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OCR for page 102
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.
