Insoluble fiber, also known as cellulose, is the part of a plant that can not be broken down by the body’s digestive process and therefore forms into stool to pass out of the body. By adding bulk to the stools, insoluble fiber acts like a scrub brush to help maintain the health of the intestines. Experts recommend 20 – 35 grams of total fiber daily, with the majority of it coming from insoluble fiber sources. The following foods have the highest insoluble fiber content.

Bran Flakes

There is a reason why Grandma swears by her daily bowl of All Bran. Containing nine grams of insoluble fiber per half cup serving, it is the single food with the highest amount of insoluble fiber. Try it with milk for breakfast, mixed with yogurt for a snack, or crumbled on top of a casserole at dinner. Note: not appropriate for those who need to avoid gluten! Read on for gluten-free sources of insoluble fiber.

Lentils

Lentils are a nearly perfect food as they are low in fat while high in protein, vitamins, and fiber, especially insoluble fiber. In a half cup of cooked lentils, there are seven grams of insoluble fiber, roughly 25 percent of your daily requirement.

Popcorn

Popcorn (unbuttered) is known as a dieter’s treat because it mainly consists of insoluble fiber. This means you can eat quite a bit of popcorn without absorbing many calories as the fiber just passes directly through the body and into the stool. In four cups of popcorn, there are three grams of insoluble fiber.

Root Vegetables

Insoluble fiber is concentrated in the skins of root vegetables, such as turnips, sweet potatoes and parsnips. When cooked with their skins, just a half cup serving of turnips contains three grams of insoluble fiber. A half cup serving of cooked sweet potatoes contains 2.5 grams, while the same amount of parsnips contains two grams of insoluble fiber.

Apples

Apples can vary in their content of insoluble fiber, depending on the size and variety of the apple. Most sources site a range of two to four grams of insoluble fiber in one apple. The insoluble fiber is in the skin of the apple, thus applesauce only has one gram of insoluble fiber in a half cup serving.

Almonds

In one ounce of almonds (equal to about twenty nuts), there are two grams of insoluble fiber. Nuts and seeds in general are good sources of both insoluble and soluble fibers, but exercise caution with the portion size as they also are dense in calories from fat.