Which foods are insoluble fiber sources?

• A. Bananas, citrus fruits, olive oil
• B. Oats, peas, beans, apples
• C. Whole-wheat flour, wheat bran, nuts, beans and vegetables such as cauliflower, green beans, and potatoes.
• D. Rice, corn, millet, oats

Answer: (C)

Insoluble fiber sources are foods that provide non-digestible parts like cellulose and lignin which don’t dissolve in water and help bulk up stool and speed intestinal transit. The best match includes bran and whole-grain products, nuts, and vegetables with edible skins, all of which contribute significant insoluble fiber.

Whole-wheat flour and wheat bran are classic insoluble fiber staples. Nuts add insoluble fiber components, and beans provide insoluble fiber in the parts of the seed that aren’t easily broken down. Vegetables such as cauliflower, green beans, and potatoes (especially with the skin) also supply insoluble fiber. Together, these foods embody the type of fiber that increases stool bulk and promotes regularity.

Other options lean more toward soluble fiber, which forms gels in the gut rather than adding bulk. Oats are high in soluble beta-glucan; apples and citrus fruits contain soluble pectin; olive oil has no fiber; rice and millet have relatively less insoluble fiber.