Nutritional Importance

Some carbohydrate types of specific importance in human nutrition are sugars and sugar alcohols, starch, and dietary fiber.

Sugars

The Food and Agriculture Organization (FAO) and the World Health Organization (WHO) expert consultation on carbohydrates use the term 'sugar' to describe monosaccharides and disaccharides. Sugars can be separated analytically from the food matrix by gas-liquid chromatography (GLC), high performance liquid chromatography, and enzymatic methods. Sugars are widely used in the food industry as sweeteners and preservatives. They improve the texture, body, palatability, and viscosity of foods and beverages.

The UK Department of Health distinguishes between 'intrinsic' and 'extrinsic' sugars. Intrinsic sugars are defined as those that occur naturally as part of the plant cell walls. Extrinsic sugars were defined as added sugars or those present when the ch2oh HO-f-A O

CH2OH

CH2OH

HO I ch2oh

HO I ch2oh

CH2OH

CH2OH

CH2OH

O HO

CH2OH

O HO

CH2OH

O HO

CH2OH

O HO

CH2OH

CH2OH

Figure 3 (A) Five units of an a-1,4-D-glucopyranose chain from a starch molecule (amylose). (B) Four units of a ,3-1,4-D-glucopyranose chain from a cellulose molecule.

food matrix has been disrupted. An additional term, 'non-milk' extrinsic sugar, is used to distinguish between milk and other extrinsic sugars. However, these terms have not been widely accepted.

can be found in cereals, potatoes, legumes, and other vegetables, with amylopectin comprising 80-85% and amylose 15-20% of total starch.

Sugar Alcohols

Sugar alcohols are monosaccharide and disaccharide derivatives, such as sorbitol and xylitol, which are extensively used as sweeteners in the food industry. They have received increased attention because of their desirable properties of relative sweetness and limited digestion and absorption.

Starch

The most important, abundant, and digestible poly-saccharide in human nutrition is starch. Starch comprises large chains of a-linked glucose residues, in the form of amylose or amylopectin. Amylose is a linear unbranched form of starch, which consists of a-1,4-linked glucose units (Figure 3A). Amylopectin is a branched-chain polymer, which consists of a-1,6-linked glucose units. Both forms of starch

Dietary Fiber

There are several definitions of fiber, and no consensus exists among international organizations. The Association of Official Analytical Chemists International defines fiber as nondigestible animal and plant carbohydrates, based on the analytical methods for fiber separation using an enzymatic-gravimetric method (Table 5).

According to the new definition of the expert panel on macronutrients appointed by the Institute of Medicine of the National Academies of Science (Table 5), dietary fiber comprises intact nondigestible carbohydrates and lignin derived from plant sources. Functional fiber consists of nondigestible carbohydrates, derived from either plant or animal sources, that have shown favorable health outcomes for humans. Total fiber consists of both dietary and functional fiber. With this new definition of dietary and functional

Table 4 Some nutritionally important polysaccharides

Class

Species

Significance

Glucans

Starch

Storage polysaccharide in plants

Glycogen

Short-term storage form of glucose in animal tissues

Cellulose

Major structural component of plant cell walls

Galactans

Major constituents of noncellulosic matrix of plant cell wall

Xylans

Constituents of mature plant tissues

Mannans

Storage forms in several plants

Uronans

Galacturonans

Major components of water-soluble pectic fraction of plants

Mannuronans

Components of algal polysaccharides

Guluronans

Components of algal polysaccharides

Starch

Amylose,

Most common digestible

amylopectin

plant polysaccharides

Nonstarch

Cellulose

Major component of plant cell wall

Pectin

Constituent of plant cell wall, food additive

Hemicellulose

Constituent of plant cell wall

Gums, mucilages

Plant hydrocolloids, food additives

Algal

Constituents of algae and

polysaccharides

seaweed, food additives

fiber, new analytical methods should be developed and implemented to quantify accurately the total fiber component of foods.

Dietary and functional fiber cannot be digested by mammalian enzymes, and therefore they pass almost intact through the small intestine. Fiber consumption has potential health benefits, including the promotion of general gastrointestinal health and the prevention of several noncommunicable diseases.

Table 5 Current definitions of dietary fiber

Source Definitions

AOACa Fiber: nondigestible animal and plant carbohydrates IOMb Dietary fiber: intact nondigestible carbohydrates and lignin, derived from plant sources Functional fiber: nondigestible carbohydrates derived from either plant or animal sources that have shown favourable health outcomes for humans Total fiber: dietary and functional fiber aAOAC: Association of Official Analytical Chemists bIOM: Institute of Medicine

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