NUTR 344 EXAM-4 Fiber

Dietary fiber

Non-digestible carbohydrates and lignin that are intrinsic and intact in plants, resisting digestion and absorption in the small intestine and undergoing complete or partial fermentation in the large intestine.

Functional fiber

Isolated, non-digestible carbohydrates that have been extracted or synthesized and have demonstrated beneficial physiological effects in humans, added to foods or taken as supplements.

Total fiber

The sum of dietary fiber (naturally occurring) and functional fiber (isolated/added), representing the complete fiber content of a food or diet.

Soluble fiber

Fiber that dissolves in water to form a viscous gel, slowing gastric emptying and glucose absorption, and helping lower blood cholesterol by binding bile acids; found in oats, legumes, and fruit.

Insoluble fiber

Fiber that does not dissolve in water and largely resists fermentation, adding bulk to stool and speeding intestinal transit time; found in wheat bran and vegetable skins.

Cellulose

An insoluble, unbranched polysaccharide made of glucose units linked by β-1,4 glycosidic bonds, which humans cannot digest due to lacking the enzyme to break these bonds; it adds bulk to stool.

Functional cellulose

Modified forms of cellulose (like methylcellulose or carboxymethylcellulose) that are processed and added to foods or supplements for their fiber-like functional properties, such as thickening or bulking.

Hemicellulose

A heterogeneous group of polysaccharides found in plant cell walls alongside cellulose, composed of various sugar units with a more branched structure, exhibiting both soluble and insoluble properties depending on composition.

Pectins

Soluble, gel-forming polysaccharides found in the cell walls of fruits (especially apples and citrus), commonly used as thickeners and known for lowering cholesterol and slowing glucose absorption.

Pectin backbone

The main structural chain of pectin, composed primarily of galacturonic acid units linked by α-1,4 glycosidic bonds, with side chains of other sugars branching off.

Lignin

A complex, non-carbohydrate polymer (not a polysaccharide) found in plant cell walls that provides structural rigidity; it is insoluble and completely resistant to digestion and fermentation.

Gums

Soluble, highly viscous polysaccharides secreted by plants (like guar gum) or seaweeds, often used as food additives for their thickening and stabilizing properties, and known to slow digestion and lower cholesterol.

Fructans (including inulin)

Soluble, fermentable oligosaccharides/polysaccharides composed of fructose chains, found in foods like chicory root and onions, acting as prebiotics that fuel beneficial gut bacteria.

Resistant starch

Starch that escapes digestion in the small intestine and reaches the large intestine intact, where it is fermented by gut bacteria, behaving functionally like soluble fiber; found in green bananas and cooked-then-cooled potatoes/rice.

Chitin

A structural polysaccharide found in the exoskeletons of insects and crustaceans and in fungal cell walls, composed of N-acetylglucosamine units; it is not digestible by humans and can serve as a source of dietary fiber.

Chitosan

A deacetylated derivative of chitin, sometimes used as a functional fiber supplement marketed for binding dietary fat and cholesterol in the gut, though evidence for its efficacy is limited.

Psyllium

A soluble, viscous fiber derived from the seed husks of the Plantago plant, commonly used as a functional fiber supplement (e.g., Metamucil) to relieve constipation and lower cholesterol.

Fiber and diabetes prevention

Soluble fiber slows carbohydrate digestion and glucose absorption, blunting post-meal blood glucose spikes and improving insulin sensitivity, which lowers the risk of developing type 2 diabetes.

Fiber and cardiovascular disease prevention

Soluble fiber binds bile acids and cholesterol in the intestine, promoting their excretion and lowering LDL cholesterol, while also improving blood pressure and reducing systemic inflammation.

Fiber and obesity/weight management

High-fiber foods increase satiety and meal volume without adding significant calories, slow gastric emptying, and are typically less energy-dense, helping to reduce overall caloric intake.

Fiber and gastrointestinal disorders

Adequate fiber intake increases stool bulk and softness, reduces transit time, and helps prevent or manage constipation, diverticular disease, and hemorrhoids, while fermentable fibers support a healthy gut microbiome.

Fiber on food labels

• “Excellent source of fiber” = 20% RDA

• “Good source of fiber” = 10% RDA

Fiber health claims

• Diets low in fat and rich in high-fiber foods (or rich in fruits and vegetables) may reduce the risk of certain cancers

• Diets low in saturated fat (or low in fat) and rich in soluble fiber (or rich in whole oats and psyllium seed husk) may reduce the risk of heart disease

• Diets low in total fat, saturated fat, and cholesterol and rich in whole grains and other plant foods may help reduce the risk of heart disease

Fiber processing

Fiber resists enzymatic digestion in the stomach and small intestine, passing largely intact into the large intestine, where soluble and fermentable fibers are broken down by gut bacteria into short-chain fatty acids (like butyrate, acetate, and propionate), while insoluble fiber remains largely unfermented and adds bulk to stool for elimination.

Dietary fibers (naturally occurring, intact in plants)

Cellulose, hemicellulose, pectins, lignin, gums, fructans (inulin), resistant starch, chitin (in fungal cell walls).

Functional fibers (isolated/synthesized and added to foods or supplements)

Functional cellulose (methylcellulose, carboxymethylcellulose), chitosan, psyllium, isolated pectin, isolated gums, isolated inulin/fructans, resistant starch (isolated forms), polydextrose.

Examples of soluble fibers

Pectins, gums, psyllium, fructans (inulin), some hemicelluloses, beta-glucans (in oats and barley).

Examples insoluble fibers

Cellulose, lignin, chitin, most hemicelluloses, resistant starch (though it behaves more like soluble fiber functionally).