reduce levels of LDL ("bad") cholesterol, associated with heart disease.

We often hear about the importance of adding high-fiber foods or fiber supplements to our diet. But when the different types of fiber get brought up, things can get more confusing. There are two types of fiber: soluble fiber and insoluble fiber. Both offer unique benefits and value and can be found in everyday foods like oats (soluble fiber) and whole grains (insoluble fiber). But what's the difference between soluble and insoluble fiber, exactly, and which one should you focus on adding to your diet?

We asked a dietitian that exact question, and she filled us in on what differentiates soluble from insoluble fiber, along with what foods are high in each, so you can incorporate these healthy nutrients into your diet.

What Is Soluble Fiber?

"Soluble fiber is a type of dietary fiber that dissolves in water to form a gel-like substance," says Lauren Manaker, MS, RDN. "This quality allows it to bind with fatty acids and prolong stomach emptying times, which may help to regulate sugar absorption and lower certain cholesterol." Soluble fiber is often found in a variety of foods including fruits, vegetables, legumes, and grains (more on that later!).

"It feeds beneficial gut bacteria, contributing significantly to overall gut health and enhancing the body's immune function," Manaker tells PS. Plus, it can help prevent certain health conditions including heart disease and diabetes, as well as certain gastrointestinal disorders.

What Is Insoluble Fiber?

Insoluble fiber does not dissolve in water. "Instead, it adds bulk to the stool and helps to move food through the digestive system more quickly, aiding in regular bowel movements and preventing constipation," Manaker says. Foods that contain insoluble fiber include whole grains, wheat bran, nuts, seeds, and the skin of many fruits and vegetables.

Soluble Fiber vs. Insoluble Fiber

"When comparing soluble and insoluble fiber, the main differences lie in their physical properties and benefits to digestive health," Manaker explains.

Again: soluble fiber dissolves in water, forming a gel-like material that plays a role in lowering blood cholesterol and glucose levels. Insoluble fiber does not dissolve in water, adding bulk to the diet which can help promote the passage of food through the digestive system.

Both types of fiber affect the GI system. Soluble fiber slows the digestion of food, which is why it can benefit glucose levels; it reduces sudden spikes in glucose caused by foods digesting super quickly. That doesn't mean soluble fiber will constipate you, though. In fact, it can help make bowel movements softer and easier to pass. Insoluble fiber, though, helps foods move more quickly through the digestive system, which also prevents constipation.

Soluble Fiber Foods

Want to incorporate more soluble fiber into your diet? Manaker suggests the following foods, all rich in soluble fiber:

  • Oats
  • Peas
  • Beans
  • Apples
  • Citrus fruits
  • Carrots
  • Barley
  • Psyllium

Insoluble Fiber Foods

If it's insoluble fiber you need more of, Manaker lists the following to stock up on:

  • Whole grains
  • Wheat bran
  • Nuts
  • Beans
  • Cauliflower
  • Green beans
  • Potatoes

Which Is Better?: Soluble Fiber vs. Insoluble Fiber

Soluble and insoluble fiber both offer unique benefits to the body, so you ideally want to get a healthy mix of both in your diet. You can talk to your doctor about whether one or the other may better serve your unique health needs, but "each serves important, complementary functions in maintaining overall health," Manaker says.

Women should be getting about 25 grams of fiber daily, according to the National Institutes of Health. And while most experts suggest just aiming to hit this number without worrying too much about exactly how much of each type you're getting, UCSF Health notes that a good goal is to get about a quarter of your total intake from soluble fiber, and the rest from insoluble.

Alexis Jones is the senior health and fitness editor at PS. Her areas of expertise include women's health and fitness, mental health, racial and ethnic disparities in healthcare, and chronic conditions. Prior to joining PS, she was the senior editor at Health magazine. Her other bylines can be found at Women's Health, Prevention, Marie Claire, and more.