What is insoluble fiber?
Insoluble fiber is a component of dietary fiber, consisting of water insoluble, metabolically inert components. For this reason, insoluble fiber is often known as a bulking agent, aiding in the digestive process while providing health benefits related to reducing serum lipid levels. Insoluble fiber is not a source of calories.
Insoluble fiber is best defined as the organic fraction of the diet that is indigestible or slowly digesting and occupies space in the gastrointestinal tract.1 Additionally, insoluble fiber is defined as a mixture of chemically complex polysaccharides which cannot be digested by the small intestine. Resistant starch is considered an insoluble fiber.
As an integral component of plant cells, insoluble fiber is one of the oldest nutritive and structural components on Earth. For instance, cellulose is a form of insoluble fiber. Additionally, chitin, a component of fungi, insects and crustaceans, is a form of insoluble fiber. Xanthan gum, a popular stabilizer in food processing and fine dining restaurants, is a form as well.
During 1985 to 1988, the methodologies were developed for differentiating insoluble and soluble dietary fiber.2 In 1991, AOAC Official Method of Analysis 991.42 for insoluble dietary fiber was adopted.2
Extracting insoluble fiber from a raw material depends on the nature of the raw material. For instance, foxtail millet is sometimes used as a source of insoluble fiber. In this process, foxtail millets are sorted and cleaned to remove foreign materials. The cleaned seeds are washed using water, drained and dried in an air oven at 35°C. The grains are milled and sieved through a 100 mesh to obtain flour. Parts of the flour (500 g) are defatted overnight with n-hexane (w/v) according the ratio (1:5) at room temperature (25°C). The defatted materials are dried in an air oven at 25°C before being packed into polyethylene bags.3
Commonly, grains are milled and defatted, followed by a process to remove the water insoluble portion of the flour. Since insoluble fiber is not easily dispersed in water, extraction methods remain rather straightforward.
Insoluble sugar provides “bulk” for stool formation and speeds up the movement of food and waste through the digestive system, which can help prevent constipation. Both soluble fiber and insoluble fiber make you feel full, which may help you eat less and stay satisfied longer. FDA suggests the Daily Value for fiber is 25 g per day based on 2,000 calories diet.2
Since insoluble fibers cannot be directly digested, they have no real nutritive value to the body. However, as a non-digestible material, insoluble fiber provides bulk as semi-digested foods move through the body. For this reason, insoluble fiber is exceptionally beneficial to the human frame. As an integral part of the human diet, there are very little dangers involved in consuming insoluble fiber in reasonable quantities.
Dietary fiber reduces the risk of predominant diseases among Western people. Clinical evidence has established the role of high fiber intakes in reducing serum lipids of hyperlipidemic individuals, improving glycemic control and insulin sensitivity in diabetic individuals, facilitating weight loss and reducing need for insulin or oral agents in obese diabetic individuals, and maintaining a healthy gastrointestinal tract.4
Insoluble fiber and resistant starch consists mainly of cellulose, hemicellulose and lignin.5
Insoluble fiber can be from a variety of foods, including:
- nuts and seeds
- wheat bran
- whole grain foods
The incorporation of fibers in bread reduces loaf volume and increases firmness by diluting gluten.6 Enzymes, like xylanase, can be added to decrease the negative effect from fiber by degrading the cell wall polysaccharides.7 Smaller sized fiber also has less negative effect on bakery quality. Bread prepared with smaller size fibers from sugarcane, is more tender and elastic than with coarser fibers, and organoleptic characteristics (those related to color, odor, and taste) increased with particle size reduction.8 The other methods include using the alkaline hydrogen peroxide treated lignocellulosic material9 or adding gluten.7
Insoluble fiber is GRAS. Depending on the type of insoluble fiber, care might be required in regards to labeling requirements. For instance, Xanthan gum is commonly labeled as E415, a formal food additive. Additionally, cellulose is commonly labelled as E460. Consult the FDA Code of Federal Regulations (CFR) for specific labeling requirements.
- Mertens, DR. “Challenges in Measuring Insoluble Dietary Fiber.” Journal of Animal Science. 2003. 81:3233-3249.
- DeVries, J. W., L. Prosky, B. Li, and S. Cho. “A Historical Perspective on Defining Dietary Fiber.” Cereal Foods World (1999): 367-69.
- Banqoura M, Ming Z, Nsor-Atindana J, Xue Z, Tolno M, Wei P. “Extraction and Fractination of Insoluble Fibers from Foxtail Millet (Setaria italic).” American Journal of Food Technology. 2011. 6(12): 1034-1044.
- Anderson J, Smith B, Gustafson, N. “Health Benefits and Practical Aspects of High-Fiber Diets.” American Journal of Clinical Nutrition. 1994;59(suppl):1242s-7s.
- “Dietary Fiber.” Accessdata. Fda. Gov. Food and Drug Administration, 6 June 2016.