You’ve likely heard that you need to get more fiber in your diet, but you might not realize that dietary fiber comes in two varieties: soluble and insoluble. 

When you think fiber, you’re probably thinking about non-soluble fiber, also known as roughage. Foods like wheat bran and whole grains bulk up the stool in your intestines, keeping you regular among other crucial duties. 

Soluble fiber plays a similar role in regulating your digestion, but works differently from non-soluble fiber. Here’s what you need to know about the lesser-known dietary fiber. 

What is soluble fiber? 

Dietary fiber is the non-digestible carbohydrate component of a plant-based food. Insoluble fiber stays in its bulky form as it travels through your body, helping food pass through the stomach and intestines more easily. Whole grains, wheat bran, and certain vegetables all contain lots of insoluble fiber.

By contrast, soluble fiber attracts water to your gut, according to the National Library of Medicine, dissolving into a gel-like substance during digestion. Eventually, most soluble fibers reach the large intestine and become fermented by bacteria, which use the resulting short-chain fatty acids to produce nutrients that improve digestive health. 

The outlier is psyllium, a non-fermentable soluble fiber. By not fermenting, it retains water and has the same stool-regulating effect as non-soluble fiber. Common soluble fiber sources include beans, lentils, and oat bran. 

A third type of fiber, resistant starch, occupies a middle ground: It passes through the stomach to the large intestine undissolved, but ferments like a soluble fiber once it gets there, providing the same digestive benefits. Plant foods contain both soluble and insoluble fiber, although the proportion of each type varies by food. 

What does soluble fiber do for me?

Soluble fiber is important to your health, digestive and otherwise. The short-chain fatty acids produced by soluble fiber fermentation feed the good bacteria in your gut, improving gastrointestinal health. The kinetics of soluble fiber digestion also have far-reaching effects beyond your gut. 

For instance, diets rich in soluble fiber have been known to lower LDL (bad) cholesterol levels. Scientists believe that the gel-like fiber traps cholesterol particles in the small intestine and prevents them from being reabsorbed in the large intestine, where they would otherwise raise LDL levels. This association with lower cholesterol has prompted the FDA to endorse soluble fiber as a mechanism for decreasing your risk of coronary heart disease.  The same viscous properties of soluble fiber also prevent your blood glucose levels from spiking after eating, according to the FDA. A New England Journal of Medicine study found that type 2 diabetics who began eating high-fiber diets (25 grams each of soluble and insoluble) each day were able to control their blood glucose more effectively than those who ate 8 grams of soluble and 16 grams of insoluble. These digestion-slowing effects also explain why high-fiber diets tend to make you feel full for a longer period of time. 

How much soluble fiber do I need?

In all likelihood, the answer is "more." According to the Academy of Nutrition and Dietetics, only 5 percent of Americans get enough dietary fiber. The USDA recommendation for dietary fiber is 14 grams per 1,000 calories, which works out to 28 grams for a 2,000-calorie diet and 42 grams for a 3,000-calorie diet. 

The ratio of recommended insoluble to soluble fiber intake is about 3:1, so someone on a 2,500-calorie diet only needs 9 grams of soluble fiber per day. However, you might want to go above the recommendation to enjoy the cholesterol-lowering and blood sugar-regulating benefits found in the research, which has largely been conducted using high-fiber diets. 

Top 19 sources of soluble fiber

Because plant-based foods contain both types of fiber, and because you don’t need as much soluble fiber as insoluble, you’ll likely get enough soluble fiber by increasing your overall fiber intake to hit the USDA recommendation. But if you’re curious to know which foods contain the most soluble fiber, check out this list, sourced from Dietitians of Canada. 

As you can see, adding soluble fiber to your diet is easy (and delicious), but be aware that increasing your fiber intake too quickly can cause GI distress, so add it to your diet gradually over several weeks.