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TABLE 1 Dietary Reference Intakes for Total Fibera by Life
Stage Group
DRI values (g/1,000 kcal) [g/day]b
AIc
males females
Life stage group
NDd
0 through 6 mo ND
7 through 12 mo ND ND
1 through 3 y 14 [19] 14 [19]
4 through 8 y 14 [25] 14 [25]
9 through 13 y 14 [31] 14 [26]
14 through 18 y 14 [38] 14 [26]
19 through 30 y 14 [38] 14 [25]
31 through 50 y 14 [38] 14 [25]
51 through 70 y 14 [30] 14 [21]
> 70 y 14 [30] 14 [21]
Pregnancy
< 18 y 14 [28]
19 through 50 y 14 [28]
Lactation
< 18 y 14 [29]
19 through 50 y 14 [29]
a Total Fiber i s the combination of D ietary Fiber , the edible, nondigestible
carbohydrate and lignin components as they exist naturally in plant foods, and
Functional Fiber, which refers to isolated, extracted, or synthetic fiber that has proven
health benefits.
b Values in parentheses are example of the total g/day of total fiber calculated from
g/1,000 kcal multiplied by the median energy intake (kcal/1,000 kcal/day) from the
Continuing Survey of Food Intakes by Individuals (CSFII 1994–1996, 1998).
c AI = Adequate Intake. If sufficient scientific evidence is not available to establish
an Estimated Average Requirement (EAR), and thus calculate a Recommended Dietary
Allowance (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 ND = Not determined.
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PART II: FIBER 111
FIBER
T
he term Dietary Fiber describes the carbohydrates and lignin that are
intrinsic and intact in plants and that are not digested and absorbed in
the small intestine. Functional Fiber consists of isolated or purified car-
bohydrates that are not digested and absorbed in the small intestine and that
confer beneficial physiological effects in humans. Total Fiber is the sum of Di-
etary Fiber and Functional Fiber. Fibers have different properties that result in
different physiological effects, including laxation, attenuation of blood glucose
levels, and normalization of serum cholesterol levels.
Since data were inadequate to determine an Estimated Average Require-
ment (EAR) and thus calculate a Recommended Dietary Allowance (RDA) for
Total Fiber, an Adequate Intake (AI) was instead developed. The AIs for Total
Fiber are based on the intake levels that have been observed to protect against
coronary heart disease (CHD). The relationship of fiber intake to colon cancer
is the subject of ongoing investigation and is currently unresolved. A Tolerable
Upper Intake Level (UL) was not set for fiber. DRI values are listed by life stage
group in Table 1.
Dietary Fiber is found in most fruits, vegetables, legumes, and grains. Di-
etary and Functional Fibers are not essential nutrients; therefore, inadequate in-
takes do not result in biochemical or clinical symptoms of a deficiency. As part
of an overall healthy diet, a high intake of Dietary Fiber will not cause adverse
effects in healthy people.
DEFINITIONS OF FIBER
Dietary Fiber, Functional Fiber, and Total Fiber
This publication defines Total Fiber as the combination of Dietary Fiber, the
edible, nondigestible carbohydrate and lignin components as they exist natu-
rally in plant foods, and Functional Fiber, which refers to isolated, extracted, or
synthetic fiber that has proven health benefits. Nondigestible means that the
material is not digested and absorbed in the human small intestine (see Box 1
for definitions). Fiber includes viscous forms that may lower serum cholesterol
concentrations (e.g., oat bran, beans) and the bulking agents that improve lax-
ation (e.g., wheat bran).
Dietary Fiber in foods is usually a mixture of the polysaccharides that are
integral components of plant cell walls or intracellular structures. Dietary Fiber
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
112
BOX 1 Definitions of Fibera
• Dietary Fiber consists of nondigestible carbohydrates and lignin
that are intrinsic and intact in plants.
• Functional Fiber consists of isolated nondigestible carbohydrates
that have beneficial physiological effects in humans.
• Total Fiber is the sum of Dietary Fiber and Functional Fiber.
a In the United States, dietary fiber is defined for regulatory purposes by a
number of analytical methods that are accepted by the Association of Official Ana-
lytical Chemists International (AOAC). In Canada, a distinction is made between
dietary fiber (defined as the endogenous components of plant material in the diet
that are resistant to digestion by enzymes produced by man) and novel fibers,
whose definition is similar to functional fiber. Novel fibers must be demonstrated to
have beneficial effects to be considered as fiber for the purposes of labeling and
claims.
sources contain other macronutrients (e.g., digestible carbohydrate and pro-
tein) normally found in foods. For example, cereal brans, which are obtained
by grinding, are anatomical layers of the grain consisting of intact cells and
substantial amounts of starch and protein. Other examples include plant
nonstarch polysaccharides (e.g., cellulose, pectin, gums, and fibers in oat and
wheat bran), plant carbohydrates (e.g., inulin, fructans), lignin, and some resis-
tant starch.
Functional Fiber may be isolated or extracted using chemical, enzymatic, or
aqueous steps, such as synthetically manufactured or naturally occurring iso-
lated oligosaccharides and manufactured resistant starch. In order to be classi-
fied as a Functional Fiber, a substance must demonstrate a beneficial physiologi-
cal effect. Potential Functional Fibers include isolated nondigestible plant (e.g.,
pectin and gums), animal (e.g., chitin and chitosan), or commercially produced
(e.g., resistant starch, polydextrose) carbohydrates.
FIBER AND THE BODY
Function
Different fibers have different properties and thus varying functions. They aid
in laxation and promote satiety, which may help reduce energy intake and there-
fore the risk of obesity. They can also attenuate blood glucose levels, normalize
serum cholesterol levels, and reduce the risk of CHD. For example, viscous
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PART II: FIBER 113
fibers can interfere with the absorption of dietary fat and cholesterol, as well as
the enterohepatic recirculation of cholesterol and bile acids, which may result
in reduced blood cholesterol concentrations and a reduced risk of CHD.
Absorption, Metabolism, and Excretion
Once consumed, Dietary Fiber and Functional Fiber pass relatively intact into
the large intestine. Along the gastrointestinal tract, the properties of different
fibers result in varying physiological effects:
Gastric emptying and satiety: Viscous fiber delays gastric emptying, thereby
slowing the process of absorption in the small intestine. This can cause a feeling
of fullness, as well as delayed digestion and absorption of nutrients, including
energy. Delayed gastric emptying may also reduce postprandial blood glucose
concentrations and potentially have a beneficial effect on insulin sensitivity.
Fermentation: Microflora in the colon can ferment fibers to carbon dioxide,
methane, hydrogen, and short-chain fatty acids. Foods rich in hemicellulose
and pectin, such as fruits and vegetables, contain Dietary Fiber that is more
completely fermented than foods rich in celluloses, such as cereals. The con-
sumption of Dietary and certain Functional Fibers, particularly those that are
poorly fermented, is known to improve fecal bulk and laxation and ameliorate
constipation.
Contribution of fiber to energy: When fiber is anaerobically fermented by
micro-flora of the colon, the short-chain fatty acids that are produced are ab-
sorbed as an energy source. Although the exact yield of energy from fiber in
humans remains unclear, current data indicate that the yield is between 1.5
and 2.5 kcal/g.
Physiological effects of isolated and synthetic fibers: Table 2 summarizes the
beneficial physiological effects of certain isolated and synthetic fibers. Note that
the discussion of these potential benefits should not be construed as endorse-
ments of the fibers. For each fiber source listed, evidence relating to one of the
three most commonly accepted benefits of fibers is presented: laxation, nor-
malization of blood lipid levels, and attenuation of blood glucose responses.
DETERMINING DRIS
Determining Requirements
There is no biochemical assay that can be used to measure Dietary Fiber or
Functional Fiber nutritional status. Blood fiber levels cannot be measured be-
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
114
TABLE 2 The Physiological Effects of Isolated and Synthetic Fibers
Potential Effect on
Attenuation
Normalization of Blood Other
of Blood Lipid Glucose Physiological
Laxation Levels Responses Effects
Cellulose Increases stool No effect on blood Did not decrease —
weight; may lipid levels or a postprandial
decrease transit slight increase glucose response.
time. in them.
Chitin and There was no Numerous animal No known reports Some animal studies
Chitosan evidence for a studies suggested in humans. have shown that
laxative effect in that chitin and chitosan reduces fat
humans. chitosan may absorption and may
decrease lipid promote weight
absorption. loss. However,
However, this has human studies have
not always been found no effect of
observed in chitosan
controlled human supplementation
studies. More on weight.
research is needed.
Guar Gum Little effect on Numerous studies Viscous fibers, —
fecal bulk or have shown an including guar
laxation. 11–16 percent gum, produced
reduction in blood significant
cholesterol levels reductions in
with guar gum glycemic response
supplementation. in 33 of 50 studies.
In addition, guar
gum has been
shown to decrease
triacylglycerol
concentrations and
blood pressure.
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PART II: FIBER 115
TABLE 2 Continued
Potential Effect on
Attenuation
Normalization of Blood Other
of Blood Lipid Glucose Physiological
Laxation Levels Responses Effects
Inulin, A few studies Studies with inulin Some, but not all, Numerous human
Oligofructose, have shown a or oligofructose studies suggest studies show that
and Fructooligo- small increase have provided that inulin and the ingestion of
saccharides in fecal bulk mixed results. fructooligo- fructooligo-
and stool saccharides saccharides
frequency with reduce fasting increases
ingestion of insulin fecal Bifidbacteria.
inulin or concentrations or This bacteria strain
oligofructose. fasting blood has been shown to
glucose. have beneficial
health effects in
animals, but the
potential benefits to
humans are not well
understood.
Extracted b-
Oat Products In a large study of Some research —
and b-Glucans glucans have adults with multiple suggests that oat
minimal effects risk factors for heart bran reduces
on fecal bulk. disease, including postprandial rises
Oat bran high LDL cholesterol in blood glucose
increases stool levels, oat cereal levels.
weight by consumption was
supplying rapidly linked to a dose-
fermented dependent reduction
viscous fiber to in LDL cholesterol.
the colon for Other research also
bacterial growth. suggests that oat
products help lower
LDL cholesterol.
continued
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
116
TABLE 2 Continued
Potential Effect on
Attenuation
Normalization of Blood Other
of Blood Lipid Glucose Physiological
Laxation Levels Responses Effects
Pectin A meta-analysis Pectin has been Viscous fibers, —
of about 100 shown to lower including pectin,
studies showed cholesterol to have significantly
that pectin is not varying degrees. reduced glycemic
an important There was some response in 33 of
fecal-bulking evidence that this 50 studies.
agent. effect was due to
increased excretion
of bile acids and
cholesterol.
Polydextrose Polydextrose In one study, — —
was shown to polydextrose
increase fecal lowered HDL (high
mass and density lipoprotein)
sometimes stool cholesterol levels.
frequency.
Findings on the
effect of
polydextrose on
fecal bacterial
production are
mixed.
Psyllium There is A number of studies When added to a —
extensive have shown that meal, psyllium has
literature on the psyllium lowers been shown to
laxative effect of total and LDL decrease the rise
psyllium, which cholesterol levels of postprandial
is the active via the stimulation glucose levels
ingredient in of bile acid and to reduce the
some over the production. glycemic index
counter of foods.
laxatives.
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PART II: FIBER 117
TABLE 2 Continued
Potential Effect on
Attenuation
Normalization of Blood Other
of Blood Lipid Glucose Physiological
Laxation Levels Responses Effects
Resistant No evidence to One study showed One animal study —
Dextrins support a that resistant and two human
laxative effect. maltodextrin helps studies suggest
reduce blood that resistant
cholesterol and maltodextrins
triacylglycerol reduce fasting
levels. and postprandial
blood glucose
levels.
Resistant Increased fecal Several animal In one study, —
Starch bulk due to studies have adding resistant
increased starch shown that starch to bread
intake has been resistant starch at various levels
reported. lowers blood was shown to
Because cholesterol and reduce the
resistant starch triacylglycerol glycemic index in
is partly levels. In humans, a dose-dependent
fermented in the resistant starch manner.
colon, intake does not appear
may lead to an to provide the
increased cholesterol-
production of lowering effects of
short-chain viscous fiber, but
fatty acids. rather acts more
like nonviscous
fiber.
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
118
cause fiber is not absorbed. Therefore, the potential health benefits of fiber
consumption have been considered in determining DRIs.
Since information was insufficient to determine an EAR and thus calculate
an RDA, an AI was instead developed. The AIs for Total Fiber are based on the
intake level observed to protect against CHD based on epidemiological, clini-
cal, and mechanistic data. The reduction of risk of diabetes can be used as a
secondary endpoint to support the recommended intake level. The relationship
of fiber intake to colon cancer is the subject of ongoing investigation and is
currently unresolved. Recommended intakes of Total Fiber may also help ame-
liorate constipation and diverticular disease, provide fuel for colonic cells, re-
duce blood glucose and lipid levels, and provide a source of nutrient-rich, low
energy-dense foods that could contribute to satiety, although these benefits were
not used as the basis for the AI.
There is no AI for fiber for healthy infants aged 0 to 6 months who are fed
human milk because human milk does not contain Dietary Fiber. During the 7-
to 12-month age period, solid food intake becomes more significant, and so
Dietary Fiber intake may increase. However, there are no data on Dietary Fiber
intake in this age group and no theoretical reason to establish an AI. There is
also no information to indicate that fiber intake as a function of energy intake
differs during the life cycle.
Criteria for Determining Fiber Requirements,
by Life Stage Group
Life stage group Criterion
NDa
0 through 6 mo
7 through 12 mo ND
1 through 70 y Intake level shown to provide the greatest protection against
coronary heart disease (14 g/1,000 kcal) ¥ median energy
intake level from CSFII (1994-1996, 1998)
(kcal/1,000 kcal/day)
Pregnancy and Intake level shown to provide the greatest protection against
coronary heart disease (14 g/1,000 kcal) ¥ median energy
Lactation
intake level from CSFII (1994–1996, 1998)
(kcal/1,000 kcal/day)
a Not determined.
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PART II: FIBER 119
The UL
The Tolerable Upper Intake Level (UL) is the highest daily nutrient intake that
is likely to pose no risk of adverse effects for almost all people. Although occa-
sional adverse gastrointestinal symptoms are observed when consuming some
of the isolated or synthetic fibers, serious chronic adverse effects have not been
observed. A UL was not set for Dietary Fiber or Functional Fiber. Due to the
bulky nature of fibers, excess consumption is likely to be self-limited.
DIETARY SOURCES
Dietary Fiber is found in most fruits, vegetables, legumes, and grains. Nuts,
legumes, and high-fiber grains typically contain fiber concentrations of more
than 3 percent Dietary Fiber, or greater than 3 g/100 g of fresh weight. Dietary
Fiber is present in the majority of fruits, vegetables, refined grains, and miscel-
laneous foods such as ketchup, olives, and soups, at concentrations of 1 to 3
percent or 1 g/100 g to 3 g/100 g of fresh weight.
Dietary Supplements
This information was not provided at the time the DRI values for fiber were set.
Bioavailability
Fiber is not absorbed by the body.
Dietary Interactions
Foods or diets that are rich in fiber may alter mineral metabolism, especially
when phytate is present. Most studies that assess the effect of fiber intake on
mineral status have looked at calcium, magnesium, iron, or zinc (see Table 3).
INADEQUATE INTAKE AND DEFICIENCY
Dietary and Functional Fibers are not essential nutrients, so inadequate intakes
do not result in biochemical or clinical symptoms of a deficiency. A lack of these
fibers in the diet, however, can cause inadequate fecal bulk and may detract
from optimal health in a variety of ways depending on other factors, such as the
rest of the diet and the stage of the life cycle.
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS
120
TABLE 3 Potential Interactions of Dietary Fiber with Other Dietary
Substances
Substance Potential Interaction Notes
FIBER AFFECTING OTHER SUBSTANCES
Calcium Decreased calcium absorption Some types of fiber have been shown to significantly
when ingested with Dietary increase fecal excretion of calcium. However, most
Fiber human studies have reported no effect.
Magnesium Decreased magnesium Studies report no effect on magnesium balance or
absorption when ingested absorption.
with Dietary Fiber
Iron Reduced iron absorption In one study, the addition of 12 g/day of bran to a meal
when ingested with Dietary decreased iron absorption by 51–74 percent, which
Fiber was not explained by the presence of phytate. Other
studies suggest that the effect of bran on iron
absorption is due to phytate content rather than fiber.
Zinc Reduced zinc absorption Most studies also include levels of phytate that are
when ingested with Dietary high enough to affect zinc absorption. Metabolic
Fiber balance studies in adult males consuming 4 oat bran
muffins daily show no changes in zinc balance.
ADVERSE EFFECTS OF CONSUMPTION
Although occasional adverse gastrointestinal symptoms were observed with the
consumption of Dietary and Functional Fibers, serious chronic adverse effects
have not been observed. The most potentially deleterious effects may arise from
the interaction of fiber with other nutrients in the gastrointestinal tract. Addi-
tionally, the composition of Dietary Fiber varies, making it difficult to link a
specific fiber with a particular adverse effect, especially when phytate is also
present. It has been concluded that as part of an overall healthy diet, a high
intake of Dietary Fiber will not cause adverse effects in healthy people. In addi-
tion, the bulky nature of fiber tends to make excess consumption self-limiting.
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PART II: FIBER 121
KEY POINTS FOR FIBER
A new set of definitions for fiber has been developed for
3
Dietary Fiber, Functional Fiber, and Total Fiber. The term Dietary
Fiber describes the nondigestible carbohydrates and lignin that
are intrinsic and intact in plants. Functional Fiber consists of
the isolated nondigestible carbohydrates that have beneficial
physiological effects in humans. Total Fiber is the sum of
Dietary Fiber and Functional Fiber. Nondigestible means not
digested and absorbed in the human small intestine.
There is no biochemical assay that reflects Dietary Fiber or
3
Functional Fiber nutritional status. Blood fiber levels cannot be
measured because fiber is not absorbed.
Since data were inadequate to determine an EAR and thus
3
calculate an RDA for Total Fiber, an AI was instead developed.
The AI for fiber is based on the median fiber intake level
3
observed to achieve the lowest risk of CHD.
A UL was not set for Dietary Fiber or Functional Fiber.
3
Dietary Fiber is found in most fruits, vegetables, legumes, and
3
grains.
Dietary and Functional Fibers are not essential nutrients,
3
therefore inadequate intakes do not result in biochemical or
clinical symptoms of a deficiency.
As part of an overall healthy diet, a high intake of Dietary Fiber
3
will not cause adverse effects in healthy people.
