Dietary fibers: their definition nutritional properties1’ 2 Peter
J. Van
Soest,3
and
Ph.D.
ABSTRACT upon
Fiber
its composition
might
not
composition. vegetable
be
is a variable and
shared
by
mentable.
with
properties.
another
Separate
in foods. fermentable
and
chemically
purified
A class
of soluble
substances
Fibers with
fibers
defective.
replacement more
support
Since
crude need
of standard and
tables
promotion
fiber
values
is the
of food than
are
reanalysis
as
all
composition.
nutritional
wood
required
and
may effects present
poorly
related by
Accomplishment being
to
which
gums
foodstuffs
depending
by one
type
describe
not
relatively
to the
of these
true
fiber
purposes Nutr.
but
are
in the diet. which is
methods
J. Clin.
groups: are less unfer-
fibers,
they can elicit official method,
appropriate Am.
received.
are
be true
of fiber quantity,
classified into three residue: brans, which
cellulose,
and
qualities
possessed
are
of the similar method, the
erratic of
is presently
its
be generally indigestible
pectins
seriously
major
can low
such
including
part of the dietary fiber complex because need is the replacement of the crude fiber
a second
to properties
methods
considered A major food,
respect
Biological
type.
and quality of fiber fibers, which are highly
fermentable:
material
physical
will
value
of
and
the
require
31: S12-S20
1978.
Our knowledge of the role of plant fiber in nutrition has had a curious history. The present intensity of interest succeeds long periods of apathy punctuated by an occasional investigation. Because of the ignorance and confusion stemming from a persistent belief in the inertness and nonnutritive character of dietary fiber in human nutrition, this topic has been assigned a very low priority in the order of nutritional investigations. Terms such as fiber, cellulose, and lignin are not often found in the indices of our food and nutrition textbooks, and, when they are found, the information given is usually inaccurate. It is important to emphasize prevailing misconceptions because they affect the present climate of fiber research. Historically, much of the misinformation about fiber has resulted from the continued use of the crude fiber method, which is the currently approved Association of Official Analytical Chemists (AOAC) procedure required in any legal contest over the composition of human food or animal feed. It is the basis for current nutrient labeling and advertising of fiber-containing foods. Problem The origin S12
of crude crude (1) and
fiber
‘From the Department of Animal Science and Diviof Nutritional Sciences, Cornell University, Ithaca, York 14853. 2 Presented at the Senate Nutrition Committee’s Hearing on Dietary Fiber and Health, March 31, 1977. Professor of Animal Nutrition.
sion New
method
fiber method is of uncertain has been in use for at least 150 The American
years. The earliest published analysis that is extant was done on Indian corn by John Gorham of Harvard in 1820 (2). Many authors attribute crude fiber to the German chemist H. Einhof; however, recent historical research does not support this (1), and Emhofs published values obtained by a maceration procedure (3) correspond to modern cell wall values (Table 1). Cell wall values, which represent the sum of lignin, cellulose, and hemicellulose, are higher in varying degrees depending on the food source. The error in the crude fiber method arises from the sequential extraction with hot dilute acid followed by dilute alkali. In this extraction, 50 to 90% of the lignin, 85% of the hemicellulose, and 0 to 50% of the cellulose is dissolved (9, 10). The error through loss is variable depending on the proportions of hgnm, cellulose, and hemicehlulose in the fiber and can be as high as 700%. In the case of wheat bran, the most common source of fiber in human food, the true fiber value is about
Journal
of Clinical
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
Nutrition
31: OCTOBER
1978,
pp. S12-S20.
Printed
in U.S.A.
DEFINITION
OF
FIBER
four times that indicated by the crude fiber value. For over a century we have known about the losses resulting from the crude fiber method and its failure to recover lignin and other genuine components of fiber (10, 11). Nevertheless, the Wiley Committee of the AOAC was instrumental in obtaining its approval as a legal official method in 1887 (12). Since that time the main effort of the AOAC has been to insure analytical reproducibility within and between laboratories, and many food chemists in FDA, state regulatory agencies, and industry remain unaware of the deficiencies of the crude fiber method. Over the past 50 years attempts have been made to develop improved fiber methods, none of which has managed to dislodge the crude fiber method. This effort has been dominated, but not exclusively, by the fields of ruminant nutrition and grassland husbandry, where fiber utilization has been a main objective of research on forage quality (13). The ruminant field has also supplied the microbiological methods and most of our present knowledge concerning gastrointestinal fermentations (14), a body of technical knowledge now becoming increasingly applied in research on monogastric animals, including man. Replacement of crude fiber with a relevant system of food analysis requires more than method development. The definition of dietary fiber must first be settled, and this will require compromise. Once a standard definition is agreed upon, methodology can follow. The methodology itself must be directed toward two separate goals that are not entirely compatible: for research purposes one needs a detailed system of structural analysis that is definitive in characterizing individual TABLE 1 Some early fiber
values
Reference
(3) (2) (4) (5)” (6) (7) (8) “Wolff’s
compared
Date
whole
1806 1820 1846 1856 1956 1963 1973 (5) values
with barley
modern
analyses:
METHODOLOGY
Sl3
plant fiber sources; for surveys or quality control work the methods must be rapid and convenient even though some detail may be sacrificed. Whatever system is adopted, if it is to be competitive with the crude fiber method, it must permit the handling of large numbers of samples yet, at the same time, yield more than a single measurement. Fibers are variable in their composition and properties, and it is not possible to describe the characteristics and amount of fiber in a single value. Among the better systems of analysis that will fulfill the need for rapid yet precise measurement is the detergent method (10) developed to overcome the deficiencies in the crude fiber method. Defmition
of fiber
Dietary fiber has been defmed as plant cell wall and as nonnutritive residues. These terms, while similar in meaning, are by no means synonymous. “Unavailable carbohydrate” is a term used in the same context as nonnutritive residues. “Nonnutritive residues” is a broader classification including all substances resistant to animal digestive enzymes (Fig. 1) while “plant cell wall” refers to botanical structure. The plant cell wall includes most of the matter in plants that is resistant to animal enzymes. The plant cell wall does contain a small amount of protein that is partially digestible. Resistance to digestion by animal enzymes is not synonymous with indigestibility. Carbohydrates that escape animal digestion and reach the lower bowel are subject to microbial fermentation, the extent of which is dependent upon the type of fiber, its potential fermentation rate, and residence time (14). Approximately 50% of the fermentation products are volatile fatty acids, principally acetic,
(values
Corn
Oats
21.3
32.8
a re on dry
matter
Potatoes
basis)
(peeled)
Rye
5.6
22
Wheat
13.8
3.3 5.3 4.9 6.0 21 are
from
summaries
16.1 12-16 13.5
2.4 3.1 12 of earlier
31 literature.
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
2.4 2.7 2.2
2.2 1.8-2.4 4.7 “Neutral-detergent
fiber.
1.6 2.9 2.1-2.6 14
S14
VAN
SUGAR
SOEST
STARCH
PROTEIN DIET
-
LIPID
PECTIN,
Cl) LU
GUMS
LU
>
CELLULOSE
HEM ICELLULOSE ‘LU
I-
LIGNIN z
FECES
-
BACTERIAL BACTER
CELL
WALLS
IA
ENDOGENOUS
MATTER
FIG. 1. Illustration of the overlapping of dietary and fecal components in relation to dietary fiber and nonnutritive residues. Dietary fiber is defined here as insoluble matrix substance of plant origin. Proteins and lipids in feces may not be of dietary origin. Pectin, gums, hemicellulose, and cellulose may be extensively fermented and have low recovery in feces. The undigested fraction of the diet appearing in the feces may contain digestible matter (relative to animal or microbial enzymes) that has escaped digestion through transit. Bacterial matter is primarily of nondietary origin (22).
propionic, and butyric acids (15), which animal studies show to be efficiently absorbed and used in animal metabolism (16-19). The role of gastrointestinal fermentations in supplying metabohizable energy presents the greatest difficulty in accepting the nonnutritive residue concept. The term should be abandoned. The supposed inability of humans to absorb and utilize volatile fatty acids is probably incorrect and is very defmitely not supported by research with other monogastric species including primates (20). If man is indeed unique in his inability to utilize fermentation acids, then a serious question must be raised as to which animal species is a suitable experimental model for human studies. The apparent fiber content of food can be increased by cooking or other heating processes, including baking, frying, extrusion, etc. (Table 2), that involve the nonenzymatic browning reaction between amino acids and degradation products of sugars (21). The Maillard polymer is a brown, insoluble, indigestible substance with the physical properties of lignin. It is quantitatively recovered in the residue from neutral detergent or acid detergent extraction and in crude hignin. Breakfast cereals and other browned products
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
TABLE 2 Composition
of toasted
Sample
products
ADF
N
(21)
ADF
6.25
ADF
N/total
N
Crude
lignin
dry basis
Untoasted bread Toast Potato chips Tortilla chips Corn flakes Wheat Chex
0.6
0.1
0.7
0.2
1.5 4.1 2.7 3.9 6.5
0.6 0.6 0.7 2.5 3.8
3.7 9.0 7.8 31.0 32.0
0.8 0.9 1.5 1.8 3.5
contain significant amounts of this material (22). The browning causes large portions of protein to become unavailable to digestion. In the case of Wheat Chex and corn flakes this amounts to one-third the total crude protein. Since most people over-consume protein, this loss should not be looked at in a negative sense but as an interesting characteristic of dietary fiber in these products. Gums
and pectin
A problem of chemical analysis is presented by components that are resistant to animal enzymes but are not part of the insoluble plant cell wall matrix. Pectins are a part
DEFINITION
OF
FIBER
of the plant cell wall but can be removed by nonhydrolytic reagents. An example is the preparation of jelly from apples or berries. Water-soluble gums are a diverse group. For example, some gums are derived from bacteria, while others such as methyl cellulose are chemically prepared. The galactans that occur in beans are storage products from the view of plant metabolism but are resistant to animal digestive enzymes (23). Most of these substances are probably highly fermentable, and fecal recovery will be low. The interest in including the gums as part of the dietary fiber complex is related to their probable role in imparting to the diet some of the properties associated with fiber (24). This possibility has stimulated development of enzymatic methods for fiber. No good method exists at the present time for recovery of soluble gums. In feces, analysis is confounded by inclusion of microbial polysaccharides of the bacterial cell wall (25, 26). The neutral-detergent fiber, which isolates only insoluble plant cell walls, is the only available method for removing microbial cell walls; however, gums and pectins are also removed in the process (27). A suitable method should be developed for the soluble substances apart from the insoluble fiber. These two classes probably elicit different dietary and nutritional effects. Components
of plant
fiber
The major components of plant cell walls are cellulose, hemicellulose, and lignin (28). Cellulose is a $ glucan isomeric with starch and is remarkable for its msolubility. Hemicellulose, which is largely extractable with acids and alkalis, is a very complex carbohydrate containing residues of various sugars and a variety of glycosidic linkages. In grasses hemicellulose is ester-linked to lignin (29), and in dicotyledonous species the bond may be glycosidic. In grasses the bond between lignin and carbohydrate is easily cleaved with alkali, rendering a large portion of hemicellulose water soluble (30). Deignified cellulose and hemicellulose become completely accessible to fermentation. The protective effect of lignin is related partly to bonding and partly to physical association in the cell wall complex (31, 32). Lignin and several minor components in-
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
S15
METHODOLOGY
cluding cutin, protein, and mineral components of the plant cell wall constitute the main noncarbohydrate fractions. The crude lignin fraction incorporates the Maillard polymers formed through heat damage in the nonenzymatic browning reaction. True lignin is polyphenolic in structure, has a condensed nonhydrolyzable structure, and is part of the truly indigestible matrix (33). Mineral components of fiber are important. Calcium pectates in legumes and vegetables are probably fairly available forms of calcium (34). Phytic acid is not a true fiber fraction and constitutes a storage reserve of phosphorus for the plant seedling. Some plants biologically accumulate opaline silica in the plant cell wall to serve as a protective factor (35, 36). Most cereals contain appreciable amounts of silicon, but rice is the cumulator plant par excellence. The opaline silica matrix could serve as a binder for other elements such as iron (37). The role of plant fiber in the availability of trace elements is a complicated one because of the probable existence of unavailable forms in the cell wall matrix and also the possible binding to the cation exchange of the fiber surface. The cation exchange properties of plant fiber are widely variable (Table 3) depending on source and preparation (18, 38). Purified TABLE Hydration capacity Isolated
3 capacity, of various tibet’
bulk plant Hydration . pacity
volume, and fibers (18) ca-
Bulk
cation
volume
mI/g
Potato Rutabaga Broccoli Celery Onions Cucumber Ryegrass Bagasse Cellulose Alfalfa Wheat straw Orchardgrass Corn bran Wheat bran Solka Floc a Cold neutral-detergent through treated binding paper.
26.8 21.7 18.5 18.1 17.4 16.5 14.7 8.6 8.6 7.2 6.8 6.7 6.6 6.5 4.7 extracted
exchange
Cation cx. change mEq/g
8.3 7.2 7.1 9.5 11.3 7.9 12.3 4.6 8.7 8.1 8.9 8.2 3.8 6.2 3.7 plant
0.62 1.56 0.96 1.35 1.00 0.96 0.26 0.03 0.00 0.30 0.15 0.27 0.38 0.22 0.06 tissue ground
20 mesh. Potato, corn bran, and wheat bran with amylase to remove starch. “Calcium of isolated fiber residue. C Whatman 41 filter
VAN
S16
cellulose has no binding ability whatsoever-hence its use as filter paper in chemical separations. Anion exchange capacity is related to the lignin, pectin, and Maillard polymers, the latter probably possessing considerable anion exchange capacity through its nitrogen content. Sources
and
types
of dietary
fiber
The composition and properties of plant fiber vary greatly according to source, species of plant, and physiological stage of growth. Fibers from related species within a plant family, such as cereal brans, are relatively similar and divergent from dicotyledonous sources. Most vegetables and fruits are dicots and represent an immature stage of growth in contrast to the brans, which are mature and senescent plant products. Generally, hgnification and other factors promoting the development of a truly indigestible fraction increase with plant age (13). The environment in which a plant is grown has a significant effect upon the composition of the plant. The amount and composition of plant cell walls are dominantly affected by environmental temperature and to a lesser degree by fertilization, moisture, soil, and light. The fiber from tropical plants tends to be less fermentable because of the effect of environmental temperature upon hignification (39, 40). Nitrogen fertilization can alter the proportions of hignin and hemicellulose. Most of these studies have been done in agronomic studies of forage. However, the pattern probably extends to cereals and vegetables (41). It has been popular to substitute purified cehluloses for natural sources of fiber in nutritional experiments. Recently cellulose derived from wood has been introduced into commercial bread. Purified cehluloses differ markedly from natural celluloses in physical and biological properties (Table 3) and may be considered to represent a separate class- of fibers that are chemically processed and altered (23, 31, 42). Bagasse from sugar cane and other similar industrial by-products may be considered members of this group. Purified celluloses differ from natural sources first because they are derived from different botanical sources, and second through the alteration caused by the chemical
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
SOEST
dehignification. Purified celluloses are seldom truly pure and can contain residual hemicellulose, hignin, and mineral components that are very difficult to remove. Dehignification results in complete fermentability in the long term, but with long lag at a slow rate (14, 31). Purified celluloses may have use in ruminant feeding (32), but their fermentive digestibility in man is probably quite low (43). Since they exhibit very fine particle size, high bulk density, and very low hydration capacity and cation exchange properties, their value as dietary fiber sources can be questioned. From an experimental point of view it is unwarranted to suppose that the isolation and separate feeding of purified cellulose or other cell wall components will elicit the same responses as the original fiber. Fermentability and physical properties are radically altered by preparatory treatments. Properties
of fiber
The properties of fiber that are of nutritional significance include bulk density, hydration capacity, binding properties, and fermentabihity. Plant fiber through its bulk effect promotes faster transit or rate of passage (10, 14). Animal studies show that fine grinding of the fiber increases the feed density and alters passage and the character of gastrointestinal fermentations (44). Coarse particles tend to pass through the digestive tract more slowly than fine ones, and elimination of coarse fiber may reduce the differential sorting of food residues. Grinding of wheat bran decreases its bulk volume primarily through collapse and demolition of the cell wall structure (Fig. 2). Hydration capacity is a more complex function of the surface area, which is increased on grinding, and of the interior cell space, which is decreased upon grinding (10, 18). Survey of various fiber sources indicates a wide range in physical properties of density, hydration, and cation exchange (Table 3). Generally vegetable fiber has the highest hydration and exchange properties, which are correlated (38), while cereal brans and forages are intermediate. As noted, purified sources of fiber rank very low in this scale of quality. It should be warned that these physical properties as measured may not be representative of those occurring in the lower
DEFINITION
OF
FIBER
bowel, because bacteria may ferment components contributing to the cation exchange or hydration capacity. Bacteria may also cause a reduction in fiber structure through fermentive digestion, which is offset by production of microbial cell mass that has its own hydration and absorption capacities (45). The bacteria thus replace fiber that has been fermented. Fermentability The most important property of fiber is its ability to serve as a substrate for microbial gastrointestinal fermentation. It is through this mechanism that energy in carbohydrates resistant to animal digestive enzymes becomes available to animal metabolism. Since the microorganisms that ferment cellulose, hemicellulose, and pectin are characteristic (46), fermentable fiber plays a role in establishing the environment of the lower digestive tract through the microorganisms and their products. HYDRATION DRYB&LI(
8
6
11
10
20
110
flJ
particle of wheat
size bran
ESH
bulk
FIG. 2. The relationship volume and hydration
TABLE 4 Fermentability
by rumen
between capacity
organisms
and
lignification
Maximal Lag
Cauliflower Onions Corn bran Wheat bran Alfalfa Bagasse Whatman cellulose Cotton “Lag determined slope of natural log
fibers .
source
4 5 5 3 4 4 9 17 by the zero of residual
intercept of the regression fermentable fiber upon
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
Sl7
Fibers vary in their capacity to support fermentation, and a number of factors influences this capacity. The fermentable portion of fiber resides primarily in the polysaccharide fractions, and most natural sources exhibit an obligately indigestible fraction and an available pool (31, 47). The obhigately unavailable portion is determined by the hgnm and cutin content that is associated with an amount of cellulose and hemicehlulose about 2#{189} times the hignin content (48, 49). The fermentabihity of the available portion is relatively independent of the unavailable portion (49) and is regulated by intrinsic factors within the cellulosic carbohydrates themselves (31, 42, 50, 51). The limitation of fermentation ratio is probably due more to intermolecular associations than to clycosidic linkage, although the latter may be important in hemicelluloses (51). In addition to the maximal rate of fermentation, a lag in fermentation is observed even with rumen organisms, which is associated with the nature of the substrate (Table 4). The range in observed fermentation rates is an order of magnitude. The consequence is that fermentation is rate limited by the character of the fiber. Highly lignified fibers are less digestible, and purified cellulose of low rate of digestion is relatively unfermented (Table 5) despite its biological availability (43). The amount and extent of fiber fermentation are dependent on the microbial population, which in turn is dependent upon the retention time at the fermentation site and the potential fermentation rate. The competition between digestion and passage rates determines the portion
of vegetable
Fermentation
.
Fiber
with (18).
METHODOLOGY
(18)
extent
Ratio
.
.
.
lignin/cellulose
IS-hr
.
digestion
coefli-
.
Rate’
0.42 0.23 0.10 0.06 0.12 0.04 0.07 0.04
0.94 0.91 0.94 0.43 0.59 0.45 0.94 0.98
of log residual fermentable time. C Maximal digestion
0.05 0.09 0.12 0.47 0.30 0.31 0.03 0.00 fiber against time. at 72 to 100 hr.
0.93 0.57 0.54 0.33 0.32 0.23 0.26 0.18 “Regression
S18
VAN
TABLE 5 Digestibility
of fiber
in various
animal
SOEST
species Digestib
Animal
Substrate Cell
Cow
Normal
Horse
Man
100% alfalfa 50% alfalfa:50% grain 20% alfalfa:80% grain Normal diets Cereal-based diets 100% alfalfa 75% brome 75% orchard grass Alfalfa Alfalfa Alfalfa Brome Brome Brome Orchard grass Orchard grass Orchard grass Wheat bran Alfalfa leaf meal Carrots Corn germ meal Cotton seed hulls Sugar beet pulp Peas Cabbage Agar-Agar Cellu-flour All Bran All Bran Lettuce Tomatoes Cabbage Celery Oranges Apples Mixed diet
Man
No
Beaver Dog Meadow
vole
Rat Sheep Swine Rat Sheep Swine Rat Sheep Swine Man
Range Mean
Range Mean
Man
“Methods the total
of Williams cell wall.
and
Olmstead
(43)
include
32-71 56 47 59 71
44-84 68 58 73 81
41 34 21 36 20 45 35 9 67 43 8 70 46 30 9 74 60 18 65 53 66 59 11
47 39 24 38 47 46 43 Il 71 47 6 76 47 35 6 84 63 23 89 84 80 59 29
pectin
of available carbohydrate lost to feces. Either fast transit or inherently slow digestion will limit extent of fermentation and the development of a microbial population. Under a
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
ility
of Reference
Cellulose
Crude
fiber
3-79 52 45 54 63 30 18 34 18 29 21 50 40 1 68 38 1 67 44 30 14 67 57 17 55 44 55
(55) (56)
(57) (58) (59)
(60)
(60)
(60)
(43)”
54-80 67 94-98
7 0 6 29 0 42 29 24 57 54-85 71 15-55
72-85
15-44
(63)
83
22
(63)
56-61
67-71
29-35
(64)
57
68
51
3 1-72 48
fruit or vegetables except potato Fruit, vegetables and whole wheat bread Fruit, vegetables and whole wheat bread, high level Corn, soybeaii, wheat bran, citrus fruit, purified cellulose All Bran
Man
and
diets
wall
Hemicellulose
and
feed
microbial
cell
wall
0 5 0 10 14 17 28 71
(61)
(62) (63)
Wrick, unpublished in the
hemicellulose
fraction
given set of conditions the microbial population probably adapts relatively rapidly (52). Thus, it is not feasible to expect improvement in celluholytic capacity of microorgan-
DEFINITION
OF
FIBER
isms through genetic manipulation or mutation (52). The worry that genetic manipulation to introduce cellulolytic capacity might prove a hazard (53) is unrealistic. Plant cell walls have evolved in plants as protective structures and are related to their ecological survival (54). Digestibility
METHODOLOGY 12.
13. 14. 15.
offibers
The capacity for fermentive digestion determines the ability of herbivorous animals to derive energy from plant fiber. The ruminant is the best adapted and exhibits generally the highest fiber digestibilities. Monogastric animals are to a varying degree less capable, the relative level depending upon the type of fiber (Table 5). Vegetable cell walls are highly fermented in human subjects while cereal brans tend to form an intermediate group. Forages and purified celluloses probably have a low availability to humans. Monogastric animals, human included, probably fare better on hemicehlulose than on cellulose in the same food (60), while ruminants digest cellulose and hemicehlulose to about an equal extent (64).
16.
17.
18.
19.
20.
21.
References 1. TYLER, C. Albrecht Thaer’s hay equivalents: fact or fiction. Nutr. Abst. Rev. 45: 1, 1975. 2. GORHAM, J. Chemical analysis of Indian corn. New Engl. J. Med. Surg. 4: 320, 1820. 3. EINHOF, H. Bemerkungen #{252}ber die Nahrungsf#{228}higkeit verschiedener vegetabelischen Produkte. Ann. Ackerbaues 4: 627, 1806. 4. HORSFORD, E. N. Value of different kinds of vegetable food, based upon the amount of nitrogen. Philosophical Mag. Ser. 3. 29: 365, 1846. 5. WOLFF, E. Die naturgesetzlichen Grundlagen des Ackerbaues (3rd ed). Leipzig: Otto Weigand, 1856. 6. MoRRISON, F. B. Feeds and Feeding (22nd ed). Clinton, Iowa: Morrison, 1956. 7. WATF, B. K., AND A. L. MERRILL. Composition of foods-raw processed prepared. Agriculture Handbook No. 8. Washington, D.C.: USDA, 1963. 8. VAN SOEST, P. J. Revised estimates of the net energy value of feeds. Proc. Cornell Nutr. Conf. 1973, pp. 11-23. 9. PALOHEIMO, L. Some persistent misconceptions concerning crude fiber and the nitrogen free extract. J. Sci. Agric. Soc. Finland 25: 16, 1953. 10. VAN SOEST, P. J. Physico-chemical aspects of fibre digestion. In Procedures of the IV International Symposium on Ruminant Physiology, edited by I. W. McDonald and A. C. I. Warner. The University of New England Publishing Unit, 1975, p. 351. 11. HENNEBERG, W., AND F. STOHMANN. Begr#{252}ndung einer rationellen F#{252}tterung der Wiederka#{252}er. Braunschweig: Schwetschke und S#{246}hne,vol. 2, 1864.
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
22.
23.
24.
25.
26. 27.
28.
29.
30.
31.
S19
Association of Official Agricultural Chemists. Official Methods of Analyses, edited by E. C. Richardson. Washington, D.C., 1887. RAYMOND, W. F. The nutritive value offorage crops. Advan. Agron. 21: 2, 1969. HUNGATE, R. E. The Rumen and Its Microbes. New York: Academic Press, 1966. WOLIN, M. J. Interactions between bacterial species of the rumen digestion and metabolism in the ruminant. In: Procedures of the IV International Symposium on Ruminant Physiology, edited by I. W. McDonald and A. C. I. Warner. The University of New England Publishing Unit, 1975, pp. 134-148. ARGENZIO, R. A., AND M. SOUTHWORTH. Sites of organic acid production and absorption in gastrointestinal tract of the pig. Am. J. Phys. 228: 454, 1974. ARGENZIO, R. A., M. SOUTHWORTH AND C. E. STEVENS. Sites of organic acid production and absorption in the equine gastrointestinal tract. Am. J. Phys. 226: 1043, 1974. VAN SoasT, P. J., AND J. B. ROBERTSON. Chemical and physical properties of dietary fibre. Presented at the Miles Symposium on Dietary Fibre, sponsored by the Nutrition Society of Canada, June 14, 1976, in press. YANG, M. 0., K. MANOHARAN AND 0. MICKELSEN. Nutritional contribution of volatile fatty acids from the cecum of rats. J. Nutr. 100: 545, 1970. BAUCHOP, T., AND R. W. MARTUCCI. Ruminant-like digestion of the Langur monkey. Science 161: 698, 1968. VAN SOEST, P. J. Use of detergents in analysis of fibrous feeds. II. Study of effects of heating and drying on yield of fiber and lignin in forages. J. Assoc. Off. Agri. Chem. 48: 785, 1965. VAN SOEST, P. J., AND J. B. ROBERTSON. Analytical problems of fiber. Presented at the Symposium on Carbohydrate Metabolism. Anaheim, California: Institute Food Technologists, in press. LEWIS, B. Physical and biological properties of the structural carbohydrates. Am. J. Clin. Nutr. 31: S82, 1978. TROWELL, H. Defmition of dietary fiber and hypothesis that it is a protective factor in certain diseases. Am. J. Chin. Nutr. 29: 417, 1976. MASON, V. C. Some observations on the distribution and origin of nitrogen in sheep faeces. J. Agri. Sci. Camb. 73: 99, 1969. GALL, L. S. Normal fecal flora of man. Am. J. Clin. Nutr. 23: 1457, 1970. BAILEY, R. W., AND M. ULYATr. Pasture quality and ruminant nutrition. II. Carbohydrate and lignin composition of detergent-extracted residues from pasture grasses and legumes. New Zealand J. Agr. Res. 13: 591, 1970. BAILEY, R. W. Structural carbohydrates. In: Chemistry and Biochemistry of Herbage, edited by 0. W. Butler and R. W. Bailey. London: Academic Press, 1973, vol. 1, pp. 157-206. HARTLEY, R. D. Carbohydrate esters of ferulic acid as components of cell walls of Lolium multflorum. Phytochemistry 12: 661, 1972. SULLIVAN, J. T., T. 0. PHILLIPS AND D. G. ROUTLEY. Water-soluble hemicelluloses of grass holocellulose. J. Agr. Food Chem. 8: 152, 1960. VAN S0E5T, P. J. The uniformity and nutritive avail-
S20
VAN ability
32.
33. 34.
of cellulose. Federation P. J., AND D. R. and nutritive characteristics wastes. Federation Proc. 33: HARTLEY, R. D. The lignin walls. Am. J. Clin. Nutr. 31: VAN
S0EST,
Proc. MERTENS.
32:
1804,
SOEST
1973.
48.
Composition
of low quality 1942, 1974. fraction of S90, 1978.
cellulosic plant
cell
L. W. The effect of dietary fiber on mineral Ph.D. Thesis, Cornell University, 1976. JONES, L. H. P., AND K. A. HANDRECK. Silica in soils, plants and animals. Advan. Agron. 19: 107, 1967. VAN SOEST, P. J., AND L. H. P. JONES. Effect of silica in forages upon digestibility. J. Dairy Sci. 51: 1644, 1968. Hopps, H. C., E. M. CARLISLE, J. A. MCKEAGUE, R. SIEVER AND P. J. VAN SOEST. Silicon. In: Geochemistry and the Environment, edited by E. T. Mertz. Washington, D.C.: National Academy of Sciences, in press. MCCONNELL, A. A., M. A. EASTWOOD AND W. D.
49.
STILES,
absorption.
35.
36.
37.
38.
MITCHELL.
foodstuffs Sci. Food
39.
40.
41.
Physical
characteristics
of
43.
44.
45.
46.
47.
52.
53.
that
54.
Agr.
D.
Yield
and
chemical
components
tropical mental Cornell
42.
51.
vegetable
could influence bowel function. J. 25: 1457, 1974. DEINUM, B., A. J. H. VAN Es AND P. J. VAN SOEST. Climate nitrogen and grass. II. The influence of light intensity, temperature, and nitrogen on the in vivo digestibility of grass and the prediction of these effects from some chemical procedures. Neth. J. Agr. Sci. 16: 217, 1968. DEINUM, B. Effect of age, leaf number and temperature on cell wall and digestibility of maize. In: Carbohydrate Research in Plants and Animals, edited by P. W. van Adrichem. Miscellaneous Papers 12. The Netherlands: Landbouwhogeschool Wageningen, 1976, p. 29. SCHMIDT,
50.
56.
57.
of
vegetable species as influenced by environvariables and plant nutrition. Ph.D. Thesis, University, 1971. STONE, J. E., A. M. SCALLAN, E. DONEFER AND E. AHLGREN. Digestibility as a simple function of a molecule of similar size to a cellulase enzyme. Advan. Chem. Ser. 95: 219, 1969. WILLIAMS, R. D., AND W. H. OMSTEAD. The effect of cellulose, hemicellulose and lignin on the weight of the stool: a contribution to the study of laxation in man. J. Nutr. 11: 433, 1936. Mooiu, L. A. General principals involved with ruminants and effect of feeding pelleted or wafered forage to dairy cattle. J. Animal Sci. 23: 230, 1964. FITT, T. J., K. HUTTON, A. THOMPSON AND D. 0. ARMSTRONG. Binding of magnesium ions by isolated cell walls of rumen bacteria and the possible relation to hypomagnesemia. Proc. Nutr. Soc. 31: lOOA, 1972. BRYANT, M. P. Nutritional features and ecology of predominant anaerobic bacteria of the intestinal tract. Am. J. Clin. Nutr. 27: 1313, 1974. WALDO, D. P., L. W. SMITH AND E. L. Cox. Model of cellulose disappearance from the rumen. J. Dairy Sci. 55: 125, 1972.
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
55.
58.
59.
60.
61.
62.
L. W., H. K. GOERING AND C. H. GORDON. Relationships of forage compositions with rates of cell wall digestion and indigestibility of cell walls. J. Dairy Sci. 55: 1 140, 1972. MERTENS, D. R. Application of theoretical mathematical models to cell wall digestion and forage intake in ruminants. Ph.D. Thesis, Cornell University, 1973. WooD, T. M. Cellulose and celluloysis. World Rev. Nutr. Dietet. 12: 227, 1970. DEKKER, R. F. H. Hemicellulose degradation in the ruminant. In: Carbohydrate Research in Plants and Animals, edited by P. J. Van Adrichem. Miscellaneous Paper 12. The Netherlands: Landbowhogesschool Wageningen, 1976, p. 43. HUNGATE, R. E. Quantitative aspects of the rumen fermentation. In: Physiology of Digestion in the Ruminant, edited by R. W. Dougherty. Washington, D.C.: Butterworths, 1965, pp. 311, 365. WADE, N. Dicing with nature: three narrow escapes. Science 195: 378, 1977. MOORE, J. E., AND G. 0. Morr. Structural inhibitors of quality in tropical grasses. In: Antiquality Components of Forages, edited by A. G. Matches. CSSA PubI. No. 4. Madison, Wisconsin: Crop Science Society of America, 1973, pp. 33-49. ARROYO-AGUILU, J. A., AND J. L. EVANS. Nutrient digestibility of low-fiber rations in the ruminant animal. J. Dairy Sci. 55: 1266, 1972. HINTZ, H. F., D. E. HOGUE, E. F. WALKER, JR., J. E. LOWE AND H. F. SCHRYVER. Apparent digestion in various segments of the digestive tract of ponies fed diets with varying roughage-grain ratios. J. Animal Sci. 32: 245, 1971. HOOVER, W. H., AND S. D. CLARKE. Fiber digestion in the beaver. J. Nutr. 102: 9, 1972. VI5EK, W. J., AND J. B. ROBERTSON. Dried Brewers Grains in Dog Diets. Proc. Cornell Nutr. Conf., 1973, pp. 40-49. KEYS, J. E., JR., AND P. J. VAN SOEST. Digestibility of forages by the meadow vole (Microtuspennsvlvanicus). J. Dairy Sci. 53: 1502, 1970. KEYS, J. E., P. J. VAN S0EST AND E. P. YOUNG. Comparative study of the digestibility of forage cellulose and hemicellulose in ruminants and non-ruminants. J. Animal Sci. 29: 11, 1969. HOPPERT, C. A., AND A. J. CLARK. Digestibility and effect on laxation of crude fiber and cellulose in certain common foods. J. Am. Dietet. Assoc. 21: 157, 1945. HUMMEL, F. C., M. I. SHEPHERD AND I. 0. MACY. SMITH,
Disappearance
the 63.
of
digestive
SOUTHGATE, Caloric
conversion
sessment
of the
energy
cellulose
and
tracts of children. D. A. T., AND
value
factors. factors
of
human
hemicellulose
J. Nutr. J. V. G. An
used diets.
in the Brit.
from
25: 59, 1943. A. DURNIN.
experimental calculation
J. Nutr.
reasof
24:
the
517,
1970. 64.
J.
SULLIVAN,
forage
plants.
T. Studies J. Animal
of the hemicelluloses Sci. 25: 83, 1966.
of
J. Van
Soest,3
and
Ph.D.
ABSTRACT upon
Fiber
its composition
might
not
composition. vegetable
be
is a variable and
shared
by
mentable.
with
properties.
another
Separate
in foods. fermentable
and
chemically
purified
A class
of soluble
substances
Fibers with
fibers
defective.
replacement more
support
Since
crude need
of standard and
tables
promotion
fiber
values
is the
of food than
are
reanalysis
as
all
composition.
nutritional
wood
required
and
may effects present
poorly
related by
Accomplishment being
to
which
gums
foodstuffs
depending
by one
type
describe
not
relatively
to the
of these
true
fiber
purposes Nutr.
but
are
in the diet. which is
methods
J. Clin.
groups: are less unfer-
fibers,
they can elicit official method,
appropriate Am.
received.
are
be true
of fiber quantity,
classified into three residue: brans, which
cellulose,
and
qualities
possessed
are
of the similar method, the
erratic of
is presently
its
be generally indigestible
pectins
seriously
major
can low
such
including
part of the dietary fiber complex because need is the replacement of the crude fiber
a second
to properties
methods
considered A major food,
respect
Biological
type.
and quality of fiber fibers, which are highly
fermentable:
material
physical
will
value
of
and
the
require
31: S12-S20
1978.
Our knowledge of the role of plant fiber in nutrition has had a curious history. The present intensity of interest succeeds long periods of apathy punctuated by an occasional investigation. Because of the ignorance and confusion stemming from a persistent belief in the inertness and nonnutritive character of dietary fiber in human nutrition, this topic has been assigned a very low priority in the order of nutritional investigations. Terms such as fiber, cellulose, and lignin are not often found in the indices of our food and nutrition textbooks, and, when they are found, the information given is usually inaccurate. It is important to emphasize prevailing misconceptions because they affect the present climate of fiber research. Historically, much of the misinformation about fiber has resulted from the continued use of the crude fiber method, which is the currently approved Association of Official Analytical Chemists (AOAC) procedure required in any legal contest over the composition of human food or animal feed. It is the basis for current nutrient labeling and advertising of fiber-containing foods. Problem The origin S12
of crude crude (1) and
fiber
‘From the Department of Animal Science and Diviof Nutritional Sciences, Cornell University, Ithaca, York 14853. 2 Presented at the Senate Nutrition Committee’s Hearing on Dietary Fiber and Health, March 31, 1977. Professor of Animal Nutrition.
sion New
method
fiber method is of uncertain has been in use for at least 150 The American
years. The earliest published analysis that is extant was done on Indian corn by John Gorham of Harvard in 1820 (2). Many authors attribute crude fiber to the German chemist H. Einhof; however, recent historical research does not support this (1), and Emhofs published values obtained by a maceration procedure (3) correspond to modern cell wall values (Table 1). Cell wall values, which represent the sum of lignin, cellulose, and hemicellulose, are higher in varying degrees depending on the food source. The error in the crude fiber method arises from the sequential extraction with hot dilute acid followed by dilute alkali. In this extraction, 50 to 90% of the lignin, 85% of the hemicellulose, and 0 to 50% of the cellulose is dissolved (9, 10). The error through loss is variable depending on the proportions of hgnm, cellulose, and hemicehlulose in the fiber and can be as high as 700%. In the case of wheat bran, the most common source of fiber in human food, the true fiber value is about
Journal
of Clinical
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
Nutrition
31: OCTOBER
1978,
pp. S12-S20.
Printed
in U.S.A.
DEFINITION
OF
FIBER
four times that indicated by the crude fiber value. For over a century we have known about the losses resulting from the crude fiber method and its failure to recover lignin and other genuine components of fiber (10, 11). Nevertheless, the Wiley Committee of the AOAC was instrumental in obtaining its approval as a legal official method in 1887 (12). Since that time the main effort of the AOAC has been to insure analytical reproducibility within and between laboratories, and many food chemists in FDA, state regulatory agencies, and industry remain unaware of the deficiencies of the crude fiber method. Over the past 50 years attempts have been made to develop improved fiber methods, none of which has managed to dislodge the crude fiber method. This effort has been dominated, but not exclusively, by the fields of ruminant nutrition and grassland husbandry, where fiber utilization has been a main objective of research on forage quality (13). The ruminant field has also supplied the microbiological methods and most of our present knowledge concerning gastrointestinal fermentations (14), a body of technical knowledge now becoming increasingly applied in research on monogastric animals, including man. Replacement of crude fiber with a relevant system of food analysis requires more than method development. The definition of dietary fiber must first be settled, and this will require compromise. Once a standard definition is agreed upon, methodology can follow. The methodology itself must be directed toward two separate goals that are not entirely compatible: for research purposes one needs a detailed system of structural analysis that is definitive in characterizing individual TABLE 1 Some early fiber
values
Reference
(3) (2) (4) (5)” (6) (7) (8) “Wolff’s
compared
Date
whole
1806 1820 1846 1856 1956 1963 1973 (5) values
with barley
modern
analyses:
METHODOLOGY
Sl3
plant fiber sources; for surveys or quality control work the methods must be rapid and convenient even though some detail may be sacrificed. Whatever system is adopted, if it is to be competitive with the crude fiber method, it must permit the handling of large numbers of samples yet, at the same time, yield more than a single measurement. Fibers are variable in their composition and properties, and it is not possible to describe the characteristics and amount of fiber in a single value. Among the better systems of analysis that will fulfill the need for rapid yet precise measurement is the detergent method (10) developed to overcome the deficiencies in the crude fiber method. Defmition
of fiber
Dietary fiber has been defmed as plant cell wall and as nonnutritive residues. These terms, while similar in meaning, are by no means synonymous. “Unavailable carbohydrate” is a term used in the same context as nonnutritive residues. “Nonnutritive residues” is a broader classification including all substances resistant to animal digestive enzymes (Fig. 1) while “plant cell wall” refers to botanical structure. The plant cell wall includes most of the matter in plants that is resistant to animal enzymes. The plant cell wall does contain a small amount of protein that is partially digestible. Resistance to digestion by animal enzymes is not synonymous with indigestibility. Carbohydrates that escape animal digestion and reach the lower bowel are subject to microbial fermentation, the extent of which is dependent upon the type of fiber, its potential fermentation rate, and residence time (14). Approximately 50% of the fermentation products are volatile fatty acids, principally acetic,
(values
Corn
Oats
21.3
32.8
a re on dry
matter
Potatoes
basis)
(peeled)
Rye
5.6
22
Wheat
13.8
3.3 5.3 4.9 6.0 21 are
from
summaries
16.1 12-16 13.5
2.4 3.1 12 of earlier
31 literature.
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
2.4 2.7 2.2
2.2 1.8-2.4 4.7 “Neutral-detergent
fiber.
1.6 2.9 2.1-2.6 14
S14
VAN
SUGAR
SOEST
STARCH
PROTEIN DIET
-
LIPID
PECTIN,
Cl) LU
GUMS
LU
>
CELLULOSE
HEM ICELLULOSE ‘LU
I-
LIGNIN z
FECES
-
BACTERIAL BACTER
CELL
WALLS
IA
ENDOGENOUS
MATTER
FIG. 1. Illustration of the overlapping of dietary and fecal components in relation to dietary fiber and nonnutritive residues. Dietary fiber is defined here as insoluble matrix substance of plant origin. Proteins and lipids in feces may not be of dietary origin. Pectin, gums, hemicellulose, and cellulose may be extensively fermented and have low recovery in feces. The undigested fraction of the diet appearing in the feces may contain digestible matter (relative to animal or microbial enzymes) that has escaped digestion through transit. Bacterial matter is primarily of nondietary origin (22).
propionic, and butyric acids (15), which animal studies show to be efficiently absorbed and used in animal metabolism (16-19). The role of gastrointestinal fermentations in supplying metabohizable energy presents the greatest difficulty in accepting the nonnutritive residue concept. The term should be abandoned. The supposed inability of humans to absorb and utilize volatile fatty acids is probably incorrect and is very defmitely not supported by research with other monogastric species including primates (20). If man is indeed unique in his inability to utilize fermentation acids, then a serious question must be raised as to which animal species is a suitable experimental model for human studies. The apparent fiber content of food can be increased by cooking or other heating processes, including baking, frying, extrusion, etc. (Table 2), that involve the nonenzymatic browning reaction between amino acids and degradation products of sugars (21). The Maillard polymer is a brown, insoluble, indigestible substance with the physical properties of lignin. It is quantitatively recovered in the residue from neutral detergent or acid detergent extraction and in crude hignin. Breakfast cereals and other browned products
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
TABLE 2 Composition
of toasted
Sample
products
ADF
N
(21)
ADF
6.25
ADF
N/total
N
Crude
lignin
dry basis
Untoasted bread Toast Potato chips Tortilla chips Corn flakes Wheat Chex
0.6
0.1
0.7
0.2
1.5 4.1 2.7 3.9 6.5
0.6 0.6 0.7 2.5 3.8
3.7 9.0 7.8 31.0 32.0
0.8 0.9 1.5 1.8 3.5
contain significant amounts of this material (22). The browning causes large portions of protein to become unavailable to digestion. In the case of Wheat Chex and corn flakes this amounts to one-third the total crude protein. Since most people over-consume protein, this loss should not be looked at in a negative sense but as an interesting characteristic of dietary fiber in these products. Gums
and pectin
A problem of chemical analysis is presented by components that are resistant to animal enzymes but are not part of the insoluble plant cell wall matrix. Pectins are a part
DEFINITION
OF
FIBER
of the plant cell wall but can be removed by nonhydrolytic reagents. An example is the preparation of jelly from apples or berries. Water-soluble gums are a diverse group. For example, some gums are derived from bacteria, while others such as methyl cellulose are chemically prepared. The galactans that occur in beans are storage products from the view of plant metabolism but are resistant to animal digestive enzymes (23). Most of these substances are probably highly fermentable, and fecal recovery will be low. The interest in including the gums as part of the dietary fiber complex is related to their probable role in imparting to the diet some of the properties associated with fiber (24). This possibility has stimulated development of enzymatic methods for fiber. No good method exists at the present time for recovery of soluble gums. In feces, analysis is confounded by inclusion of microbial polysaccharides of the bacterial cell wall (25, 26). The neutral-detergent fiber, which isolates only insoluble plant cell walls, is the only available method for removing microbial cell walls; however, gums and pectins are also removed in the process (27). A suitable method should be developed for the soluble substances apart from the insoluble fiber. These two classes probably elicit different dietary and nutritional effects. Components
of plant
fiber
The major components of plant cell walls are cellulose, hemicellulose, and lignin (28). Cellulose is a $ glucan isomeric with starch and is remarkable for its msolubility. Hemicellulose, which is largely extractable with acids and alkalis, is a very complex carbohydrate containing residues of various sugars and a variety of glycosidic linkages. In grasses hemicellulose is ester-linked to lignin (29), and in dicotyledonous species the bond may be glycosidic. In grasses the bond between lignin and carbohydrate is easily cleaved with alkali, rendering a large portion of hemicellulose water soluble (30). Deignified cellulose and hemicellulose become completely accessible to fermentation. The protective effect of lignin is related partly to bonding and partly to physical association in the cell wall complex (31, 32). Lignin and several minor components in-
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
S15
METHODOLOGY
cluding cutin, protein, and mineral components of the plant cell wall constitute the main noncarbohydrate fractions. The crude lignin fraction incorporates the Maillard polymers formed through heat damage in the nonenzymatic browning reaction. True lignin is polyphenolic in structure, has a condensed nonhydrolyzable structure, and is part of the truly indigestible matrix (33). Mineral components of fiber are important. Calcium pectates in legumes and vegetables are probably fairly available forms of calcium (34). Phytic acid is not a true fiber fraction and constitutes a storage reserve of phosphorus for the plant seedling. Some plants biologically accumulate opaline silica in the plant cell wall to serve as a protective factor (35, 36). Most cereals contain appreciable amounts of silicon, but rice is the cumulator plant par excellence. The opaline silica matrix could serve as a binder for other elements such as iron (37). The role of plant fiber in the availability of trace elements is a complicated one because of the probable existence of unavailable forms in the cell wall matrix and also the possible binding to the cation exchange of the fiber surface. The cation exchange properties of plant fiber are widely variable (Table 3) depending on source and preparation (18, 38). Purified TABLE Hydration capacity Isolated
3 capacity, of various tibet’
bulk plant Hydration . pacity
volume, and fibers (18) ca-
Bulk
cation
volume
mI/g
Potato Rutabaga Broccoli Celery Onions Cucumber Ryegrass Bagasse Cellulose Alfalfa Wheat straw Orchardgrass Corn bran Wheat bran Solka Floc a Cold neutral-detergent through treated binding paper.
26.8 21.7 18.5 18.1 17.4 16.5 14.7 8.6 8.6 7.2 6.8 6.7 6.6 6.5 4.7 extracted
exchange
Cation cx. change mEq/g
8.3 7.2 7.1 9.5 11.3 7.9 12.3 4.6 8.7 8.1 8.9 8.2 3.8 6.2 3.7 plant
0.62 1.56 0.96 1.35 1.00 0.96 0.26 0.03 0.00 0.30 0.15 0.27 0.38 0.22 0.06 tissue ground
20 mesh. Potato, corn bran, and wheat bran with amylase to remove starch. “Calcium of isolated fiber residue. C Whatman 41 filter
VAN
S16
cellulose has no binding ability whatsoever-hence its use as filter paper in chemical separations. Anion exchange capacity is related to the lignin, pectin, and Maillard polymers, the latter probably possessing considerable anion exchange capacity through its nitrogen content. Sources
and
types
of dietary
fiber
The composition and properties of plant fiber vary greatly according to source, species of plant, and physiological stage of growth. Fibers from related species within a plant family, such as cereal brans, are relatively similar and divergent from dicotyledonous sources. Most vegetables and fruits are dicots and represent an immature stage of growth in contrast to the brans, which are mature and senescent plant products. Generally, hgnification and other factors promoting the development of a truly indigestible fraction increase with plant age (13). The environment in which a plant is grown has a significant effect upon the composition of the plant. The amount and composition of plant cell walls are dominantly affected by environmental temperature and to a lesser degree by fertilization, moisture, soil, and light. The fiber from tropical plants tends to be less fermentable because of the effect of environmental temperature upon hignification (39, 40). Nitrogen fertilization can alter the proportions of hignin and hemicellulose. Most of these studies have been done in agronomic studies of forage. However, the pattern probably extends to cereals and vegetables (41). It has been popular to substitute purified cehluloses for natural sources of fiber in nutritional experiments. Recently cellulose derived from wood has been introduced into commercial bread. Purified cehluloses differ markedly from natural celluloses in physical and biological properties (Table 3) and may be considered to represent a separate class- of fibers that are chemically processed and altered (23, 31, 42). Bagasse from sugar cane and other similar industrial by-products may be considered members of this group. Purified celluloses differ from natural sources first because they are derived from different botanical sources, and second through the alteration caused by the chemical
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
SOEST
dehignification. Purified celluloses are seldom truly pure and can contain residual hemicellulose, hignin, and mineral components that are very difficult to remove. Dehignification results in complete fermentability in the long term, but with long lag at a slow rate (14, 31). Purified celluloses may have use in ruminant feeding (32), but their fermentive digestibility in man is probably quite low (43). Since they exhibit very fine particle size, high bulk density, and very low hydration capacity and cation exchange properties, their value as dietary fiber sources can be questioned. From an experimental point of view it is unwarranted to suppose that the isolation and separate feeding of purified cellulose or other cell wall components will elicit the same responses as the original fiber. Fermentability and physical properties are radically altered by preparatory treatments. Properties
of fiber
The properties of fiber that are of nutritional significance include bulk density, hydration capacity, binding properties, and fermentabihity. Plant fiber through its bulk effect promotes faster transit or rate of passage (10, 14). Animal studies show that fine grinding of the fiber increases the feed density and alters passage and the character of gastrointestinal fermentations (44). Coarse particles tend to pass through the digestive tract more slowly than fine ones, and elimination of coarse fiber may reduce the differential sorting of food residues. Grinding of wheat bran decreases its bulk volume primarily through collapse and demolition of the cell wall structure (Fig. 2). Hydration capacity is a more complex function of the surface area, which is increased on grinding, and of the interior cell space, which is decreased upon grinding (10, 18). Survey of various fiber sources indicates a wide range in physical properties of density, hydration, and cation exchange (Table 3). Generally vegetable fiber has the highest hydration and exchange properties, which are correlated (38), while cereal brans and forages are intermediate. As noted, purified sources of fiber rank very low in this scale of quality. It should be warned that these physical properties as measured may not be representative of those occurring in the lower
DEFINITION
OF
FIBER
bowel, because bacteria may ferment components contributing to the cation exchange or hydration capacity. Bacteria may also cause a reduction in fiber structure through fermentive digestion, which is offset by production of microbial cell mass that has its own hydration and absorption capacities (45). The bacteria thus replace fiber that has been fermented. Fermentability The most important property of fiber is its ability to serve as a substrate for microbial gastrointestinal fermentation. It is through this mechanism that energy in carbohydrates resistant to animal digestive enzymes becomes available to animal metabolism. Since the microorganisms that ferment cellulose, hemicellulose, and pectin are characteristic (46), fermentable fiber plays a role in establishing the environment of the lower digestive tract through the microorganisms and their products. HYDRATION DRYB&LI(
8
6
11
10
20
110
flJ
particle of wheat
size bran
ESH
bulk
FIG. 2. The relationship volume and hydration
TABLE 4 Fermentability
by rumen
between capacity
organisms
and
lignification
Maximal Lag
Cauliflower Onions Corn bran Wheat bran Alfalfa Bagasse Whatman cellulose Cotton “Lag determined slope of natural log
fibers .
source
4 5 5 3 4 4 9 17 by the zero of residual
intercept of the regression fermentable fiber upon
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
Sl7
Fibers vary in their capacity to support fermentation, and a number of factors influences this capacity. The fermentable portion of fiber resides primarily in the polysaccharide fractions, and most natural sources exhibit an obligately indigestible fraction and an available pool (31, 47). The obhigately unavailable portion is determined by the hgnm and cutin content that is associated with an amount of cellulose and hemicehlulose about 2#{189} times the hignin content (48, 49). The fermentabihity of the available portion is relatively independent of the unavailable portion (49) and is regulated by intrinsic factors within the cellulosic carbohydrates themselves (31, 42, 50, 51). The limitation of fermentation ratio is probably due more to intermolecular associations than to clycosidic linkage, although the latter may be important in hemicelluloses (51). In addition to the maximal rate of fermentation, a lag in fermentation is observed even with rumen organisms, which is associated with the nature of the substrate (Table 4). The range in observed fermentation rates is an order of magnitude. The consequence is that fermentation is rate limited by the character of the fiber. Highly lignified fibers are less digestible, and purified cellulose of low rate of digestion is relatively unfermented (Table 5) despite its biological availability (43). The amount and extent of fiber fermentation are dependent on the microbial population, which in turn is dependent upon the retention time at the fermentation site and the potential fermentation rate. The competition between digestion and passage rates determines the portion
of vegetable
Fermentation
.
Fiber
with (18).
METHODOLOGY
(18)
extent
Ratio
.
.
.
lignin/cellulose
IS-hr
.
digestion
coefli-
.
Rate’
0.42 0.23 0.10 0.06 0.12 0.04 0.07 0.04
0.94 0.91 0.94 0.43 0.59 0.45 0.94 0.98
of log residual fermentable time. C Maximal digestion
0.05 0.09 0.12 0.47 0.30 0.31 0.03 0.00 fiber against time. at 72 to 100 hr.
0.93 0.57 0.54 0.33 0.32 0.23 0.26 0.18 “Regression
S18
VAN
TABLE 5 Digestibility
of fiber
in various
animal
SOEST
species Digestib
Animal
Substrate Cell
Cow
Normal
Horse
Man
100% alfalfa 50% alfalfa:50% grain 20% alfalfa:80% grain Normal diets Cereal-based diets 100% alfalfa 75% brome 75% orchard grass Alfalfa Alfalfa Alfalfa Brome Brome Brome Orchard grass Orchard grass Orchard grass Wheat bran Alfalfa leaf meal Carrots Corn germ meal Cotton seed hulls Sugar beet pulp Peas Cabbage Agar-Agar Cellu-flour All Bran All Bran Lettuce Tomatoes Cabbage Celery Oranges Apples Mixed diet
Man
No
Beaver Dog Meadow
vole
Rat Sheep Swine Rat Sheep Swine Rat Sheep Swine Man
Range Mean
Range Mean
Man
“Methods the total
of Williams cell wall.
and
Olmstead
(43)
include
32-71 56 47 59 71
44-84 68 58 73 81
41 34 21 36 20 45 35 9 67 43 8 70 46 30 9 74 60 18 65 53 66 59 11
47 39 24 38 47 46 43 Il 71 47 6 76 47 35 6 84 63 23 89 84 80 59 29
pectin
of available carbohydrate lost to feces. Either fast transit or inherently slow digestion will limit extent of fermentation and the development of a microbial population. Under a
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
ility
of Reference
Cellulose
Crude
fiber
3-79 52 45 54 63 30 18 34 18 29 21 50 40 1 68 38 1 67 44 30 14 67 57 17 55 44 55
(55) (56)
(57) (58) (59)
(60)
(60)
(60)
(43)”
54-80 67 94-98
7 0 6 29 0 42 29 24 57 54-85 71 15-55
72-85
15-44
(63)
83
22
(63)
56-61
67-71
29-35
(64)
57
68
51
3 1-72 48
fruit or vegetables except potato Fruit, vegetables and whole wheat bread Fruit, vegetables and whole wheat bread, high level Corn, soybeaii, wheat bran, citrus fruit, purified cellulose All Bran
Man
and
diets
wall
Hemicellulose
and
feed
microbial
cell
wall
0 5 0 10 14 17 28 71
(61)
(62) (63)
Wrick, unpublished in the
hemicellulose
fraction
given set of conditions the microbial population probably adapts relatively rapidly (52). Thus, it is not feasible to expect improvement in celluholytic capacity of microorgan-
DEFINITION
OF
FIBER
isms through genetic manipulation or mutation (52). The worry that genetic manipulation to introduce cellulolytic capacity might prove a hazard (53) is unrealistic. Plant cell walls have evolved in plants as protective structures and are related to their ecological survival (54). Digestibility
METHODOLOGY 12.
13. 14. 15.
offibers
The capacity for fermentive digestion determines the ability of herbivorous animals to derive energy from plant fiber. The ruminant is the best adapted and exhibits generally the highest fiber digestibilities. Monogastric animals are to a varying degree less capable, the relative level depending upon the type of fiber (Table 5). Vegetable cell walls are highly fermented in human subjects while cereal brans tend to form an intermediate group. Forages and purified celluloses probably have a low availability to humans. Monogastric animals, human included, probably fare better on hemicehlulose than on cellulose in the same food (60), while ruminants digest cellulose and hemicehlulose to about an equal extent (64).
16.
17.
18.
19.
20.
21.
References 1. TYLER, C. Albrecht Thaer’s hay equivalents: fact or fiction. Nutr. Abst. Rev. 45: 1, 1975. 2. GORHAM, J. Chemical analysis of Indian corn. New Engl. J. Med. Surg. 4: 320, 1820. 3. EINHOF, H. Bemerkungen #{252}ber die Nahrungsf#{228}higkeit verschiedener vegetabelischen Produkte. Ann. Ackerbaues 4: 627, 1806. 4. HORSFORD, E. N. Value of different kinds of vegetable food, based upon the amount of nitrogen. Philosophical Mag. Ser. 3. 29: 365, 1846. 5. WOLFF, E. Die naturgesetzlichen Grundlagen des Ackerbaues (3rd ed). Leipzig: Otto Weigand, 1856. 6. MoRRISON, F. B. Feeds and Feeding (22nd ed). Clinton, Iowa: Morrison, 1956. 7. WATF, B. K., AND A. L. MERRILL. Composition of foods-raw processed prepared. Agriculture Handbook No. 8. Washington, D.C.: USDA, 1963. 8. VAN SOEST, P. J. Revised estimates of the net energy value of feeds. Proc. Cornell Nutr. Conf. 1973, pp. 11-23. 9. PALOHEIMO, L. Some persistent misconceptions concerning crude fiber and the nitrogen free extract. J. Sci. Agric. Soc. Finland 25: 16, 1953. 10. VAN SOEST, P. J. Physico-chemical aspects of fibre digestion. In Procedures of the IV International Symposium on Ruminant Physiology, edited by I. W. McDonald and A. C. I. Warner. The University of New England Publishing Unit, 1975, p. 351. 11. HENNEBERG, W., AND F. STOHMANN. Begr#{252}ndung einer rationellen F#{252}tterung der Wiederka#{252}er. Braunschweig: Schwetschke und S#{246}hne,vol. 2, 1864.
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
22.
23.
24.
25.
26. 27.
28.
29.
30.
31.
S19
Association of Official Agricultural Chemists. Official Methods of Analyses, edited by E. C. Richardson. Washington, D.C., 1887. RAYMOND, W. F. The nutritive value offorage crops. Advan. Agron. 21: 2, 1969. HUNGATE, R. E. The Rumen and Its Microbes. New York: Academic Press, 1966. WOLIN, M. J. Interactions between bacterial species of the rumen digestion and metabolism in the ruminant. In: Procedures of the IV International Symposium on Ruminant Physiology, edited by I. W. McDonald and A. C. I. Warner. The University of New England Publishing Unit, 1975, pp. 134-148. ARGENZIO, R. A., AND M. SOUTHWORTH. Sites of organic acid production and absorption in gastrointestinal tract of the pig. Am. J. Phys. 228: 454, 1974. ARGENZIO, R. A., M. SOUTHWORTH AND C. E. STEVENS. Sites of organic acid production and absorption in the equine gastrointestinal tract. Am. J. Phys. 226: 1043, 1974. VAN SoasT, P. J., AND J. B. ROBERTSON. Chemical and physical properties of dietary fibre. Presented at the Miles Symposium on Dietary Fibre, sponsored by the Nutrition Society of Canada, June 14, 1976, in press. YANG, M. 0., K. MANOHARAN AND 0. MICKELSEN. Nutritional contribution of volatile fatty acids from the cecum of rats. J. Nutr. 100: 545, 1970. BAUCHOP, T., AND R. W. MARTUCCI. Ruminant-like digestion of the Langur monkey. Science 161: 698, 1968. VAN SOEST, P. J. Use of detergents in analysis of fibrous feeds. II. Study of effects of heating and drying on yield of fiber and lignin in forages. J. Assoc. Off. Agri. Chem. 48: 785, 1965. VAN SOEST, P. J., AND J. B. ROBERTSON. Analytical problems of fiber. Presented at the Symposium on Carbohydrate Metabolism. Anaheim, California: Institute Food Technologists, in press. LEWIS, B. Physical and biological properties of the structural carbohydrates. Am. J. Clin. Nutr. 31: S82, 1978. TROWELL, H. Defmition of dietary fiber and hypothesis that it is a protective factor in certain diseases. Am. J. Chin. Nutr. 29: 417, 1976. MASON, V. C. Some observations on the distribution and origin of nitrogen in sheep faeces. J. Agri. Sci. Camb. 73: 99, 1969. GALL, L. S. Normal fecal flora of man. Am. J. Clin. Nutr. 23: 1457, 1970. BAILEY, R. W., AND M. ULYATr. Pasture quality and ruminant nutrition. II. Carbohydrate and lignin composition of detergent-extracted residues from pasture grasses and legumes. New Zealand J. Agr. Res. 13: 591, 1970. BAILEY, R. W. Structural carbohydrates. In: Chemistry and Biochemistry of Herbage, edited by 0. W. Butler and R. W. Bailey. London: Academic Press, 1973, vol. 1, pp. 157-206. HARTLEY, R. D. Carbohydrate esters of ferulic acid as components of cell walls of Lolium multflorum. Phytochemistry 12: 661, 1972. SULLIVAN, J. T., T. 0. PHILLIPS AND D. G. ROUTLEY. Water-soluble hemicelluloses of grass holocellulose. J. Agr. Food Chem. 8: 152, 1960. VAN S0E5T, P. J. The uniformity and nutritive avail-
S20
VAN ability
32.
33. 34.
of cellulose. Federation P. J., AND D. R. and nutritive characteristics wastes. Federation Proc. 33: HARTLEY, R. D. The lignin walls. Am. J. Clin. Nutr. 31: VAN
S0EST,
Proc. MERTENS.
32:
1804,
SOEST
1973.
48.
Composition
of low quality 1942, 1974. fraction of S90, 1978.
cellulosic plant
cell
L. W. The effect of dietary fiber on mineral Ph.D. Thesis, Cornell University, 1976. JONES, L. H. P., AND K. A. HANDRECK. Silica in soils, plants and animals. Advan. Agron. 19: 107, 1967. VAN SOEST, P. J., AND L. H. P. JONES. Effect of silica in forages upon digestibility. J. Dairy Sci. 51: 1644, 1968. Hopps, H. C., E. M. CARLISLE, J. A. MCKEAGUE, R. SIEVER AND P. J. VAN SOEST. Silicon. In: Geochemistry and the Environment, edited by E. T. Mertz. Washington, D.C.: National Academy of Sciences, in press. MCCONNELL, A. A., M. A. EASTWOOD AND W. D.
49.
STILES,
absorption.
35.
36.
37.
38.
MITCHELL.
foodstuffs Sci. Food
39.
40.
41.
Physical
characteristics
of
43.
44.
45.
46.
47.
52.
53.
that
54.
Agr.
D.
Yield
and
chemical
components
tropical mental Cornell
42.
51.
vegetable
could influence bowel function. J. 25: 1457, 1974. DEINUM, B., A. J. H. VAN Es AND P. J. VAN SOEST. Climate nitrogen and grass. II. The influence of light intensity, temperature, and nitrogen on the in vivo digestibility of grass and the prediction of these effects from some chemical procedures. Neth. J. Agr. Sci. 16: 217, 1968. DEINUM, B. Effect of age, leaf number and temperature on cell wall and digestibility of maize. In: Carbohydrate Research in Plants and Animals, edited by P. W. van Adrichem. Miscellaneous Papers 12. The Netherlands: Landbouwhogeschool Wageningen, 1976, p. 29. SCHMIDT,
50.
56.
57.
of
vegetable species as influenced by environvariables and plant nutrition. Ph.D. Thesis, University, 1971. STONE, J. E., A. M. SCALLAN, E. DONEFER AND E. AHLGREN. Digestibility as a simple function of a molecule of similar size to a cellulase enzyme. Advan. Chem. Ser. 95: 219, 1969. WILLIAMS, R. D., AND W. H. OMSTEAD. The effect of cellulose, hemicellulose and lignin on the weight of the stool: a contribution to the study of laxation in man. J. Nutr. 11: 433, 1936. Mooiu, L. A. General principals involved with ruminants and effect of feeding pelleted or wafered forage to dairy cattle. J. Animal Sci. 23: 230, 1964. FITT, T. J., K. HUTTON, A. THOMPSON AND D. 0. ARMSTRONG. Binding of magnesium ions by isolated cell walls of rumen bacteria and the possible relation to hypomagnesemia. Proc. Nutr. Soc. 31: lOOA, 1972. BRYANT, M. P. Nutritional features and ecology of predominant anaerobic bacteria of the intestinal tract. Am. J. Clin. Nutr. 27: 1313, 1974. WALDO, D. P., L. W. SMITH AND E. L. Cox. Model of cellulose disappearance from the rumen. J. Dairy Sci. 55: 125, 1972.
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S12/4656076 by University of Minnesota, Twin Cities user on 12 May 2018
55.
58.
59.
60.
61.
62.
L. W., H. K. GOERING AND C. H. GORDON. Relationships of forage compositions with rates of cell wall digestion and indigestibility of cell walls. J. Dairy Sci. 55: 1 140, 1972. MERTENS, D. R. Application of theoretical mathematical models to cell wall digestion and forage intake in ruminants. Ph.D. Thesis, Cornell University, 1973. WooD, T. M. Cellulose and celluloysis. World Rev. Nutr. Dietet. 12: 227, 1970. DEKKER, R. F. H. Hemicellulose degradation in the ruminant. In: Carbohydrate Research in Plants and Animals, edited by P. J. Van Adrichem. Miscellaneous Paper 12. The Netherlands: Landbowhogesschool Wageningen, 1976, p. 43. HUNGATE, R. E. Quantitative aspects of the rumen fermentation. In: Physiology of Digestion in the Ruminant, edited by R. W. Dougherty. Washington, D.C.: Butterworths, 1965, pp. 311, 365. WADE, N. Dicing with nature: three narrow escapes. Science 195: 378, 1977. MOORE, J. E., AND G. 0. Morr. Structural inhibitors of quality in tropical grasses. In: Antiquality Components of Forages, edited by A. G. Matches. CSSA PubI. No. 4. Madison, Wisconsin: Crop Science Society of America, 1973, pp. 33-49. ARROYO-AGUILU, J. A., AND J. L. EVANS. Nutrient digestibility of low-fiber rations in the ruminant animal. J. Dairy Sci. 55: 1266, 1972. HINTZ, H. F., D. E. HOGUE, E. F. WALKER, JR., J. E. LOWE AND H. F. SCHRYVER. Apparent digestion in various segments of the digestive tract of ponies fed diets with varying roughage-grain ratios. J. Animal Sci. 32: 245, 1971. HOOVER, W. H., AND S. D. CLARKE. Fiber digestion in the beaver. J. Nutr. 102: 9, 1972. VI5EK, W. J., AND J. B. ROBERTSON. Dried Brewers Grains in Dog Diets. Proc. Cornell Nutr. Conf., 1973, pp. 40-49. KEYS, J. E., JR., AND P. J. VAN SOEST. Digestibility of forages by the meadow vole (Microtuspennsvlvanicus). J. Dairy Sci. 53: 1502, 1970. KEYS, J. E., P. J. VAN S0EST AND E. P. YOUNG. Comparative study of the digestibility of forage cellulose and hemicellulose in ruminants and non-ruminants. J. Animal Sci. 29: 11, 1969. HOPPERT, C. A., AND A. J. CLARK. Digestibility and effect on laxation of crude fiber and cellulose in certain common foods. J. Am. Dietet. Assoc. 21: 157, 1945. HUMMEL, F. C., M. I. SHEPHERD AND I. 0. MACY. SMITH,
Disappearance
the 63.
of
digestive
SOUTHGATE, Caloric
conversion
sessment
of the
energy
cellulose
and
tracts of children. D. A. T., AND
value
factors. factors
of
human
hemicellulose
J. Nutr. J. V. G. An
used diets.
in the Brit.
from
25: 59, 1943. A. DURNIN.
experimental calculation
J. Nutr.
reasof
24:
the
517,
1970. 64.
J.
SULLIVAN,
forage
plants.
T. Studies J. Animal
of the hemicelluloses Sci. 25: 83, 1966.
of
