Dietary Carbohydrates

The primary role of carbohydrates (sugars and starches) is to provide energy to cells in the body, particularly the brain, which is the only carbohydrate-dependent organ in the body. The Recommended Dietary Allowance (RDA) for carbohydrate is set at 130 g/d for adults and children based on the average minimum amount of glucose utilized by the brain. This level of intake, however, is typically exceeded to meet energy needs while consuming acceptable intake levels of fat and protein. The median intake of carbohydrates is approximately 220 to 330 g/d for men and 180 to 230 g/d for women.

Classification of Dietary Carbohydrates
Carbohydrates can be subdivided into several categories based on the number of sugar units present. A monosaccharide consists of one sugar unit such as glucose or fructose. A disaccharide (e.g., sucrose, lactose, and maltose) consists of two sugar units. Oligosaccharides, containing 3 to 10 sugar units, are often breakdown products of polysaccharides, which contain more than 10 sugar units. Oligosaccharides such as raffinose and stachyose are found in small amounts in legumes. Examples of polysaccharides include starch and glycogen, which are the storage forms of carbohydrates in plants and animals, respectively. Finally, sugar alcohols, such as sorbitol and mannitol, are alcohol forms of glucose and fructose, respectively.

Definition of Sugars

The term "sugars" is traditionally used to describe mono- and disaccharides. Sugars are used as sweeteners to improve the palatability of foods and beverages and for food preservation. In addition, sugars are used to confer certain functional attributes to foods such as viscosity, texture, body, and browning capacity. The monosaccharides include glucose, galactose, and fructose, while the disaccharides include sucrose, lactose, maltose, and trehalose. Some commonly used sweeteners contain trisaccharides and higher saccharides. Corn syrups contain large amounts of these saccharides; for example, only 33 percent or less of the carbohydrates in some corn syrups are mono- and disaccharides; the remaining 67 percent or more are trisaccharides and higher saccharides. This may lead to an underestimation of the intake of sugars if the trisaccharides and higher saccharides are not included in an analysis.

Extrinsic and Intrinsic Sugars
The terms extrinsic and intrinsic sugars originate from the United Kingdom Department of Health. Intrinsic sugars are defined as sugars that are present within the cell walls of plants (i.e., naturally occurring), while extrinsic sugars are those that are typically added to foods. An additional phrase, "non-milk extrinsic sugars," was developed due to the lactose in milk also being an extrinsic sugar. The terms were developed to help consumers differentiate sugars inherent to foods from sugars that are not naturally occurring in foods.

Added Sugars
The U.S. Department of Agriculture (USDA) has defined "added sugars" for the purpose of analyzing the nutrient intake of Americans using nationwide surveys, as well as for use in the Food Guide Pyramid. The Food Guide Pyramid, which is the food guide for the United States, translates recommendations on nutrient intakes into recommendations for food intakes. Added sugars are defined as sugars and syrups that are added to foods during processing or preparation. Major sources of added sugars include soft drinks, cakes, cookies, pies, fruitades, fruit punch, dairy desserts, and candy. Specifically, added sugars include white sugar, brown sugar, raw sugar, corn syrup, corn-syrup solids, high-fructose corn syrup, malt syrup, maple syrup, pancake syrup, fructose sweetener, liquid fructose, honey, molasses, anhydrous dextrose, and crystal dextrose. Added sugars do not include naturally occurring sugars such as lactose in milk or fructose in fruits.

Since USDA developed the added sugars definition, the added sugars term has been used in the scientific literature. Although added sugars are not chemically different from naturally occurring sugars, many foods and beverages that are major sources of added sugars have lower micronutrient densities compared with foods and beverages that are major sources of naturally occurring sugars. Currently, U.S. food labels contain information on total sugars per serving, but do not distinguish between sugars naturally present in foods and added sugars.

Definition of Starch

Starch consists of less than 1,000 to many thousands of a-linked glucose units. Amylose is the linear form of starch that consists of a-(1,4) linkages of glucose polymers. Amylopectin consists of the linear α-(1,4) glucose polymers, as well as branched 1-6 glucose polymers. The amylose starches are compact, have low solubility, and are less rapidly digested. They are prone to retrogradation (hydrogen bonding between amylose units) to form resistant starches (RS3). The amylopectin starches are digested more rapidly, presumably because of the more effective enzymatic attack of the more open-branched structure.

Definition of Glycemic Response, Glycemic Index, and Glycemic Load

Foods containing carbohydrate have a wide range of effects on blood glucose concentration during the time course of digestion (glycemic response), with some resulting in a rapid rise followed by a rapid fall in blood glucose concentration, and others resulting in a slow extended rise and a slow extended fall. Prolonging the time over which glucose is available for absorption in healthy individuals greatly reduces the postprandial glucose response. Holt and coworkers (1997), however, reported that the insulin response to consumption of carbohydrate foods is influenced by the level of the glucose response, but varies among individuals and with the amount of carbohydrate consumed. Adults with type 1 or type 2 diabetes have been shown to have similar glycemic responses to specific foods, whereas glycemic responses were shown to vary with severity of diabetes. Individuals with lactose maldigestion have reduced glycemic responses to lactose-containing items.

The glycemic index (GI) is a classification proposed to quantify the relative blood glucose response to foods containing carbohydrate. It is defined as the area under the curve for the increase in blood glucose after the ingestion of a set amount of carbohydrate in an individual food (e.g., 50 g) in the 2-hour postingestion period as compared with ingestion of the same amount of carbohydrate from a reference food (white bread or glucose) tested in the same individual, under the same conditions, using the initial blood glucose concentration as a baseline. The average daily dietary GI of a meal is calculated by summing the products of the carbohydrate content per serving for each food, times the average number of servings of that food per day, multiplied by the GI, and all divided by the total amount of carbohydrate . Individual foods have characteristic values for GI, although within-subject and between-subject variability is relatively large. Because GI has been determined by using 50-g carbohydrate portions of food, it is possible that there is a nonlinear response between the amount of food ingested, as is the case for fructose and the glycemic response.

TABLE 6-3 Glycemic Index (GI) of Common Foods

Source: REF FILE CF-205

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