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TABLE 1 Dietary Reference Intakes for Dietary Carbohydrates: Sugars and Starches by Life Stage Group DRI values (g/day) EARa RDAb AIc Life stage groupd 0 through 6 mo 60 7 though 12 mo 95 1 through 3 y 100 130 4 through 8 y 100 130 9 through 13 y 100 130 14 through 18 y 100 130 19 through 30 y 100 130 31 through 50 y 100 130 51 through 70 y 100 130 > 70 y 100 130 Pregnancy All ages 135 175 Lactation All ages 160 210 a EAR = Estimated Average Requirement. An EAR is the average daily nutrient intake level estimated to meet the requirements of half of the healthy individuals in a group. b RDA = Recommended Dietary Allowance. An RDA is the average daily dietary intake level sufficient to meet the nutrient requirements of nearly all (97–98 percent) healthy individuals in a group. c AI = Adequate Intake. If sufficient scientific evidence is not available to establish an EAR, and thus calculate an RDA, an AI is usually developed. For healthy breast-fed infants, the AI is the mean intake. The AI for other life stage and gender groups is believed to cover the needs of all healthy individuals in the group, but a lack of data or uncertainty in the data prevents being able to specify with confidence the percentage of individuals covered by this intake. d All groups except Pregnancy and Lactation represent males and females.
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PART II: DIETARY CARBOHYDRATES: SUGARS AND STARCHES 103 DIETARY CARBOHYDRATES: SUGARS AND STARCHES T he primary role of carbohydrates (i.e., sugars and starches) is to provide energy to all of the cells in the body. Carbohydrates are divided into several categories: monosaccharides, disaccharides, oligosaccharides, polysaccharides, and sugar alcohols. The requirements for carbohydrates are based on the average minimum amount of glucose that is utilized by the brain. Evidence was insufficient to set a Tolerable Upper Intake Level (UL) for carbohydrates. However, a maximal intake level of 25 percent or less of total calories from added sugars is sug- gested. This suggestion is based on trends indicating that people with diets at or above this level of added sugars are more likely to have poorer intakes of impor- tant essential nutrients. DRI values are listed by life stage group in Table 1. Nondiet soft drinks are the leading source of added sugars in U.S. diets, followed by sugars and sweets, sweetened grains, fruit ades, sweetened dairy products, and breakfast cereals and other grains. Most carbohydrates occur as starches in food. Grains and certain vegetables are major contributors. Other sources include corn, tapioca, flour, cereals, pop- corn, pasta, rice, potatoes, and crackers. Fruits and darkly colored vegetables contain little or no starch. The amount of dietary carbohydrate that confers optimal health in humans is unknown. A significant body of data suggests that more slowly absorbed starchy foods that are less processed, or have been processed in traditional ways, may have health advantages over those that are rapidly digested and absorbed. CARBOHYDRATE AND THE BODY Function The primary role of carbohydrates (i.e., sugars and starches) is to provide en- ergy to the cells in the body. The only cells that have an absolute requirement for glucose are those in the central nervous system (i.e., the brain) and those cells that depend upon anaerobic glycolysis, such as red blood cells. Normally, the brain uses glucose almost exclusively for its energy needs.
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 104 Classification of Dietary Carbohydrates Carbohydrates are classified by their number of sugar units: monosaccharides, such as glucose or fructose, consist of one sugar unit; disaccharides, such as sucrose, lactose, and maltose, consist of two sugar units; oligosaccharides, such as raffinose and stachyose, contain 3 to 10 sugar units and may be produced by the breakdown of polysaccharides; and polysaccharides, such as starch and glycogen, contain more than 10 sugar units and are the storage forms of carbo- hydrates in plants and animals, respectively. Sugar alcohols, such as sorbitol and mannitol, are alcohol forms of glucose and fructose, respectively. SUGARS AND ADDED SUGARS The term “sugars” is traditionally used to describe the monosaccharides and disaccharides. Monosaccharides include glucose, galactose, and fructose. Di- saccharides include sucrose, lactose, maltose, and trehalose. Sugars are used to sweeten or preserve foods and to give them certain functional attributes, such as viscosity, texture, body, and browning capacity. “Added sugars” are defined as sugars and syrups that are added to foods during processing or preparation. They do not include naturally occurring sug- ars, such as lactose in milk or fructose in fruits. Major food sources of added sugars include soft drinks, cakes, cookies, pies, fruit ades, 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, molas- ses, anhydrous dextrose, and crystal dextrose. Although added sugars are not chemically different from naturally occur- ring 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. STARCHES Starch is a carbohydrate polymer found in grains, legumes, and tubers. It is a polysaccharide composed of less than 1,000 to many thousands of a-linked glucose units and its two forms are amylase and amylopectin. Amylose is the linear form of starch, while amylopectin consists of linear and branched glu- cose polymers. In general, amylose starches are compact, have low solubility, and are less rapidly digested. Amylopectin starches are more rapidly digested, presumably because of their more open-branched structure.
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PART II: DIETARY CARBOHYDRATES: SUGARS AND STARCHES 105 Absorption, Metabolism, and Storage The breakdown of starch begins in the mouth, where enzymes act on the link- ages of amylase and amylopectin. The digestion of these linkages continues in the intestine, where more enzymes are released, breaking amylase and amy- lopectin into shorter glucose chains of varying lengths. Specific enzymes that are bound to the intestinal brush border membrane hydrolyze the glucose chains into monosaccharides, which are then absorbed into the bloodstream via active transport or facilitated diffusion mechanisms. Other sugars are also hydrolyzed to monosaccharide units before absorption. Once absorbed, sugars (glucose, galactose, and fructose) are transported throughout the body to cells as a source of energy. Glucose is the major fuel used by most of the body’s cells. Blood glucose concentration is highly regu- lated by the release of insulin, and the uptake of glucose by adipocytes and muscle cells is dependent on the binding of insulin to a membrane-bound in- sulin receptor. Galactose and fructose are taken up by the liver (when blood circulates past it) where they are metabolized. Galactose is mostly converted to glycogen for storage. Fructose is transformed into intermediary metabolites or converted to a precursor for glycogen synthesis. When blood glucose is high and cellular energy demand is low, glucose can be converted to glycogen for storage (in skeletal muscle and liver), a process called glycogenesis. Glycogenesis is acti- vated in the skeletal muscle by a rise in insulin concentration that occurs after the consumption of carbohydrate. It is activated in the liver by an increase in circulating monosaccharide or insulin concentrations. Glycogen is present in the muscle for storage and utilization and in the liver for storage, export, and the maintenance of blood glucose concentrations. When blood glucose levels become too low, glycogenolysis occurs, which is the release of glucose from glycogen stores in the liver. Following glycogenolysis, the body can export glucose from the liver to maintain normal blood glucose concentrations and be used by other tissues. Muscle glycogen is mainly used in the muscle. Gluconeogenesis, the production of glucose from a noncarbohydrate source (amino acids or glycerol), can occur during fasting (or in the absence of dietary carbohydrate), thus allowing the liver to continue to release glucose to main- tain adequate blood glucose concentrations. Glycemic Index A significant body of data suggests that more slowly absorbed starchy foods that are less processed, or have been processed in traditional ways, may have health advantages over those that are rapidly digested and absorbed. The former have been classified as having a low glycemic index (GI) and reduce the diet’s
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 106 glycemic load. GI is a measure of the increase in blood glucose in the two hours after eating a given amount (e.g., 50 g) of a carbohydrate relative to its response to a reference carbohydrate (white bread or glucose). The glycemic load is an indicator of the glucose response or insulin demand that is induced by total carbohydrate intake. Dietary GI and glycemic load have relatively predictable effects on circulating glucose, hemoglobin A1c, insulin, triacylglycerol, high den- sity lipoprotein (HDL) cholesterol, and urinary C-peptide concentrations. As such, it is theoretically plausible to expect a low GI diet to reduce risk of Type II diabetes and cardiovascular disease. However, the sufficient evidence needed to recommend substantial dietary changes based on GI is not available. DETERMINING DRIS Determining Requirements The requirements for carbohydrates are based on the average minimum amount of glucose that is utilized by the brain. Because brain size remains fairly con- stant after 1 year of age and approximates adult size, the EAR and RDA are identical for all age and gender groups after age 12 months, except pregnant and lactating women. The recommended amount also prevents ketosis, which is a rise in keto acid production in the liver to provide the brain with an alterna- tive fuel in times of low glucose availability. Criteria for Determining Carbohydrate Requirements, by Life Stage Group Life stage group Criterion 0 through 6 mo Average content of human milk 7 through 12 mo Average intake from human milk + complementary foods 1 through 18 y Extrapolation from adult data > 18 y Brain glucose utilization Pregnancy 14 through 18 y Adolescent female EAR plus fetal brain glucose utilization 19 through 50 y Adult female EAR plus fetal brain glucose utilization Lactation 14 through 18 y Adolescent female EAR plus average human milk content of carbohydrate 19 through 50 y Adult female EAR plus average human milk content of carbohydrate
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PART II: DIETARY CARBOHYDRATES: SUGARS AND STARCHES 107 The AMDR The AMDR for carbohydrates for both adults and children is 45–65 percent of total calories (see Part II, “Macronutrients, Healthful Diets, and Physical Activity”). The UL The Tolerable Upper Intake Level (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse effects for almost all people. Evidence was insufficient to set a UL for carbohydrates. However, a maximal intake level of 25 percent or less of total energy from added sugars is suggested, based on trends indicating that people with diets at or above this level of added sugars are more likely to have poorer intakes of important essential nutrients. DIETARY SOURCES Foods According to U.S. Department of Agriculture food consumption survey data from 1994 to 1996, nondiet soft drinks were the leading source of added sugars in U.S. diets, accounting for one-third of added sugar intake. This was followed by sugars and sweets (16 percent), sweetened grains (13 percent), fruit ades and drinks (10 percent), sweetened dairy products (9 percent), and breakfast cereals and other grains (10 percent). Together, they account for 90 percent of the added sugars that are consumed in the United States. Most carbohydrates occur as starches in food. Grains and certain vegetables are major contributors. Grain sources include corn, tapioca, flour, cereals, pop- corn, pasta, rice, potatoes, and crackers. Fruits and darkly colored vegetables contain little or no starch. INADEQUATE INTAKE AND DEFICIENCY The amount of dietary carbohydrate that confers optimal health in humans is unknown. The ability of humans to endure weeks of starvation after endog- enous glycogen supplies are exhausted is indicative of the body’s ability to sur- vive without an exogenous supply of glucose. However, adapting to a fat and protein fuel requires considerable metabolic adjustments. In Western urban societies, one particular concern is the long-term effect of a diet so low in carbohydrate that it induces a chronically increased produc- tion of keto acids. Such a diet may lead to bone mineral loss, hypercholester- olemia, increased risk of urolithiasis, and impaired development and function of the central nervous system. It also may adversely affect a person’s sense of
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 108 well-being and fail to provide adequate glycogen stores. The latter is required for hypoglycemic emergencies and for maximal short-term power production by muscles. ADVERSE EFFECTS OF OVERCONSUMPTION Data are mixed on potential adverse effects of overconsuming carbohydrate (i.e., sugars and starches), which include dental caries, behavioral changes, can- cer, risk of obesity, and risk of hyperlipidemia. For more information on the association between carbohydrates and chronic disease, see Part II, “Macronu- trients, Healthful Diets, and Physical Activity.”
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PART II: DIETARY CARBOHYDRATES: SUGARS AND STARCHES 109 KEY POINTS FOR DIETARY CARBOHYDRATES: SUGARS AND STARCHES Carbohydrates (sugars and starches) provide energy to the 3 cells in the body. The requirements for carbohydrate are based on the average 3 minimum amount of glucose that is utilized by the brain. Evidence was insufficient to set a UL for carbohydrates. 3 A maximal intake level of 25 percent or less of total energy 3 from added sugars is suggested, based on trends indicating that people with diets at or above this level of added sugars are more likely to have poorer intakes of important essential nutrients. Nondiet soft drinks are the leading source of added sugars in 3 U.S. diets, followed by sugars and sweets, sweetened grains, fruit ades, sweetened dairy products, and breakfast cereals and other grains. Most carbohydrates occur as starches in food. Grains and 3 certain vegetables are major contributors. Grain sources include corn, tapioca, flour, cereals, popcorn, pasta, rice, potatoes, and crackers. The amount of dietary carbohydrate that confers optimal health 3 in humans is unknown. Of particular concern is the long-term effect of a diet so low in 3 carbohydrate that it induces a chronically increased production of keto acids. Such a diet may lead to bone mineral loss, hypercholesterolemia, increased risk of urolithiasis, and impaired development and function of the central nervous system. Data are mixed on potential adverse effects of overconsuming 3 carbohydrate.