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What is fiber?

Fiber is a carbohydrate polymer that cannot be digested. The type people can consume is called dietary fiber. Fiber is key in nutrition and an adequate amount daily intake is vital to digestive health.

The rule of thumb for human consumption is that for every 1,000 calories consumed, 14 grams of fiber should be consumed as well, to maintain a healthy digestive system.

  • Two terms, dietary and functional, denote certain characteristics of different types of fiber. Dietary fiber is made up of the combination of non-digestible carbohydrates and lignin, including non-starch polysaccharides from a plant source such as cellulose and fiber, found in oat as well as wheat bran.
  • American Association of Cereal Chemists (AACC) defined dietary fiber as the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine.1 It is also called bulk or roughage.
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Origin

“Dietary fiber” was first used by Hipsley in 1953 as a shorthand term for the non-digestible constituents that make up the plant cell wall.2 In 1981, there was a consensus on the dietary fiber definition and analytical approach at AOAC Spring Workshop in Ottawa, Canada.2 In 1985, AOAC Official Method of Analysis 985.29 for dietary fiber started.2 In 1991, AOAC Official Method of Analysis 991.42 for insoluble dietary fiber was adopted.2 After two international surveys to reaffirm consensus on dietary fiber definition, AACC released the dietary fiber definition in 1999.2

Nutrition

Soluble fiber can interfere with the absorption of dietary fat and cholesterol. This, in turn, can help lower low-density lipoprotein (LDL or “bad”) cholesterol levels in the blood. Soluble fiber also slows digestion and the rate at which carbohydrates and other nutrients are absorbed into the bloodstream. This can help control the level of blood glucose (often referred to as blood sugar) by preventing rapid rises in blood glucose following a meal.

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.

The FDA suggests the Daily Value for fiber is 28 g per day based on 2,000 calories diet.3 Most Americans don’t get the recommended amount of dietary fiber. Dietary fiber is considered a “nutrient of public health concern” because low intakes are associated with potential health risks.3 Food with 5% Daily Value or less of dietary fiber per serving is low fiber, while food with 20% Daily Value or more of dietary fiber per serving is high fiber. The goal is to get 100% of the Daily Value for dietary fiber on most days.

Types of fiber

There are two types of dietary fiber:3

  • Soluble fiber: dissolves in water to form a thick gel-like substance in the stomach. It is broken down by bacteria in the large intestine and provides some calories. Soluble fiber can be found in a variety of foods like beans and peas, fruits, oats, nuts and seeds, and vegetables.
  • Insoluble fiber: doesn’t dissolve in water and passes through the gastrointestinal tract relatively intact and therefore, and is not a source of calories. Examples of include whole wheat, whole grains, wheat bran, corn bran, seeds, raisins, and green leafy vegetables. In addition, insoluble, or dietary fiber, includes inulin, or chicory root fiber, cellulose fiber, and beta glucans. The FDA reported five non-digestible carbohydrates in the dietary fibers category – cellulose, guar gum, hydroxypropylmethyl cellulose, locust bean gum and pectin – have beneficial physiological effects to human health.2

Function

In baking, the major source of fiber originates from whole grains and the usage of whole grains in breads, muffins, crackers, and tortillas. Fiber originating from cereal grains is deemed “cereal fiber.” This is used in healthy breads or baked items to provide a more nutritious product. Whole-wheat bread flour, whole-wheat pastry flour, oat flour, or barley flour are sources of cereal fiber used in baking. Each imparts a sweet, nutty flavor to baked products compared to white flour.

Bakers aiming to increase fiber content in baked products without imparting color variability can utilize resistant starch, maintaining a white or cream crumb color, concurrently with boosting fiber content. Resistant starch has a neutral taste, and a low water-holding capacity, resulting in an ideal flour replacement. In addition, resistant starch increases freshness of baked snack foods, batter, as well as maintaining crispness and increasing shelf life in breakfast cereals.

Dried fruits and fruit pastes are also utilized in baking as a source of fiber, as well as a source of color and texture in muffins, breads, turnovers, bars, and crackers. In fat reduction, fiber can be used as a substitute. When substituting fat for fiber there are many advantages such as lower caloric content, longer satiety, and greater nutritious value. In addition, fiber can be used to retain moisture or add crispness. Tortillas or breads are often enriched with fiber for such functionality benefits.

Application

When dietary fiber is added to the bread formulation, different sources of have different effects on bread quality.4 The effect of wheat bran, resistant starch, and locust bean gum on bread technological quality has been studied by Almeida (2012).4

The research shows that for some parameters, such as proofing time, crust color acceptance, crust appearance acceptance, taste acceptance and aroma acceptance, fiber addition did not present a significant effect. But the high-speed mixing time, crumb color acceptance, crumb appearance acceptance and texture acceptance were influenced by the three different fiber sources studied.

FDA regulation

The FDA regulates the claim of soluble fiber and low cholesterol food for reducing the risk of coronary heart disease on the label of food products in the Code of Federal Regulations (Title 21 Part 101.81). In this article, the type of soluble fiber and the amount of daily dietary intake level are listed.5

The FDA has identified eight sources of dietary fiber that can be listed on labels:6

  • Mixed plant cell wall fibers
  • Arabinoxylan
  • Alginate
  • Inulin and inulin-type fructans
  • High amylose starch (resistant starch 2)
  • Galactooligosaccharide
  • Polydextrose
  • Resistant maltodextrin/dextrin

References

  1. American Association of Cereal Chemists. “The Definition of Dietary Fiber: Report of the Dietary Fiber Definition Committee to the Board of Directors of the American Association of Cereal Chemists.” Cereal Foods World 46 (2001): 112-26.
  2. DeVries, J. W., L. Prosky, B. Li, and S. Cho. “A Historical Perspective on Defining Dietary Fiber.” Cereal Foods World (1999): 367-69.
  3. “Dietary Fiber.” Accessdata. Fda. Gov. Food and Drug Administration, 6 June 2016.
  4. Almeida, Eveline Lopes, Yoon Kil Chang, and Caroline Joy Steel. “Dietary Fibre Sources in Frozen Part-baked Bread: Influence On technological Quality.” LWT – Food Science and Technology 53.1 (2013): 262-70.
  5. “21CFR101.81.” CFR-Code of Federal Regulations Title 21. N.p., 1 Apr. 2016.
  6. “Review of the Scientific Evidence on the Physiological Effects of Certain Non-Digestible Carbohydrates.” The Food and Drug Administration. 14 June. 2018. https://www.fda.gov/Food/LabelingNutrition/ucm610115.htm