Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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overall procedure for measuring total dietary fiber in

human foods. Nonfiber materials such as starch, most

protein, and any fiber-like artifacts remaining in the

NDF residue can be efficiently removed by rapid

treatment with a-amylase from porcine pancreas (A-

3176, Sigma Chemicals Co., St Louis, MO); the rapid

treatment consists of 5-min and 60-min incubations

at 55



C and is at least as effective as the 18-h treat-

ment at 37



C originally proposed by Schaller. The

source of the enzyme is specified since pancreatin and

other preparations of porcine pancreas a-amylase are

much less efficient. Unpurified porcine a-amylase has

unique amylolytic and proteolytic activities when

applied after the neutral detergent extraction step,

making it an ideal enzyme to use for measuring insol-

uble dietary fiber. (See Enzymes: Uses in Analysis;

Protein: Determination and Characterization; Starch:

Structure, Properties, and Determination.)

0014 The heat-stable a-amylase from Bacillus subtilis

has also been used to digest starch after the neutral

detergent extraction step, but several potential

problems arise with this enzyme. It may contain

fiber-digesting enzymes that could lead to an under-

estimation of the cellulose and hemicellulose frac-

tions. On the other hand, it does not completely

remove the nonfiber material (such as starch), which

will lead to an overestimation of the lignin and

possibly the hemicellulose fractions. The final value,

therefore, represents a compromise between these

two competing factors. Table 1 shows that, for shred-

ded wheat, the NDF residue obtained after treatment

with B. subtilis a-amylase contained 30% more ma-

terial than that obtained after treatment with porcine

pancreas a-amylase. This residue apparently con-

tained three times more lignin and four times more

protein. Both the NDF residue and the apparent

lignin content were also higher in samples of bran

flakes. Other problems are apparent with the B. sub-

tilis a-amylase. For example, toasting wholewheat

bread increased the B. subtilis a-amylase-measured

NDF and lignin values by two- and fivefold, respect-

ively (Table 1), but no such differences were noted

with the porcine pancreas a-amylase. The efficacy of

the porcine pancreas a-amylase treatment has been

verified in various foods and food products and con-

firmed by other researchers.

0015In addition to its demonstrated capabilities in

avoiding artifactual increases in the estimation of

various dietary fiber fractions, porcine pancreas

a-amylase enzyme also facilitates rapid filtration in

subsequent steps (starch and protein tend to clog the

crucibles). The neutral detergent–a-amylase combin-

ation is advantageous for research analyses since the

neutral detergent solution discriminates well between

plant and bacterial cell walls. This is important

because bacterial cell walls interfere with the fiber

analysis of fecal samples.

0016Other modifications from the original Van Soest

NDF method include the deletion of decalin, 2-

ethoxy-ethanol, and sulfite (to maximize the recovery

of lignin).

Rapid HPB Method

0017The rapid HPB method described here is the AOAC

No. 992.16 method. In order to obtain a measure of

the TDF in a sample, one needs only to combine the

insoluble dietary fiber from the NDF þ porcine pan-

creas a-amylase treatment with a specifically tailored

method for measuring soluble fiber. In the rapid HPB

method, the soluble fiber procedure was devised to

maximize the recovery of soluble fiber while exclud-

ing protein and other precipitable, nonfiber materials.

This latter characteristic is important if one wishes to

avoid time-consuming protein determinations that

are part of other procedures. The soluble fiber pro-

cedure includes a gelatinization step in acetate buffer

at pH 4.5 (121



C) followed by a treatment with heat-

stable a-amylase (100



C) to remove any starch.

Starch gelatinization is important to insure complete

starch hydrolysis. The soluble material is separated

tbl0001 Table 1 Effect of two sources of a-amylase on the apparent neutral detergent fiber (NDF) and lignin content of breakfast cereals and

bread (g per 100 g, dry weight basis)

Porcine pancreas a-amylase

Bacillus subtilis a-amylase

NDF Lignin Protein

Shredded wheat 10.2 0.8 0.4 13.4

2.7

1.7

Bran flakes 10.1 0.9 15.8 2.4

Wholewheat bread

5.2 0.3 5.9 0.4

Wholewheat bread, toasted

5.4 0.4 10.1 2.2

a-Amylase from porcine pancreas (Sigma Chemical Co., catalogue no. A-3176).

Protein measured in NDF residue (N  6.25).

From Van Soest PJ (1978) Fiber analysis tables. AmericanJournalofClinicalNutrition31: S284.

From Mongeau R, Brassard R (1980) Rapid digestion of starch and artifact fibre in the measurement of neutral detergent fibre of cereal products.

Getreide Mehl und Brot 34: 125–127.

1826 DIETARY FIBER/Determination

from insoluble fiber by filtration and is treated at



C with amyloglucosidase (to finish starch diges-

tion by hydrolyzing branched residues), and then with

protease (to hydrolyze the peptide bonds of protein).

The soluble fiber is then precipitated in 80% ethanol

at room temperature; the sugar and amino acid resi-

dues remain soluble in ethanol.

0018 Acetate buffer at pH 4.5 favors the exclusion of

protein and appears to prevent the degradation of

fiber components that may occur at high tempera-

tures. This does not occur as readily at the relatively

higher pH values used by other TDF procedures (e.g.,

phosphate buffer at pH 7). The heat-processed refer-

ence standards extracted from apple and carrot, and

used in some collaborative studies, may show lower

TDF values due to the loss of pectin during analysis.

The differences are less apparent when expressed on a

‘per g fresh weight’ basis. Without a method for

accurately measuring all cell wall components it is

difficult to provide an absolute compositional meas-

ure of the constituents of dietary fiber as measured by

different methods. However, the use of differential

procedures to obtain fiber fractions (ADF, NDF),

coupled with sulfuric acid digestion, permanganate

oxidation and total nitrogen analysis, allows an esti-

mate of the relative contribution of several plant

components to the final TDF value.

0019 Figure 2 shows a comparison between several

methods for measuring dietary fiber as well as the

relative amounts of the components retained by

each method. The rapid HPB method retains lignin

(a structural polyphenol), the cell wall-associated

protein, and a small amount of starch. The significant

amount of starch retained by the Prosky method (see

below) is readily apparent when TDF values are

measured in legumes. Table 2 shows a comparison

of four different methods for measuring TDF in green

peas. The starch in the residue was estimated by

subtracting the values from the total nonstarch poly-

saccharides (NSPs) þ lignin measured by the Englyst

procedure. The table also shows the effect of adding

porcine pancreas a-amylase.

0020The method is considered ‘rapid’ when compared

to other fiber methods, particularly when the Fibertec

E (for soluble fiber) and the Fibertec I (for insoluble

fiber) are used. For example, the maximum number

of duplicate determinations using the Prosky TDF

method is 20 per week, while 44 duplicate TDF de-

terminations per week are possible using the rapid

HPB method. This evaluation takes into account the

time required for preparing the reagents and perform-

ing the analysis, but not the time required for prepar-

ing the samples (freeze-drying, grinding).

0021Both separate and sequential rapid HPB methods

are available and Table 3 presents a comparison be-

tween them. In the sequential method, the insoluble

residue generated during the soluble fiber determin-

ation is treated with neutral detergent and porcine

a-amylase yielding the insoluble fiber residue. Thus,

a single sample generates both insoluble and soluble

fiber measurements to give the TDF value. The separ-

ate method requires two samples for analysis. Using a

Englyst

Uppsala no. 994.13

Prosky no. 985.29

Lee no. 991.43

Rapid HPB no. 992.16

Structural NSP

Structural protein

Nonstructural

NSP

Lignin

Starch

Maillard

products?

fig0002 Figure 2 Material measured as dietary fiber by unmodified methods for all food categories. The surface area of the box is

proportional to its total dietary fiber (TDF) value. It shows whether the method includes lignin and structural fiber protein or if it

excludes starch and Maillard products. The cell wall material corresponds to the structural nonstarch polysaccharides (NSP), lignin,

and structural protein. The larger amounts of starch retained by the Theander, Prosky, and Lee methods are most evident when

measuring TDF in legumes. AOAC numbers for each procedure are indicated. Adapted from Mongeau R (1995). HPB, Health Protection

Branch, Health Canada.

DIETARY FIBER/Determination 1827

single sample to measure TDF gives a more accurate

and reliable measurement because it eliminates

potential cross-contamination that could give rise to

falsely high TDF readings. For example, if the insol-

uble fiber fraction contains some small amount of

soluble fiber, it will be counted twice in the final

value (once in the insoluble fraction and once in the

soluble fraction). The far right column of Table 3

shows that the TDF values are comparable when

measured by either procedure and demonstrates the

variability in the procedure.

0022 In a comparison of 28 foods from various categor-

ies, the rapid sequential HPB method agreed well

with the Prosky TDF method (slope ¼0.99, intercept

¼0.11, r

¼0.985) and with the Englyst GLC

method (slope ¼1.03, intercept ¼0.15, r

¼0.989).

For comparison with the Englyst method, it was

necessary to measure lignin in each food and add its

weight to the total NSP (obtained from the Englyst

analysis). This gave a TDF value that was comparable

to that measured gravimetrically. The regression

parameters indicate that the rapid HPB method and

the Prosky TDF method retain similar material in the

final fraction (Figure 2). However, the negative inter-

cept shows that the Prosky TDF method gives a

higher value than the rapid HPB method when meas-

uring foods containing low amounts of fiber.

(See Carbohydrates: Classification and Properties;

Chromatography: Gas Chromatography.)

Prosky, Lee, and Asp Methods

0023The Prosky TDF method has also been referred to as

the AOAC method since 1985. The Prosky, Lee, and

Asp methods are similar but differ in the sources and

types of enzymes as well as in the type and pH value

of the buffers. The TDF values from all three methods

are comparable (see Table 2 for a comparison be-

tween the Prosky and Lee methods for green peas).

In the Prosky and Lee TDF methods, duplicate

samples are treated with a-amylase, protease, and

amyloglucosidase to remove digestible material.

Four volumes of 95% ethanol is then added and the

precipitate filtered through celite C-211 (Fisher Sci-

entific, Mississauga, Ontario, Canada). The residue

tbl0002 Table 2 Total dietary fiber (TDF) in boiled dried green peas as measured by four different methods with or without added porcine

pancreas a-amylase

Method TDFon a‘perg dry weight basis’ (%) Estimated starchcontent (%)

Rapid HPB method, separate 9.9 + 0.1 2.7

Prosky TDF method 19.0 + 0.2 11.8

Prosky TDF method þ porcine pancreas a-amylase 12.3 + 0.3 5.1

Lee TDF method 18.8 + 0.3 11.6

Lee TDF method þ porcine pancreas a-amylase 12.3 + 0.3 5.1

Englyst method (NSP þ lignin) 7.2 + 0.2

Englyst method (omit DMSO) 10.4 + 0.1 3.2

NSP, nonstarch polysaccharides; DMSO, dimethyl sulfoxide.

Starch content estimated by subtracting the NSP þ lignin value from the TDF estimate. Table adapted from Mongeau R and Brassard R (1994)

Comparison and assessment of the difference in total dietary fiber in cooked dried legumes as determined by five methods. Journal of AOAC International

77: 1197–1202.

tbl0003 Table 3 Comparison of total dietary fiber (TDF) values by gas liquid chromatography (GLC: nonstarch polysaccharides NSPs þ

lignin) or gravimetric methods (on a g 100 g

1

dry weight basis)

GravimetricTDF values

Food

NSP

Permanganate

lignin Prosky

Rapid

separate

Rapid

sequential

Apple 9.9–14.7 13.4 0.8 14.6 14.1 14.6

Beans, green 27.8–32.2 30.6 2.1 33.7 31.3 29.1

Bread, white wheat 2.6–3.3 3.0 0.6 3.5 3.0 3.9

Carrot 22.8–28.9 23.5 1.8 25.4 23.4 23.4

Corn kernels, canned 6.7–8.9 5.9 1.0 7.7 7.8 7.6

Cornflakes 0.7–2.4 1.1 < 0.1 2.5 1.1 2.6

Flour, white wheat 3.0–3.7 2.6 < 0.1 3.0 3.9 3.6

Oats, rolled 7.1–9.5 8.2 0.9 10.5 11.0 9.6

Peas, canned 20.4–22.3 – – – 23.3 20.9

Range of values reported by Anderson and Bridges (1988), Englyst and Cummings (1989), Englyst et al. (1995) and Marlett (1992): see Further Reading.

NSP as determined by the Englyst GLC method.

TDF as determined by the Prosky method and the rapid separate and sequential methods.

The separate method required analyzing two separate samples to obtain the TDF value.

1828 DIETARY FIBER/Determination

contains a substantial amount of nitrogen requiring a

total nitrogen analysis (N) and subsequent back-cal-

culation to correct for protein content (N content 

6.25). One of the duplicates is also analyzed for ash as

in other gravimetric analyses and this, too, is sub-

tracted from the residue weight. It is proposed that

the use of this protein conversion factor leads to

overestimation of dietary fiber (Table 4). The add-

itional protein determination step prevents this

method from being considered rapid. As pointed out

above and in Tables 3 and 4, the final insoluble

dietary fiber residue can retain significant amounts

of starch. In a simplified version of the Prosky

method, the protease step is skipped since protein is

measured in the final residue. This means that a larger

amount of protein is subtracted as compared with the

original Prosky procedure and this may contribute to

some variability. The nature of the celite used in

different laboratories has also been considered as a

source of variability. Other modifications have also

been proposed to improve the precision of the Prosky

method. The method comparisons in Tables 2 and 3

were confined to the results obtained with the original

version of the Prosky method.

Complete Polysaccharidic Components

(GLC Methods)

0024 Southgate published the original TDF method that

required the hydrolysis of fiber into its individual

polysaccharidic components. In Southgate’s method,

TDF represented NSPs plus lignin but the method

also provided values for hemicellulose and cellulose

content. The method used 85% (v/v) alcohol to pre-

cipitate soluble starch residues followed by gelatin-

ization and starch digestion with an appropriate

enzyme. The original enzyme used by Southgate

(takadiastase) had both amylolytic and proteolytic

activities. However, in the 1970s, this enzyme became

unavailable and other enzymes were used as replace-

ments. It is apparent that some of these replacement

enzymes are often less effective (Tables 2 and 3) but

the a-amylase from porcine pancreas appears to be

adequate. The original Southgate method measured

the NSPs colorimetrically but now more specific

methods are available such as GLC (See Spectros-

copy: Visible Spectroscopy and Colorimetry.)

0025The more modern chemical methods use a combin-

ation of enzyme and/or chemical treatments to

remove digestible material. The residue is then hydro-

lyzed by acid prior to measurement of the sugar con-

tent. The acid treatment needs to be severe enough to

assure complete hydrolysis but mild enough to pre-

vent monomer degradation. The neutral fiber poly-

saccharidic constituents are measured chemically,

usually by GLC after derivitization to a measurable

form (e.g., by forming alditol acetates). Chemical

derivitization is not required when high-performance

liquid chromatography (HPLC) is used to measure

the residue concentration. Acidic sugars (uronic

acids) are measured colorimetrically. The total NSP

content is obtained by appropriate calculations and

TDF is calculated as the sum of NSPs þlignin. The

main GLC methods have been published by Ander-

son, Englyst, and Theander and their coworkers.

Marlett uses a modified Uppsala method. The GLC

methods are different in several aspects, including the

starch gelatinization step, enzyme treatments, and

conditions of acid hydrolysis. This explains why the

methods may be in agreement for some foods, but not

for others. (See Chromatography: High-performance

Liquid Chromatography.)

Englyst Method

0026Like all GLC methods, the Englyst method measures

NSPs (neutral sugars þuronic acids). However,

unlike other methods, the Englyst method is com-

pletely applicable to processed foods because it

makes use of dimethyl sulfoxide (DMSO) during the

gelatinization step to disperse the starch completely.

This is followed by treatment with a heat-stable

a-amylase and by a mixture of pullulanase-pancreatin

tbl0004 Table 4 Polysaccharide and unidentified material in the Prosky total dietary fiber (TDF) method for selected food groups

ProskyTDFmethod residue (g100 g

1

dry matter)

Food Starch Totalpolysaccharides Unidentified material

Bread 1.0 5.1 0.8

Other cereals 0.3 3.8 1.0

Green vegetables 1.5 25.6 4.6

Other vegetables 1.8 10.2 7.5

Canned vegetables 2.3 12.9 4.0

Fresh fruit 0.2 8.6 3.5

Fruit products 0.1 2.5 1.5

Nuts 0.4 6.7 2.5

Adapted from Englyst et al. (1995): see Further Reading. Values represent TDF measurements after correction for ash and ‘protein’ (N  6.25). Part of the

unidentified material represents lignin but the values are too large to be accounted for as lignin only.

DIETARY FIBER/Determination 1829

in acetate buffer at pH 5.2. This treatment completely

removes all the starch, leaving only the nonstarch

polypeptides (Figure 2). The total insoluble and

soluble NSPs are then precipitated in 80% ethanol

and collected by centrifugation prior to hydrolysis

with sulfuric acid. Neutral sugar residues are meas-

ured by GLC as alditol acetates and uronic acids are

measured colorimetrically. According to Englyst and

Cummings, the method can be performed in approxi-

mately the same amount of time as the Prosky TDF

method. However, the GLC instrument must be

equipped with an autosampler for the comparison to

be valid. A rapid version of the Englyst method also

exists with the sugar components measured colorime-

trically but this does not represent a substantial

reduction in assay time when compared to the GLC

method with an autosampler.

0027 The Englyst method is the only dietary fiber

method that does not include lignin. The role of non-

carbohydrate components (which include lignin) in in

vivo fiber digestion may be significant (See Dietary

Fiber: Properties and Sources). There is some debate

over whether some of the starch and structural pro-

tein should be considered as dietary fiber. (See Dietary

Fiber: Properties and Sources). These components

would have to be measured by other methods if they

were included in the TDF value.

Uppsala Method

0028 In the Uppsala method, there is a provision for an

initial extraction in 80% ethanol if the sample

contains very large amounts of mono-, di-, and oligo-

saccharides. This extraction appears important in re-

ducing any coprecipitation of nonpolymeric sugars

and fiber. Starch is digested in acetate buffer at pH

5.0 by incubating with a thermostable a-amylase and

then with amyloglucosidase. DMSO is not used and

starch may be less efficiently dispersed and, therefore,

less efficiently removed (Figure 2). After reprecipitat-

ing in ethanol and centrifuging, the residue is hydro-

lyzed with sulfuric acid. As in the Englyst method,

neutral polysaccharides are analyzed as alditol acet-

ates by GLC and uronic acids by decarboxylation

with special equipment or by colorimetry.

0029 Unlike the Englyst method, Klason lignin is meas-

ured gravimetrically. Potential errors can arise when

lignin is measured independently and added back to

NSPs. For example, the lignin fraction may include

other, nondigested fiber material (such as cellulose)

which would then overestimate insoluble fiber. This

was apparently the reason behind an apparent lignin

value of 0.9–3.5% (dry weight basis) measured in

apples, peas, or a food composite. This value is suspect

because low permanganate lignin values (obtained

after treatment of the insoluble fiber residue with

porcine pancreas a-amylase and subsequent work-

up to obtain an ADF residue) were obtained which

were independent of any food-processing method

(similar to the values in Table 1). Marlett has dis-

cussed the necessity of measuring lignin and the

problems related to its measurement.

Anderson Method

0030As in the Englyst method, the Anderson GLC method

uses DMSO to disperse starch prior to hydrolysis.

The starch is then digested with porcine pancreas

a-amylase in acetate buffer at pH 5.2. After precipi-

tation with four volumes of ethanol, the residue is

extracted with water at 100



C to obtain soluble and

insoluble fiber fractions. Following centrifugation,

the soluble fiber from the supernatant and the insol-

uble fiber from the pellet are hydrolyzed with sulfuric

acid. Klason lignin is measured as the loss in weight of

the nonhydrolyzed insoluble residue after ashing.

Comparison of GLC and Gravimetric

Methods for Measuring TDF

0031As already indicated, there is general agreement be-

tween the rapid HPB method and the Prosky and

Englyst methods. The regression coefficients were

0.985 and 0.989 for the Prosky and Englyst methods,

respectively. Other comparisons have also shown

good agreement between values. For example, in a

comparison of 25 different foods, TDF values as

measured by the rapid HPB method agreed with

those reported by Anderson and Bridges (modified

Englyst method) and with those reported by Marlett

(modified Uppsala method). However, more direct

comparisons are needed to determine the potential

problems with each method.

0032Table 3 presents a comparison of the TDF values

expressed on a ‘per g dry weight’ basis to magnify

differences which may be partly hidden when the

values are expressed on a ‘per g fresh weight’ basis.

In order to compare TDF values, the permanganate

lignin values need to be added to the NSP values.

Three different levels of variability are apparent from

the data shown in Table 3. The range of NSP values

obtained from three different laboratories (using three

slightly different methods and different food samples)

illustrates, in part, the intermethod variability. In add-

ition, there is some interlaboratory variability, as

shown by a comparison of four laboratories as well

as an inherent variability in the samples (including

stage of maturation of the product; (See Dietary

Fiber: Properties and Sources). Direct comparisons

are only possible for the results in the five right-hand

columns. These samples were composites based on

winter and summer collections of up to 20 individual

1830 DIETARY FIBER/Determination

foods over 30 months in different locations. The

results show that, when lignin is included in the final

TDF value, there is good agreement between GLC and

gravimetric methods for the indicated foods.

Nutritional Labeling and Analysis

0033 It is difficult to decide on a single method for measur-

ing the dietary fiber content of all foods. The chal-

lenge is to find a single method that works adequately

for all food groups and is independent of sample

processing. Several candidates are possible but to a

large extent, the final decision will reflect the defin-

ition of dietary fiber. (See Dietary Fiber: Properties

and Sources). The dietary fiber definition must not be

driven by available methodologies but must be ar-

rived at independently of methodological consider-

ations. As shown in Figure 2, it is possible to make a

case for including structural protein, lignin, struc-

tural, and nonstructural NSPs, as well as some starch.

Each method includes variable amounts of all these

components. In addition to these complications, it is

also possible to separate TDF into insoluble and sol-

uble components. Once again, each method uses dif-

ferent procedures to measure these fractions so that

the measured content of insoluble and soluble fiber

will differ from laboratory to laboratory. Consump-

tion of these different fiber fractions has not yet been

related to definite physiological effects, although

there appears to be a role for viscous soluble dietary

fiber in reducing the risk factors for cardiovascular

disease. The role of starch and other nonstarch poly-

mers not traditionally associated with dietary fiber

has yet to be determined as well. Therefore, the

following discussion deals only with practical consid-

erations in choosing a TDF method for food labeling.

Equipment

0034 A method requiring simple, inexpensive equipment

would be the most widely applicable because it

would be more acceptable to nations unwilling or

unable to spend considerable sums of money to moni-

tor labeling. In general, the gravimetric methods re-

quire simpler equipment and the rapid HPB method is

less costly than other gravimetric procedures because

it does not require elemental nitrogen analysis. The

Englyst colorimetric method can be completed with a

simple spectrophotometer, although this method is

less robust than the GLC method. Except for the

Englyst method, all methods require a muffle furnace

for ashing the samples. Although the gravimetric

systems require simpler equipment, a hot-water

supply is needed and analyses are greatly aided by

purchasing semiautomated filtering/refluxing equip-

ment that would increase the cost. However, these

pieces of equipment are usually less expensive to

maintain than GLCs.

Operator Time

0035Because it does not include a protein determination,

the rapid HPB method requires the least amount of

operator time per duplicate determination. Methods

that require sugar derivitization and analysis by GLC

tend to be longer than the gravimetric methods, espe-

cially when lignin measurement is included. The time

required for these methods is greatly reduced if the

GLC system includes an autosampler. Semiautomatic

equipment is available for the gravimetric determin-

ations but filtering time can be slow, especially for

samples that contain a considerable amount of starch.

Lignin

0036The Englyst method is the only one that does not

include lignin in the final TDF value. Most research-

ers consider lignin to be an important dietary fiber

constituent (See Dietary Fiber: Properties and

Sources) and, consequently, most methods include a

measure of the lignin content. In the GLC methods,

the adoption of the Klason method (residue after

sulfuric acid digestion) for measuring lignin means

that other nonfiber material such as Maillard prod-

ucts may be included in the lignin fraction. This could

lead to overestimation of the lignin content and, con-

sequently, overestimation of the insoluble dietary

fiber and TDF values. Lignin may be more accurately

measured by a combination of NDF þ porcine pan-

creas a-amylase and ADF treatments prior to perman-

ganate oxidation. This would, however, increase the

time required for analysis as well as increase the

amount of sample needed per assay.

Independence from Food Processing

0037Different methods include different amounts of starch

in the final TDF estimate (Figure 2). The only method

that does not include any starch is the Englyst method

because it completely solubilizes the starch with

DMSO prior to enzymatic hydrolysis. There are sev-

eral different types of starch, some of which arise

from food preparation such as retrograde starch and

so the inclusion of starch in a method could be prob-

lematic. An example of the potential for problems

comes from measuring the TDF values in cooked

and uncooked rice and potatoes (high-starch foods)

using the Prosky method. The Prosky method gave

higher TDF values in the cooked foods. Problems

were also noted with roasted peanuts where nonfiber

material was included with TDF as well as toasted

wholewheat bread (Table 1). Lower cooking tem-

peratures (i.e., 95



C compared to 100–120



DIETARY FIBER/Determination 1831

artificially increased the TDF content of white navy

beans when the Prosky or Lee methods were used.

Some of the problems may have been related to the

choice of enzyme used to degrade starch, as was

observed for toasted wholewheat bread (Table 1).

0038 In general, the Englyst method is independent of

the effects of food processing because it uses DMSO

to aid in removing starch and nonpolymeric sugars.

This helps exclude food preparation artifacts, such as

retrograde starch, as well as Maillard products from

the final analysis but it also means that lignin is not

included. It also means that lignin is excluded from

the final analysis. Other methods that rely on GLC

identification of NSPs include Klason lignin but this

analysis has other problems (see above). Of the gravi-

metric methods, the rapid HPB method appears to be

independent from food processing and, at the same

time, includes lignin in the final value.

Precision

0039 Method variability pooled over all foods has been

measured in a US Department of Agriculture study

using 25 duplicate food samples. Different laborator-

ies were asked to measure TDF in the samples with-

out knowing their composition or source. The

coefficient of variation for a single observation was

3.0% with the rapid HPB method, 4.7% with the

Prosky TDF method, and 8.4% for the Uppsala

method. This represents an estimation of the intrala-

boratory precision for selected unprocessed and pro-

cessed foods. More interlaboratory work comparing

precision of several methods is needed.

Remaining Problems

0040 The general agreement between the different methods

for measuring fiber is encouraging because it opens

the door to universal acceptance of a dietary fiber

method. However, many problems remain. First, an

alcohol precipitation step is used to separate fiber

material from nonfiber material. This step usually

follows starch digestion and is designed to precipitate

sugar polymers. It is possible that some of the fiber

components are not completely precipitated and that

some sugars released by starch digestion are copreci-

pitated. This may vary with the method and may be

the source of some of the intermethod variability. In

addition, the selectivity of this step in separating

sugar polymers from nonpolymeric compounds has

been questioned. In particular, polymers such as oli-

gofructose (actually a mixture of oligomers and poly-

mers) do not precipitate during the subsequent

centrifugation. The ‘dietary fiber’ nature of these

polymers is currently being debated. (See Dietary

Fiber: Properties and Sources.)

0041Second, structural fiber protein may represent up

to 10% of dietary fiber in immature plant tissues. It

is impossible, at the present time, to quantify this

protein (although the rapid HPB gravimetric method

presumably includes it). Except for the rapid HPB

method, this fraction is excluded from the reported

TDF values.

0042Third, the colorimetric method for determination

of uronic acid values (included in most GLC methods)

needs to be reexamined. This is because the uronic

acid content of vegetables was shown to be higher

when measured by a decarboxylation method.

0043Fourthly, there are a small number of foods where

significant method disagreement occurs. For example,

for soya bean fiber isolate the NSP value using

the Englyst GLC method is lower than the TDF

value measured gravimetrically. The reasons for

these discrepancies need to be examined in order to

determine the exact TDF composition measured by

each method. These comparisons will greatly improve

fiber methodology.

0044Finally, although both acetone and ether have been

used to remove fat from the samples prior to analysis,

their efficacy is questionable and only the defatting

procedure of Bligh and Dyer is efficient enough to

defat all food samples completely. Caution is war-

ranted when substituting defatting procedures since

TDF methods have been validated with their respect-

ive defatting methods and substitutions may produce

unexpected results. For example, the degree of lignin

removal with the Bligh and Dyer procedure is un-

known. There is some question as to the necessity

for complete defatting since the methods were valid-

ated with a wide variety of food samples, all with

different fat contents. It may be necessary to reexa-

mine the importance of fat removal prior to analysis

in samples containing high amounts of fat.

Conclusion

0045Comparable TDF values can be obtained for many

foods using any of the methodologies discussed

above. Some foods are more problematic and require

a careful choice of methodology. Significant causes of

error have been identified, including the potential

precipitation of incompletely digested starch and the

measurement of Klason lignin, particularly in pro-

cessed foods. The inclusion of Maillard products in

Klason lignin may significantly increase the final TDF

value. The following treatment is recommended:

NDF, porcine pancreas a-amylase, ADF, and perman-

ganate treatment. This procedure gives permanganate

lignin values that are independent of food processing

and, when added to NSPs determined by GLC, pro-

vides a detailed and known measurement of dietary

1832 DIETARY FIBER/Determination

fiber. The rapid HPB method, including the neutral

detergent and porcine a-amylase treatments, has

several advantages and would serve as a good method

for food labeling.

Seealso: Carbohydrates:ClassificationandProperties;

Cellulose; Chromatography:High-performanceLiquid

Chromatography;GasChromatography; Dietary Fiber:

PropertiesandSources; Enzymes:UsesinAnalysis;

Hemicelluloses; Lignin; Protein:Determinationand

Characterization; Spectroscopy:VisibleSpectroscopy

andColorimetry; Starch:Structure,Properties,and

Determination

Further Reading

Anderson JW and Bridges SR (1988) Dietary fiber content

of selected foods. American Journal of Clinical Nutrition

47: 440–447.

Armstrong A (1989) US Department of Agriculture Collab-

orative Study 1988. Total Dietary Fibre. HPB Labora-

tory (Canadian) Results. Statistical Report. Food

Statistics and Operational Planning. Health and Wel-

fare, Canada.

Bach Knudsen KE and Mongeau R (1995) Analytical

methods for the determination of components conven-

tionally considered as dietary fibre. In: Sørensen A,

Bach Knudsen KE, Englyst HN, Gudmand-Høyer and

Nyman M (eds), Metabolic and Physiological Aspects of

Dietary Fibre in Food. Recent Progress in the Analysis of

Dietary Fibre COST 92, pp. 205–206. Luxembourg:

Commission of the European Communitites.

Bligh EG and Dyer WJ (1959) A rapid method of total lipid

extraction and purification. Canadian Journal of Bio-

chemistry 37: 911–917.

Cho S, Devries J and Prosky L (1997) Dietary Fiber Analy-

sis and Applications. Gaithersburg, MD: AOAC Inter-

national.

Dreher ML (1987) Handbook of Dietary Fiber. An Applied

Approach. New York: Marcel Dekker.

Englyst H and Cummings J (1989) Dietary fibre and starch:

definition, classification and measurement. In: Leeds AR

(ed.) Dietary Fibre Perspectives. Reviews and Bibliog-

raphy, pp. 3–26. London: John Libbey.

Englyst HN, Quigley ME, Englyst KN, Bravo L and

Hudson GJ (1995) Dietary Fibre. Measurement by the

Englyst NSP Procedure. Measurement by the AOAC

Procedure. Explanation of Differences. Report of a

Study Commissioned by MAFF, UK. Medical Research

Council & University of Cambridge.

Jenkins DA, Kendall CWC and Vuksan V (2000) Viscous

fibers, health claims, and strategies to reduce cardio-

vascular disease risk. American Journal of Clinical

Nutrition 71: 401–402.

Marlett JA (1990) Analysis of dietary fiber in human foods.

In: Kritchevsky D, Bonfield C, Anderson JW (eds) Diet-

ary Fiber – Chemistry, Physiology and Health Effects,

pp. 31–48. New York: Plenum Press.

Marlett JA (1992) Content and composition of dietary fiber

in 117 frequently consumed foods. Journal of the

American Dietetic Association 92: 175–186.

Mongeau R (1995) Experiences with different official

analytical methods in Canada. In: Sørensen A, Bach

Knudsen KE, Englyst HN, Gudmand-Høyer E and

Nyman M (eds) Metabolic and physiological aspects of

dietary fibre in food. Recent progress in the analysis of

dietary fibre, COST 92, pp. 143–151. Luxembourg:

European Commission.

Mongeau R and Brassard R (1989) A comparison of three

methods for analyzing dietary fiber in 38 foods. Journal

of Food Composition and Analysis 2: 189–199.

Mongeau R and Brassard R (1993) Enzymatic-gravimetric

determination in foods of dietary fibre as sum of insol-

uble and soluble fibre fractions: summary of collabora-

tive study. Journal of AOAC International 76: 923–925.

Mongeau R and Brassard R (1995) Importance of cooking

temperature and pancreatic amylase in determination of

dietary fiber in dried legumes. Journal of AOAC Inter-

national 78: 1444–1449.

Mongeau R, Brassard R and Verdier P (1989) Measurement

of dietary fiber in a total diet study. Journal of Food

Composition and Analysis 2: 317–326.

Mongeau R and Brooks SPJ (2001) Chemistry and analysis

of lignin. In: Cho SS and Dreher ML (eds) Handbook of

Dietary Fiber, pp. 321–373. New York, Basel: Marcel

Dekker.

Theander O (1981) A review of the different analytical

methods and remaining problems. In: James WPT,

Theander O (eds) The Analysis of Dietary Fiber in

Food. New York: Marcel Dekker.

Theander O, A

man P, Westerlund E, Andersson R and

Pettersson D (1995) Total dietary fiber determined as

neutral sugar residues, uronic acid residues, and Klason

lignin (the Uppsala method): collaborative study. Jour-

nal of AOAC International 78: 1030–1044.

US Department of Agriculture Research Service (1999)

USDA Nutrient Database for Standard Reference. Com-

position of Foods: Raw, Processed, Prepared. Release 12.

US Department of Agriculture, Agriculture Research

Service (1999) USDA Nutrient Database for Standard

Reference, Release 13. Nutrient Data Laboratory Home

Page, www.nal.usda.gov/fnic/foodcomp.

Physiological Effects

I T Johnson, Institute of Food Research, Norwich, UK

Introduction

0001The term ‘crude fiber’ was coined in the nineteenth

century to describe the material that remained after

rigorous nonenzymic hydrolysis of animal feedstuffs.

DIETARY FIBER/Physiological Effects 1833

The possible importance of a similar undigestible

residue in human foods was proposed in the early

1970s by the physician and epidemiologist Hugh Tro-

well, but he also recognized that the crude fiber fig-

ures then available for foods had little physiological

significance and were of no value in the context of

human diets. He therefore used the term ‘dietary

fiber’ to describe the ‘remnants of plant cell walls

resistant to hydrolysis (digestion) by the alimentary

enzymes of man.’ Later, he and others redefined it

more precisely as ‘The sum of lignin and the plant

polysaccharides that are not digested by the endogen-

ous secretions of the mammalian digestive tract.’

Although some controversy still exists as to the pre-

cise definition of dietary fiber, there is no doubt that

the term embraces a large and complex mixture of

polysaccharides that share few common properties

other than resistance to digestion by the endogenous

enzymes of the human alimentary tract. The presence

of undigested cell wall fragments and dispersed

polysaccharides can alter physiological processes

throughout the gut, but the effects of different fiber

components depend upon their varied physical and

chemical properties during digestion, and upon their

susceptibility to degradation by bacterial enzymes in

the colon. It is important to realize that a single

analytical value for the fiber content of a food is a

poor guide to its physiological effects. This article will

review the main mechanisms of action of resistant

polysaccharides in the alimentary tract and their

implications for human health.

Fiber in the Alimentary Tract

0002 Food is conveyed through the alimentary tract by

rhythmic muscular activity. Digestive enzymes are

released into the lumen at intervals to facilitate the

breakdown of macromolecules into fragments that

can then be absorbed and utilized by the body. The

earliest stages of digestion begin in the mouth, where

food particles are reduced in size, lubricated with

saliva, which also contains the digestive enzyme

salivary amylase, and prepared for swallowing. In

the stomach, any large particles that remain are

broken down by rigorous muscular activity in the

presence of hydrochloric acid and proteolytic

enzymes. The need to disrupt and disperse intractable

food particles and cell walls appears to delay the

digestive process significantly. Since cell wall polysac-

charides help to determine the texture of many plant

foods, and hard foods tend to be chewed more thor-

oughly than soft foods, the presence of dietary fiber in

unrefined foods may begin to regulate digestion at a

very early stage. Apples, for example, tend to be

consumed more slowly than apple juice, and the

absorption of sugar from apples is also slower. Simi-

larly, the rate at which the starch is digested and

absorbed from cubes of cooked potato has been

shown to be much slower when they are swallowed

whole than when they are chewed normally. The

nutritional significance of these effects is not entirely

clear, but they may limit the rate at which glucose

from some foods enters the circulation.

Small Intestine

0003The small intestine is the largest of the digestive

organs, with the greatest surface area, as befits its

role as the main site of nutrient absorption. The semi-

liquid products of gastric digestion are released at

intervals into the small intestine and propelled away

from the gastroduodenal junction by peristatic move-

ments at about 1 cm min

1

. The hydrolysis of pro-

teins, triglycerides, and starch continues within the

duodenum and upper jejunum, under the influence of

pancreatic enzymes. The final stages of digestion

occur at the mucosal surface, where the products are

absorbed into the circulation, along with water and

electrolytes, via the specialized epithelial cells of the

intestinal villi. Muscular activity in the small intes-

tinal wall, together with rhythmic contractions of the

villi, insures that the partially digested chyme is well

stirred. In adults, the first fermentable residues from

a meal containing complex carbohydrates enter the

colon approximately 4.5 h after ingestion. When a

solution containing indigestible sugar is swallowed

without food, it reaches the colon about 1.5 h earlier

than when the same material is added to a solid meal

containing dietary fiber. The presence of solid food

residues slows transit, probably by delaying gastric

emptying and perhaps also by increasing the viscosity

of the chyme so that it tends to resist the peristaltic

flow. Soluble polysaccharides such as guar gum,

pectin and b-glucan from oats increase the mouth-

to-cecum transit time still further.

Carbohydrate Metabolism

0004In his early work, Trowell proposed that dietary fiber

was a major factor in the prevention of the metabolic

condition insulin-dependent diabetes. Indeed, he

argued that historically, this condition was unknown

in western Europe prior to the introduction of mech-

anized flour milling, because until then, the consump-

tion of unrefined carbohydrate foods had favored

slow absorption of glucose, which placed less strain

upon the ability of the pancreas to maintain glucose

homeostasis. Whatever its merits, this hypothesis has

encouraged a great deal of research on the effects of

cell wall polysaccharides on the digestion and absorp-

tion of carbohydrates, which in turn has shed light on

1834 DIETARY FIBER/Physiological Effects

the potential role of dietary fiber in the control of

diabetes. The ‘glycemic index’ is essentially a quanti-

tative expression of the quantity of glucose appearing

in the bloodstream after ingestion of a carbohydrate-

rich food. To calculate the index, fasted subjects are

given a test meal of the experimental food containing

a standardized quantity of carbohydrate. The change

in concentration of glucose in the blood is then meas-

ured over a period of time. The ratio of the area under

the blood-glucose curve in response to the test meal to

that produced by an equal quantity of a standard

reference food is then calculated and expressed as a

percentage. When glucose is used as the standard,

complex starchy foods often have glycemic indices

lower than 100%.

0005 The physical resistance of plant cell walls during

their passage through the gut varies considerably

from one food to another, and any cell walls that

remain intact in the small intestine will impede the

access of pancreatic amylase to starch. Even when

enzymes and their substrates do come into contact,

the presence of cell wall polysaccharides may slow the

diffusion of hydrolytic products through the partially

digested matrix in the gut lumen. Pulses tend to give

particularly low glycemic index values, probably

because legume seeds have relatively thick cell

walls, which resist destruction during processing

and cooking. This effect of fiber on carbohydrate

metabolism cannot be predicted from simple analyt-

ical values for total fiber because it reflects the struc-

ture, as opposed to the absolute quantity, of cell wall

polysaccharides within the food.

0006 Many studies on postprandial glycemia have been

conducted using isolated fiber supplements added to

glucose test-meals or to low-fiber sources of starch.

They demonstrate that, contrary to Trowell’s original

hypothesis, wheat bran and other insoluble cell wall

materials have little effect on glucose metabolism.

However certain polysaccharides, such as guar gum,

pectin, and oat beta-glucan, which form viscous solu-

tions in the stomach and small intestine, do slow the

absorption of glucose. Highly viscous food compon-

ents may delay gastric emptying and inhibit the dis-

persion of the digesta along the small intestine, but

the primary mechanism of action appears to be sup-

pression of convective stirring in the fluid layer adja-

cent to the mucosal surface. The rapid uptake of

monosaccharides by the epithelial cells tends to

reduce the concentration of glucose in this boundary

layer, so that absorption from the gut lumen becomes

rate-limited by the relatively slow process of diffu-

sion. The overall effect is to delay the assimilation of

glucose and hence suppress the glycemic response to

glucose or starchy foods in both healthy volunteers

and diabetics. A similar mechanism probably inhibits

the reabsorption of cholesterol and bile salts in the

distal ileum, and this may account for the ability

of some viscous polysaccharides to reduce plasma

cholesterol levels in humans. (See Carbohydrates:

Metabolism of Sugars.)

Mineral Metabolism

0007Polysaccharides and phenolic components of cell

walls often contain polar groups that can interact

with ionized solutes in the gastrointestinal contents.

Such molecular binding effects do occur in vitro and

might, in principle, restrict the availability of nutri-

ents and other substances for absorption in the small

intestine. Iron, zinc, and calcium are absorbed rela-

tively poorly from the human diet, and there is a long

tradition that high intakes of dietary fiber have an

adverse effect on human mineral nutrition. Studies

with subjects who, for clinical reasons, have had the

colon removed and the small bowel brought to

the abdominal surface (ileostomists) suggest that al-

though neutral polysaccharides do not bind minerals,

charged polysaccharides such as pectin can displace

cations into the colon. However, fermentation of cell

walls probably releases iron and calcium into the

lumen, whence they may be salvaged by colonic ab-

sorption, and there is little objective evidence that

dietary fiber per se has much of an adverse effect

on mineral metabolism. Indeed, highly fermentable

oligo- and polysaccharides have been reported to pro-

mote mineral absorption in some studies, presumably

by stimulating active transfer in the colon.

0008The effect of phytate (myoinositol hexaphosphate)

is generally considered to be more significant than

that of fiber. Phytate is often present in close associ-

ation with cell wall polysaccharides in unprocessed

legume seeds, oats, and other cereals. Phytate does

exert a potent binding effect on minerals and has been

shown to reduce the availability of magnesium, zinc,

and calcium for absorption in trials with human vol-

unteers. Phytate levels in foods can be reduced by the

activity of endogenous phytase, by hydrolysis with

exogenous enzymes, or by fermentation. Dephyti-

nized products may be of benefit to individuals at

risk of suboptimal mineral status. (See Phytic Acid:

Nutritional Impact.)

Large Intestine

0009The large intestine, which is located distal to the small

bowel, salvages energy from food residues and from

endogenous proteins and carbohydrates that have

escaped digestion and absorption. Its other main

role is to form and store feces. Fecal microorganisms

degrade undigested starch, and many of the poly-

saccharides that comprise dietary fiber, to yield the

DIETARY FIBER/Physiological Effects 1835