Soluble Fiber vs Insoluble Fiber: What Is the Difference?

Soluble fiber vs insoluble fiber comes down to what you want from your fiber. Soluble fiber dissolves in water and turns into a gel. Insoluble fiber does not dissolve and mostly adds bulk. Most women need more of both: women in the US eat an average of 15.2 grams of fiber a day, according to USDA survey data.

This guide helps you decide which type to lean on, and why you rarely have to choose. For the full picture of daily targets, see our complete guide to fiber for women.

Soluble fiber vs insoluble fiber: who should pick which?

Lean on soluble fiber for cholesterol and on insoluble fiber for stool bulk, and use both if you simply want to eat more fiber. Here is how that sorts out:

• You want to lower LDL cholesterol a little: choose gel-forming soluble fiber, such as oats, barley or psyllium.
• You want softer, bulkier stools: choose coarse insoluble fiber such as wheat bran, or gel-forming soluble fiber such as psyllium. Both add water to stool.
• You just want to reach your daily target: eat a mix of whole grains, beans, fruit and vegetables. You do not need to count each type.

If you have a digestive condition or take daily medicines, ask your doctor before you change your fiber.

How do soluble and insoluble fiber work differently?

Soluble fiber soaks up water and forms a gel, and insoluble fiber stays mostly intact and adds bulk. A review for clinicians puts it this way: “Soluble fiber dissolves in water and is partially fermented in the colon, while insoluble fiber does not dissolve and mainly adds bulk to the stool.”

The gel from soluble fiber slows how fast the stomach empties and binds bile acids, which is one route to a modest drop in LDL cholesterol. Insoluble fibers, which include lignin, cellulose and hemicellulose, increase stool bulk and reduce transit time through the gut.

Soluble vs insoluble	Soluble fiber	Insoluble fiber
How it works	Dissolves, forms a gel, partly fermented by gut bacteria	Does not dissolve, adds bulk
What trials found	Small LDL drops: 2 to 10 g a day in 67 trials	Heavier stools: 3.7 g more per extra gram of wheat fiber
Where it comes from	Oats, barley, beans, apples, citrus, chia seeds	Whole wheat, brown rice, nuts, leafy greens, bran
Watch out for	Gas as bacteria ferment it	Fine bran can be constipating

What do trials show for each type?

Trials show small, real effects that depend on the kind of fiber more than the label.

In a meta-analysis of 28 trials with 1,924 people, about 10.2 grams a day of psyllium, a gel-forming soluble fiber, lowered LDL cholesterol by 0.33 mmol/L. The authors graded that evidence as moderate quality because the trials did not all agree. An earlier meta-analysis of 67 controlled trials linked 2 to 10 grams a day of soluble fiber with small drops in LDL. It called the effect “small within the practical range of intake.”

On the insoluble side, a systematic review of 65 intervention studies in healthy people found that each extra gram a day of wheat fiber added 3.7 grams to daily stool weight. In people whose gut transit was slow, over 48 hours, each gram also cut transit time by 0.78 hours.

A 2017 review adds a catch. Cholesterol and blood-sugar benefits tracked how thick the fiber gets, so thin soluble fibers such as inulin and insoluble fibers such as wheat bran did not show them. A 2019 review argues fiber should be sorted by fermentability, not just solubility. Read more on how soluble fiber works and on psyllium husk for constipation.

Which foods have soluble fiber and which have insoluble fiber?

Almost every fiber-rich plant food has both, with one type usually leading. The clinician review lists these examples:

• Mostly soluble: oats, barley, rye, citrus, plums, peaches, carrots and sweet potatoes.
• Mostly insoluble: whole wheat, brown rice, bulgur, corn bran, cauliflower, celery, green beans and leafy greens.
• Listed under both: apples, pears, beans, lentils, chickpeas, chia seeds and broccoli.

Can you use both soluble and insoluble fiber?

Yes, and eating both is the simplest plan. The 2020 to 2025 Dietary Guidelines set women’s fiber targets at 28 grams a day at ages 19 to 30, 25 grams at 31 to 50, and 22 grams from 51. Those are for total fiber. No official target exists for soluble or insoluble fiber alone.

A woman aged 31 to 50 who eats the average 15.2 grams is about 9.8 grams short. Closing that gap with a mix of oats, beans, whole grains, fruit and vegetables covers both types without any counting. Gut bacteria turn fermentable fiber into short-chain fatty acids, which is one reason variety matters. See our guide to gut health for women, or browse the fiber category.

Do soluble and insoluble fiber have different side effects?

Both can cause gas and bloating when you add them too fast, and bran has its own catch. In a 2022 review of 16 trials with 1,251 adults, 66% of people with chronic constipation responded to fiber, against 41% on a placebo. Gas was more common with fiber. Results looked best with psyllium at doses above 10 grams a day, taken for at least 4 weeks.

The 2017 review also warns that some fibers can be constipating, such as “fine/smooth insoluble wheat bran particles.” An older meta-analysis of 20 papers found that people who were constipated responded less well to bran than people who were not. Add fiber slowly over a few weeks, drink water with it, and ask your doctor or pharmacist before starting a supplement. You can also read how we check the research.

Frequently asked questions

Is soluble or insoluble fiber better for constipation?

Both can help, because gel-forming soluble fiber such as psyllium and coarse insoluble fiber such as wheat bran each add water to stool. A 2022 review of 16 trials found the best results with psyllium above 10 grams a day.

Is soluble or insoluble fiber better for cholesterol?

Soluble fiber has the trial evidence. Gel-forming types such as oat beta-glucan and psyllium lowered LDL cholesterol by small amounts, while insoluble fibers such as wheat bran did not show that effect.

Do most foods have both types of fiber?

Yes. The same clinician review lists apples, pears, beans, lentils, chickpeas, chia seeds and broccoli under both soluble and insoluble fiber, so a varied diet covers both.

How much soluble and insoluble fiber do you need?

There is no official target for each type. The 2020 to 2025 guidelines set total fiber for women at 22 to 28 grams a day by age.

Written by the Health Science Mag Editorial Team.

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

This article is for education and is not medical advice. Talk to your doctor before changing your diet, supplements, or medication.