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Dietary Fiber
Dietary fiber is the nonstarch polysaccharide part of plant foods that is poorly digested by human enzymes. In the latter half of the 20th century, physicians and scientists began to understand the importance of dietary fiber in maintaining health and dietary fiber deficiencies in causing disease. Cleave first noted the increased sugar intake that resulted from decreased dietary fiber intake. Epidemiologists and gastroenterologists such as Burkett, Trowell, Painter, Walker, Heaten, and Eastwood then began to stress the importance of dietary fiber in maintaining normal gastrointestinal (GI) function and in preventing disease.
Chemistry and Properties
Food chemists analyzed plant foods first for crude fiber content, then began to correlate fiber content with various parts of plants. Because dietary fibers are extremely complex chemical compounds, it has been difficult for food scientists to decide on simple methods to identify their components in all foods. However, understanding the properties of fiber in the GI tract has facilitated making the distinction between soluble and insoluble fibers. In Englyst's classic method, after extraction, substances can be broken down as soluble, insoluble, and cellulose, which constitute the nonstarch polysaccharide component. Some resistant starch is left over from the chemical process.
Components in the plant cell walls are cellulose, noncellulose polysaccharides (soluble and insoluble), lignin, waxes, protein, and ash. Major classes of noncellulose polysaccharide are rhamnogalacturonans, arabinogalactans, β-glucans, xylans, mannans, and xyloglucans. Gums and mucilages develop but are not strictly part of the plant cell walls; they are complex heteroglycans with branch structures. Bacterial fermentation clearly shows that bacterial enzymes almost completely ferment soluble fibers but poorly ferment insoluble fibers or cellulose.
Physical Properties
Important physical properties of dietary fiber and its components are particle size and polysaccharides. Depending on how food is cooked or processed, particle size will be large or greatly reduced. In some cases, reduction in size can completely disrupt the plant cell wall. Therefore particle size is important in determining some of the properties discussed here.
Polysaccharides may be hydrophilic and may have a definite water-holding capacity. This varies with the food, and certainly, cellulose is limited to swelling property, which depends on polysaccharide type. Some polysaccharides are able to form gels, and some can become extremely viscous. These properties affect nutrient ion absorption, although the gels are completely fermented by bacterial action, freeing any substances that are trapped.
The water-holding property of insoluble fibers is particularly important for maintaining larger, softer stool. Insoluble fibers are poorly fermented by bacteria. Hence this water-holding property is helpful in maintaining a larger stool bulk throughout the colon.
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