Soluble vs. Insoluble Fiber: What Each Does and Where to Find It

The Quick Difference

Fiber comes in two kinds, and the names tell you most of the story:

• Soluble fiber mixes with water and turns to gel as you digest it.
• Insoluble fiber doesn’t dissolve in water. It adds bulk.

Here’s the part that surprises people: most high-fiber foods have both. MedlinePlus, the U.S. National Library of Medicine’s health site, says some foods just have more of one kind than the other.

What Soluble Fiber Does

Soluble fiber attracts water and turns into a gel during digestion. According to MedlinePlus, that gel slows digestion down.

Soluble Fiber Foods

MedlinePlus lists these as sources of soluble fiber:

• Oat bran and oats
• Barley
• Nuts and seeds
• Beans, lentils and peas
• Some fruits and vegetables

How much fiber those foods have in total, per serving (USDA FoodData Central):

Food	Serving	Total fiber
Chia seeds	1 oz	9.8 g
Navy beans, cooked	½ cup	9.6 g
Lentils, cooked	½ cup	7.8 g
Barley, pearled, cooked	1 cup	6.0 g
Oats, dry	½ cup	4.1 g
Almonds	1 oz	3.5 g

These are total fiber numbers, both kinds together. USDA’s standard database doesn’t split most foods into soluble and insoluble, so I’m not going to guess.

What Insoluble Fiber Does

Insoluble fiber doesn’t dissolve. MedlinePlus says it adds bulk to stool and helps food move more quickly through your stomach and intestines.

If soluble fiber is the gel, insoluble fiber is the bulk.

Insoluble Fiber Foods

MedlinePlus names wheat bran, vegetables and whole grains. That points you to foods like these (total fiber per serving, USDA FoodData Central):

Food	Serving	Total fiber
Bulgur, cooked	1 cup	8.2 g
Collard greens, cooked	1 cup	7.6 g
Whole-wheat spaghetti, cooked	1 cup	4.6 g
Spinach, cooked	1 cup	4.3 g
Brown rice, cooked	1 cup	3.2 g
Whole-wheat bread	1 slice	1.9 g

Skins matter here. Peeling fruits and vegetables takes away some of their fiber, so leave the peel on your apples and potatoes.

Do You Need to Track Both?

No. There’s no separate daily target for each kind. The official U.S. targets from the National Academies, 21 to 38 grams a day for adults, are for total fiber. (Here’s how much you need.)

Because most high-fiber foods carry both kinds, eating a mix gets you both without thinking about it. MedlinePlus puts it simply: eat a variety of vegetables, fruits, whole grains, and beans.

A good week might include oats or barley, a few servings of beans, berries, a cooked green and some whole grains. That covers both kinds. The high-fiber foods list has 44 options with grams for each.

One more nice fact: fiber is a carbohydrate, but MedlinePlus says it provides about half the calories of other carbs, around 2 calories per gram instead of 4.

Where Fiber Supplements Fit

Supplements give you whatever fiber is in them, and the label tells you what that is. Food gives you a mix of both kinds, plus everything else in the food.

My take: start with food. If you still want a supplement, the supplements section is where I’ll cover them. I only call a product “tested” after I’ve published a real test with my own photos.

Whichever way you go, add fiber slowly and drink enough water. The 30-Day Fiber Ramp shows you how.