**4. Analytical methods for studying dietary fibers**

The complexity of fibers is given by their chemical nature and polymerisation degree that they possess. This requires various analytical methods for the measurement of dietary fiber, to precisely estimate its composition in food and food by-products. Methods for the determination of dietary fiber may be divided into three categories: non-enzymaticgravimetric, enzymatic-gravimetric, and enzymatic-chemical methods. The latter includes enzymatic-colorimetric and enzymatic-chromatographic (GLC/ HPLC) methods [28]. Nowadays, the most commonly used methods for dietary fiber measurement are the enzymatic-gravimetric Association of Official Analytical Chemists (AOAC) method [36] and enzymatic-chemical method [37]. The method of Van Soest [38] is generally used in veterinary studies.

#### **5. Dietary fiber and human health**

460 The Complex World of Polysaccharides

**Table 2.** Dietary fiber constituents [4]

Substances associated with nonstarch polysaccharides

**Fiber Constituents Principal groupings Fiber components/sources** 

Waxes, cutin, Suberin Plant fibers

Fungi, yeasts, invertebrates

bean), seaweed extracts (carrageenan, alginates), plant extracts (gum acacia, gum karaya, gum tragacanth), microbial gums (xanthan, gellan),

Pectins Fruits, vegetables, legumes, sugar beet, potato Gums & Mucilages Leguminous seed plants (guar, locust

psylluim

Oligosaccharides Various plants and synthetically

Inulin Chicory, Jerusalem artichoke, sugar beet, onions

Plant fibers

Resistant starches Plants (corn, potatoes, grains,

produced (polydextrose, fructooligosaccharides, galactooligosaccharides, transgalactooligosaccharides)

Fungi, yeasts, invertebrates

legumes, bananas)

Bacterial fermentation

Lignin Lignin Woody plants

**Category Subcategory Fiber fraction Main food source** Soluble fiber Viscous β-glucans Grains (oat, barley, rye)

Nonviscous Sugars Lactulose

Insoluble fiber Cellulose Plants (vegetables, various brans)

Cutin/suberin/other plant waxes

Chitin and chitosan,

Curdlan (insoluble

The complexity of fibers is given by their chemical nature and polymerisation degree that they possess. This requires various analytical methods for the measurement of dietary fiber, to precisely estimate its composition in food and food by-products. Methods for the determination of dietary fiber may be divided into three categories: non-enzymatic-

collagen

β-glucan)

**Table 3.** Table 3. Classification of dietary fiber based on solubility

**4. Analytical methods for studying dietary fibers** 

Hemicellulose Cereal grains Lignin Woody plants

chondroitin

Animal origin fibers Chitin, chitosan, collagen,

#### **5.1. Fiber, lipid metabolism and cardiovascular disease**

The earliest and most widely researched topic related to dietary fiber and human health is reducing the risk factors for coronary heart disease [24]**.** Total serum cholesterol and low density- lipoprotein (LDL) cholesterol levels are generally accepted as biomarkers indicating of potential risk for developing the disease [5]. In consequence, research has primarily focused on their reduction as a means to diminish the risk of developing cardiovascular disease. Substantial experimental data support that blood cholesterol can be lowered using viscous soluble fibers that produce relatively high viscosity in the intestinal tract [39-41]. It is known that viscous soluble and insoluble dietary fibers can bind bile acids and micelle components, such as monoglycerides, free fatty acids, and cholesterol, which decrease the absorption and increase the fecal excretion of these entities [42,43]. For insoluble dietary fibers such as lignin or citric fiber this reducing effect is rather low compared to viscous soluble dietary fibers and is mainly based on direct binding of bile acids. In the small intestine the bile acids are bound by the insoluble dietary fibers through hydrophobic interactions and excreted from enterohepatic circulation together with the undigested insoluble dietary fibers which results in a lowering of the blood biomarkers levels [44,45]. Furthermore, free fatty acids and cholesterol bound by dietary fibers cannot be absorbed by the body and will be excreted. The biomarkers-lowering effect of viscous soluble dietary fibers such as psyllium, oat β-glucan or pectin is based on different mechanisms. The binding of water in the chyme and the resulting increase in viscosity is regarded as the main effect. This leads to a reduced diffusion rate of bile acids, which cannot be reabsorbed by the body and thus are excreted [46-48]. Besides, some studies indicate that there are also direct binding forces such as hydrophobic interactions between soluble dietary fibers and bile acids [49]. Dietary fiber also modifies lipid metabolism by influencing the expression of key genes. Acetyl-CoA carboxylase is the rate-limiting enzyme in lipogenesis and is regulated by AMP-activated protein kinase (AMPK). In a 10-week study comparing obese and lean rats, adding 5 g of P. ovata to rat chow increased the phosphorylation of AMPK, consequently inhibiting acetyl-CoA carboxylase [50]. Fructooligosaccharide (10g/100g) also has been shown to decrease the hepatic acetyl-CoA carboxylase expression in rats [51]. In view of new research [52,34] the United States Food and Drug Administration has approved health claims supporting the role of dietary fiber in the prevention of coronary heart disease [53].
