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Soluble vs. Insoluble Fiber: What’s the Difference?

By Charlie Seltzer, MD / February 20, 2016

Because the health benefits of fiber are so widespread, it’s one of the most talked about nutrients. Dietary fiber is comprised of carbohydrates that cannot be digested by humans, as well as lignin, which is a compound that forms the cell wall of plant cells.

Up until very recently, fiber was divided into two broad categories- soluble and insoluble, the classification being based on whether or not it dissolves in water.1 However, researchers are now using other classification systems as well, distinguishing between fibers that exist in whole foods (dietary fiber) and fibers that are extracted or manufactured (functional fiber).

Dietary fiber includes materials derived from plants and well as animals (i.e. chitin, which forms the shells of insects and crustaceans, though people don’t generally eat lobster shells). Functional fiber can be both man-made (i.e. fructooligosaccharides and polydextrose, which are used as food additives) and extracted from natural sources, like the chitin from crustaceans, which is found in nutritional supplements.

Although the different ways fiber is classified is interesting, from a health standpoint, it doesn’t really matter, as the health benefits of a high fiber diet are independent of the way a scientist groups them.

What Are Some Of The Health Benefits Of Fiber?

What Are The Best Sources Of Fiber?

Fruits, vegetables, whole grains, beans, lentils, nuts and seeds are all good sources of fiber. Supplemental fiber is a great, convenient way to up daily fiber intake, especially for those who are restricting calories or eat high amounts of processed food, which is essentially always stripped of fiber.
The Food and Nutrition Board of the Institute of Medicine published recommended daily fiber intakes in 2001, and those recommendations can be found here: IOM Recommended Fiber Intake.

For the vast majority of people, however, more fiber is probably better (though always check with your doctor before making dietary changes). Since abrupt increases in fiber can cause gas, bloating and abdominal pain, it is wise to increase fiber intake slowly.

If you decide to take fiber supplements, I recommend Vitafusion Fiber Gummies (2 gummies provide 5 grams of soluble fiber and 10 calories) or psyllium husk, which is a mix of soluble and insoluble fiber.
Hope this clears up any fiber confusion you may have had – feel free to leave a question or comment.

Show 4 References

  1. Solubility, or the ability to disperse in water, was originally thought to predict fibers’ physiologic effects. Research revealed this was not always the case. However, the terms soluble and insoluble fiber are still used for labeling purposes by the FDA, and are the most widely used terms by nutritionists and dietitians.
  2. Wolever TM, Tosh SM, Gibbs AL, et al. Physicochemical properties of oat beta-glucan influence its ability to reduce serum LDL cholesterol in humans: a randomized clinical trial. Am J Clin Nutr. 2010;92:723-732.
  3. Cummings JH. The effect of dietary fiber on fecal weight and composition. In: Spiller GA, ed. Fiber in Human Nutrition. 3rd ed. Boca Raton: CRC Press; 2001:183-252.
  4. Although no fiber can be absorbed, certain types are fermented by gut bacteria, forming fatty acids which can be absorbed and used as energy.

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3 Comments

  • James says:

    Is there a specific amount of fiber we should aim to consume each day? Also, in a typical garden salad (about1.5-2 cups), how many grams of fiber would be present?

    • Charlie Seltzer says:

      I would shoot for at least 40 grams per day. It is tough to give a number for the amount of fiber in a typical garden salad, as the different amounts of ingredients will influence content. A garden salad from McDonald's contains 3 grams of fiber. Hope this helps.

  • pratap says:

    thanks for the info

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