Chapter: Introduction to Human Nutrition: Digestion and Metabolism of Carbohydrates

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Dietary fiber

A definition and method of measuring fiber is important for scientific studies and for food-labeling purposes.

Dietary fiber

Major interest in dietary fiber began in the early 1970s with the proposal by Burkitt and Trowell (1975) that many Western diseases were due to a lack of fiber in the diet. However, the definition of dietary fiber has been, and continues to be, a source of scientific con-troversy. Indeed, two consecutive reports from the FAO (1997 and 1998) recommended that the term “dietary fiber” be phased out. Nevertheless, the term appears to be here to stay because it is accepted by consumers, the food industry, and governments.

 

A definition and method of measuring fiber is important for scientific studies and for food-labeling purposes. The student must be aware that the defini-tions and methods of measuring fiber have changed over time, and differ in different parts of the world. Knowledge of what is meant by the term “fiber” and what is included in the measurement is essential for proper interpretation of the scientific literature (but often is not given in the methods section of papers and reports).

 Originally, Burkitt and Trowell (1975) defined fiber as the components of plant cell walls that are indigest-ible in the human small intestine. Later, the definition was expanded to include storage polysaccharides within plant cells (e.g., the gums in some legumes). Many different methods were developed to measure dietary fiber, but they measured different things. All of the methods start with the drying and grinding of the food and extraction of the fat using an organic solvent. If the remaining material is treated with strong acid, the chemical bonds in starch and many (but not all) polysaccharides will be broken down to release their component sugars. If these are filtered away, the residue is “crude fiber.” For many years this was the way in which fiber was measured for food tables. However, acid hydrolysis breaks down many carbohydrates that would not be digested in the small intestine. So, in more modern methods, the food residue is digested with amylase to hydrolyze the starch to soluble sugars and oligosaccharides. The latter are removed by filtration or by centrifugation to leave a residue containing mainly dietary fiber, pro-teins, and inorganic materials.

 

The two main methods used to determine dietary fiber are chemical and gravimetric. In the chemical method (used in the UK), the residue is subjected to acid hydrolysis and the resultant sugars are measured colorimetrically, by gas chromatography or by high-performance liquid chromatography. The sum of all these sugars constitutes the NSP. The chemical method includes only carbohydrates in the NSP. In the gravi-metric method (used in the USA and elsewhere), the residue is dried and weighed, and the amounts of protein and mineral materials present are subtracted (after separate analyses). The gravimetric method includes the NSP, plus other noncarbohydrate com-ponents such as lignin and waxes. Recently, all coun-tries in Europe have recognized the gravimetric method as an approved method for measuring fiber in foods.

 The main areas of disagreement now with respect to fiber are whether indigestible oligosaccharides and sugars and nonplant compounds should be included and whether the definition of fiber should include a physiological component. In Japan, fructooligo-saccharides (FOSs) are classified as dietary fiber for food-labeling purposes. However, FOSs and similar compounds, being soluble in water, are not included in the dietary fiber methods, because they are filtered out along with the sugars resulting from the starch hydrolysis. Specific methods exist for FOSs and related compounds, and they could be included as fiber. Certain animal-derived compounds, such as chitin and chitosan, derived from the shells of shrimp and crabs, are indigestible, would be included in the gravi-metric fiber analysis, and could be classified as fiber. Chitin has some physiological properties, such as cholesterol lowering, which are associated with dietary

 

fiber. There are many other indigestible carbohydrate and noncarbohydrate compounds, both natural and artificial, that could be classified as “fiber” (e.g., poly-dextrose, sucrose polyester, styrofoam). Should these be included in dietary fiber? In favor of this is the argument that some of these materials have physio-logical properties associated with fiber, such as stool bulking, or effects on satiety or blood glucose and cholesterol. Against this is the feeling that dietary fiber should include only plant materials that are normally present in the diet. These are not easy issues and they have not been resolved.


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