Fiber

What is the difference between soluble and insoluble fiber?

Updated 4 Nov 2026

Short answer

Soluble fiber dissolves in water and forms a gel, and gut bacteria ferment part of it. Insoluble fiber does not dissolve and mainly adds bulk to stool.[1]

  • Soluble: pectin, gums, mucilage, fructans and some resistant starches. Oats, barley, beans, lentils, apples and citrus fruit are good sources.[1]
  • Insoluble: lignin, cellulose and hemicellulose. Whole wheat, brown rice, nuts, green beans and leafy greens are good sources.[1]

The effects differ too. Soluble fiber lowers LDL cholesterol.[2] In irritable bowel syndrome, soluble fiber helped and bran, an insoluble fiber, did not.[3] In observational studies, however, insoluble fiber tended to be linked to lower death rates more than soluble fiber.[4]

Sources (4)

  1. The Role of Dietary Fiber in Health Promotion and Disease Prevention: A Practical Guide for Clinicians. Daley SF, Vadakekut ES StatPearls, 2026. PMID 32644459
  2. Soluble Fiber Supplementation and Serum Lipid Profile: A Systematic Review and Dose-Response Meta-Analysis of Randomized Controlled Trials. Ghavami A, Ziaei R, Talebi S, Barghchi H, Nattagh-Eshtivani E, Moradi S, et al. Adv Nutr, 2023. PMID 36796439
  3. The effect of fiber supplementation on irritable bowel syndrome: a systematic review and meta-analysis. Moayyedi P, Quigley EM, Lacy BE, Lembo AJ, Saito YA, Schiller LR, et al. Am J Gastroenterol, 2014. PMID 25070054
  4. Dietary fiber intake and all-cause and cause-specific mortality: An updated systematic review and meta-analysis of prospective cohort studies. Ramezani F, Pourghazi F, Eslami M, Gholami M, Mohammadian Khonsari N, Ejtahed HS, et al. Clin Nutr, 2024. PMID 38011755

More about fiber

What is fiber?How does fiber lower cholesterol?How much fiber do you need per day?What is fiber used for?How long does fiber take to lower cholesterol?Is fiber safe?

For education; not medical advice. Talk to a doctor before you start, stop or change a supplement.