The term "fiber" encompasses a multitude of carbohydrates and lignin, which by current definitions escape hydrolysis by digestive enzymes completely or at least partially. Hence, they are not absorbed in the small intestine and reach the colon. This physiological definition cannot be transferred to a uniform chemical standard. The attempted classification is based on different degrees of solubility under different treatments (A).
Pectins can be dissolved out of plant cells with neutral solvents, which is why they are classified as soluble fiber. Cell wall components are insoluble under these conditions, and they make up the largest share of fiber consumed with foods. In an acidic environment, hemicelluloses and cellulose dissolve. Lignin, the "woody substance," remains resistant even here.
Characteristically, fiber is made from the simple monosaccharides glucose, fructose, arabinose, and ribose; additionally, it may contain certain derivatives of various monosaccharides (B) and, in lignin, some non-carbohydrate structures.
Cellulose, the most abundant organic compound in our biosphere, is made only from glucose. It differs from starch and glycogen in that the glucose sub-units are linked with p-1,4 bonds. This p-linkage causes long, straight chains to form, making it suitable as a fibrous material. Mammals do not have cellu-lases and are therefore unable to digest wood and plant fiber. Ruminants have bacteria in their digestive tract, which make p-1,4 linkage-breaking enzymes. This is why these animals can digest woody plant parts like straw, to some degree.
By far most wheat and rye fibers are hemicelluloses. They are also considered insoluble, but can become soluble in acidic or alkaline environments due to their short-chain, branched nature. Pectins, the soluble fibers in fruit, are a chemically heterogeneous mix. Galac-turonic acid is their main component, besides a number of other monomers. They form characteristic gels when mixed with water, a property that is used in food processing. Technically modified pectins are also in use. Additional fibers are, for instance: gum arabic, seed mucilage, sea weed extracts, and technically modified cellulose.
The above definition of fiber applies increasingly to new products, most of which are produced synthetically: poly-oles, high-melting fats, or fat alternatives. Starch, which was considered to be completely digestible until recently, also has a fiber component. This so-called "resistant starch" either results from the structure of the food—e. g., coarsely ground grains—or from altered amylose. For instance, when potatoes are cooked and subsequently left to cool, indigestible, "retrograded" amy-lose forms.
r A. Fiber
Cell contents, pectins
Hemicellulose, rubber, mucilaginous compounds
CH2OH Ol-L OH _ ^ CH2UH OH,
u HO CH2OH H 0 HO CHwQH HO \ CH2OH
- B. Other Important Components of Fibrous Compounds
Galacturonic acid ch2oh
COOCHe HC )H
N-Acetyl- Methylated galactosamine galacturonic acid
OH HOO2SO )H
Monomers in pectins h2coh I
HN C CH3
Ribose Galactose-4-sulfate N-Acetyl-glucosamine Phenylpropane
(Hemicellulose) (Carageenan) (Chitin) derivatives
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WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.