Soluble vs Insoluble Fiber: What's the Difference?

Soluble fiber soaks up water and turns into a gel as you digest it. That slows digestion and helps lower cholesterol. Insoluble fiber does not dissolve. It adds bulk to stool and helps food move through your gut. Most people need both, and you get both by eating a mix of beans, whole grains, fruit, vegetables, nuts and seeds.

This page uses MedlinePlus (the NIH's health library), Harvard's Nutrition Source and the FDA. The sources are listed at the bottom. It is general information, not medical advice.

Soluble vs insoluble fiber at a glance

	Soluble fiber	Insoluble fiber
In water	Dissolves and turns to gel	Does not dissolve
What it does	Slows digestion	Adds bulk to stool and helps food pass through
Main benefit	Helps lower cholesterol and blood sugar	Helps with regularity
Good sources	Oats, oat bran, barley, beans, peas, chia seeds, apples, blueberries, psyllium	Wheat bran, whole wheat, brown rice, quinoa, kale and other leafy greens, almonds, walnuts, fruit with edible skins

What soluble fiber does

MedlinePlus says soluble fiber "attracts water and turns to gel during digestion. This slows digestion." It adds that soluble fiber helps lower cholesterol, which may help prevent heart disease.

Harvard's Nutrition Source says soluble fiber can help lower both blood sugar and blood cholesterol.

The FDA lets some foods carry a heart health claim for soluble fiber. The claim needs 3 grams or more a day of beta-glucan (the soluble fiber in oats and barley) from whole oats or barley, or 7 grams or more a day of soluble fiber from psyllium husk. It also has to be part of a diet low in saturated fat and cholesterol.

For grams of soluble fiber per serving in beans, oats, fruit and more, see the soluble fiber foods chart.

What insoluble fiber does

MedlinePlus says insoluble fiber "adds bulk to the stool and appears to help food pass more quickly through the stomach and intestines."

Harvard says insoluble fiber helps food move through your digestive system and promotes regularity.

Harvard also notes that diets low in fiber, especially insoluble fiber, may raise the risk of type 2 diabetes.

Foods high in each type

Soluble fiber foods

MedlinePlus lists oat bran, barley, nuts, seeds, beans, lentils, peas, some fruits and vegetables, and psyllium, a common fiber supplement. Harvard adds oatmeal, chia seeds, apples and blueberries.

Insoluble fiber foods

MedlinePlus lists wheat bran, vegetables and whole grains. Harvard names whole wheat products (wheat bran most of all), quinoa, brown rice, legumes, leafy greens like kale, almonds, walnuts, seeds, and fruit with edible skins.

Some foods show up on both lists. Beans, nuts, seeds and fruit give you both kinds. So you do not have to pick one type. A plate with beans, a whole grain and a vegetable covers both.

To compare foods by total fiber, see 50 high-fiber foods ranked or look up any food in the fiber library.

Which one do you need more of?

Both. The main daily targets are for total fiber, not for each type. Harvard says children and adults need at least 25 to 35 grams of fiber a day, and most Americans get only about 15 grams. There is a full breakdown by age and sex in how much fiber per day, and the fiber calculator works out your own number.

The type can matter more for some goals:

• Lowering cholesterol: soluble fiber is the type MedlinePlus and the FDA link to lower cholesterol and heart health.
• Staying regular: insoluble fiber adds the bulk. Harvard says eating more fiber from fruits, vegetables and whole grains, drinking more water and regular exercise all help with constipation.
• IBS: the advice is different here. I covered what the guidelines say in tracking fiber with IBS or constipation. Talk to your doctor first.

How to add more fiber without the bloating

Harvard suggests adding fiber slowly, because a big jump can cause bloating and cramping. Drinking more fluids as you eat more fiber helps too.

Food comes first. Harvard says a supplement like psyllium can help if food alone is hard, but supplements are not meant to replace high-fiber foods.

A few easy swaps:

• Oatmeal for breakfast instead of a refined cereal (soluble).
• Brown rice or quinoa instead of white rice (insoluble).
• Beans or lentils in soup, chili or salad (both).
• Eat apples with the skin on (both).
• A handful of almonds or walnuts as a snack (both).

Do you need to track them separately?

For most people, no. The daily targets are for total fiber, and a varied diet gives you both types.

I build Grove, a fiber tracker for iPhone. Grove counts total fiber from USDA data. It does not split soluble from insoluble fiber. It shows how close you are to your daily goal and which foods got you there.

Frequently asked questions

What is the main difference between soluble and insoluble fiber?

Soluble fiber dissolves in water and turns to gel, which slows digestion and helps lower cholesterol. Insoluble fiber does not dissolve. It adds bulk to stool and helps food move through your gut.

Is oatmeal soluble or insoluble fiber?

Oats are a soluble fiber food. Harvard lists oatmeal as a source, MedlinePlus lists oat bran, and the FDA's heart health claim names beta-glucan, the soluble fiber in whole oats. For grams per bowl, see the soluble fiber foods chart.

Which fiber is better for constipation?

Insoluble fiber. It adds bulk to stool, and Harvard says it promotes regularity. Harvard also points to more water and regular exercise. If constipation goes on, see a doctor.

Which fiber lowers cholesterol?

Soluble fiber. MedlinePlus says it helps lower cholesterol, and the FDA allows a heart health claim for soluble fiber from whole oats, barley and psyllium.

Does Grove track soluble and insoluble fiber?

No. Grove tracks total fiber from USDA data. You can use the lists on this page to choose foods, then log them in Grove to see your total.

Sources

• MedlinePlus. Soluble vs. insoluble fiber. US National Library of Medicine. Reviewed August 2026.
• Harvard T.H. Chan School of Public Health, The Nutrition Source. Fiber.
• US Food and Drug Administration. 21 CFR 101.81: Health claims, soluble fiber from certain foods and risk of coronary heart disease.