Eha Nutrition Medical Definition

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It has long been recognized that both animal feedstuffs and human foods contain poorly digestible components, which do not contribute to nutrition in the classical sense of providing essential substances or metabolic energy. With the development of scientific approaches to animal husbandry in the nineteenth century, the term 'crude fiber' was coined to describe the material that remained after rigorous nonenzymic hydrolysis of feeds. During the twentieth century, various strands of thought concerning the virtues of 'whole' foods, derived from plant components that had undergone only minimal processing, began to converge, leading eventually to the dietary fiber hypothesis. Put simply, this states that the nondigesti-ble components of plant cell walls are essential for the maintenance of human health.

In the early 1970s the physician and epidemiologist Hugh Trowell recognized that the crude fiber figures available at the time for foods had little physiological significance and were of no practical value in the context of human diets. He was amongst the first to use the term dietary fiber to describe the 'remnants of plant cell walls resistant to hydrolysis (digestion) by the alimentary enzymes of man.' This definition was later refined and given the more quantitative form: ''The sum of lignin and the plant polysaccharides that are not digested by the endogenous secretions of the mammalian digestive tract.'' This definition paved the way for the development of analytical methods that could be used to define the fiber content of human foods. Broadly, these techniques are based on enzymic removal of the digestible elements in food, followed by either gravimetric analysis of the residue ('South-gate' and Association of Analytical Chemists (AOAC) methods), which results in the retention of some undigested starch, or chemical analysis ('Englyst' method), which enables a more precise separation of starch from the structural polysaccharides of the cell wall. In the latter case, the cell wall components are defined as 'nonstarch polysaccharides' (NSP). Whatever analytical approach is used, both 'dietary fiber' and nonstarch polysaccharides are shorthand terms for large and complex mixtures of polysaccharides. The components of such mixtures vary widely among foods and they often share few properties other than resistance to digestion in the small intestine. A summary of the main types of plant cell polysaccharides contained in the general definition of dietary fiber is given in Table 1.

In recent years this problem has been made more complex in some ways because of the explosion of interest in functional foods for gastrointestinal health. These often contain high levels of novel oligo- and polysaccharides, which might perhaps be regarded as analogs of dietary fiber. Fructose oligosaccharides, which are nondigestible but highly fermentable, are now often added to foods as pre-biotic substrates for the colonic microflora. Such materials may not fit the original definition of dietary fiber, but it is certainly not helpful to exclude them from the contemporary concept, which needs to expand to accommodate modern developments.

The presence of large undigested cell wall fragments, finely dispersed particulates, or soluble poly-saccharides can alter physiological processes

Table 1 Major components of dietary fiber

Food source

Polysaccharides and related substances

Fruits and

Cellulose, xyloglucans, arabinogalactans,


pectic substances, glycoproteins


Cellulose, arabinoxylans,

glucoarabinoxylans, ß-D-glucans, lignin,

and phenolic esters

Legume seeds

Cellulose, xyloglucans, galactomannans,

pectic substances


Gums (guar gum, gum arabic), alginates,


carrageenan, modified cellulose gums

(methyl cellulose, carboxymethyl cellulose)

throughout the gut. The effects of different fiber components depend upon their varied physical and chemical properties during digestion, and also upon their susceptibility to degradation by bacterial enzymes in the colon. The complex nature of the various substances covered by the general definition of dietary fiber means that a single analytical value for the fiber content of a food is a poor guide to its physiological effects. This article will review the main mechanisms of action of resistant polysacchar-ides in the alimentary tract and their implications for human health.

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