Livro DRI 2006 (Micronutrientes)

Livro DRI 2006 (Micronutrientes)


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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\u2019s
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Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
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106 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
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 A
1c
, 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
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
PART II: DIETARY CARBOHYDRATES: SUGARS AND STARCHES 107
The AMDR
The AMDR for carbohydrates for both adults and children is 45\u201365 percent
of total calories (see Part II, \u201cMacronutrients, Healthful Diets, and Physical
Activity\u201d).
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\u2019s 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\u2019s sense of
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
108 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
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, \u201cMacronu-
trients, Healthful Diets, and Physical Activity.\u201d
Copyright © National Academy of Sciences. All rights reserved.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements
http://www.nap.edu/catalog/11537.html
PART II: DIETARY CARBOHYDRATES: SUGARS AND STARCHES 109
KEY POINTS FOR DIETARY
CARBOHYDRATES: SUGARS AND
STARCHES
3 Carbohydrates (sugars and starches) provide energy to the
cells in the body.
3 The requirements for carbohydrate are based on the average
minimum amount of glucose that is utilized by the brain.
3 Evidence was insufficient to set a UL for carbohydrates.
3 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.
3 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