Guide
Soluble vs Insoluble Fiber: What’s the Difference?
Soluble fiber dissolves in water and forms a gel; insoluble fiber does not and adds bulk. Most plant foods contain both. Here is what each does and where to find it.
| Soluble fiber | Insoluble fiber | |
|---|---|---|
| In water | Dissolves, forms a gel | Does not dissolve |
| Main effects | Slows digestion, steadies blood sugar, lowers LDL cholesterol, feeds gut bacteria | Adds bulk, speeds transit, helps regularity |
| Rich sources | Oats, barley, beans, lentils, chia, flax, psyllium, apples, citrus | Wheat bran, whole grains, nuts, vegetable and fruit skins, cauliflower, green beans |
| Examples | Beta-glucan, pectin, inulin, gums | Cellulose, lignin, some hemicellulose |
Soluble fiber
Soluble fiber absorbs water in the gut and turns into a thick gel. That gel slows how quickly your stomach empties and how fast sugar enters your blood. Viscous types like beta-glucan in oats and barley bind bile acids, which is why oats can carry an approved heart-health claim for lowering cholesterol. Many soluble fibers are also fermentable, meaning gut bacteria feed on them.
Good everyday sources: Rolled oats, dry, Barley, cooked, Black beans, cooked, Lentils, cooked, Chia seeds, Flaxseed, ground, Apple, with skin, Orange.
Insoluble fiber
Insoluble fiber passes through mostly intact. It holds onto water and adds bulk, which helps food move through your gut and keeps you regular. It is concentrated in the outer layers of grains (bran) and in skins and seeds.
Good everyday sources: Wheat bran, All-Bran cereal, Whole-wheat bread, Bulgur, cooked, Almonds, Cauliflower, cooked, Green beans, cooked, Pear.
Prebiotic and resistant starch
Two more terms come up in fibermaxxing. Prebiotic fiber (like inulin in garlic, onions, leeks and Jerusalem artichokes) is fermented by beneficial bacteria. Resistant starch escapes digestion and behaves like fiber; cooked-and-cooled potatoes, rice and pasta, green bananas and beans are good sources.
Do you need to track them separately?
No. Hitting your total fiber target from a mix of beans, whole grains, fruit, vegetables, nuts and seeds covers both. Track the total in the fiber calculator.
FAQ
Is soluble or insoluble fiber better?
Neither. You need both, and most high-fiber foods contain a mix. Eating a variety of plants covers both without counting them separately.
Which fiber helps constipation?
Insoluble fiber from wheat bran, whole grains and vegetable skins adds bulk and speeds transit. Psyllium, a soluble fiber, also helps because it holds water and softens stool.
Which fiber lowers cholesterol?
Viscous soluble fiber, such as beta-glucan in oats and barley and the fiber in psyllium and beans, lowers LDL cholesterol.
Why doesn’t this site show soluble and insoluble grams?
The USDA database reports total dietary fiber for most foods, not the split. Values for soluble and insoluble fiber vary a lot between sources, so we show total fiber and describe which foods lean which way.