Dietary D. A.
fiber:
T Southgate,2
analysis
The
components
discussed
measurement species seems
in foods
would essential
in a range
of
in
relation
and
the
dietary to
diet
be a time-consuming to an understanding
of foodstuffs
of total
sources
dietary
is presented, fiber
Journal
of Clinical
Nutrition
fiber
their
sources1
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reviewed
and
the
Complete
relative Am.
and
physiological
exercise; nevertheless of any physiological
is outlined.
31: OCTOBER
are
possible
as a whole.
The total intake of dietary fiber3 is derived from two sources: the structural materials of the plant cell walls present in the foods making up the diet; and nonstructural polysaccharides that occur naturally in foods (e.g., mucilages and gums) or are used as food additives (e.g., gums, algal polysaccharides, and modified celluloses) (2). Dietary fiber is a mixture of components, the composition of the mixture depending on the types of foods in the diet. Furthermore, the composition of the cell wall in many plants is dependent on the stage of maturity of the plant and on the cultural conditions under which the plants were grown. The major components of dietary fiber are listed in Table 1. The components themselves contain a range of types of chemical structures (Table 2); the complexity of this mixture making up dietary fiber is therefore very great if these variations are included. Any analytical procedure for the measurement of total dietary fiber must represent the compromise between a complete fractionation and measurement of all the various species present and a simplified system involving grouping of the different components in some arbitrary and often empirical way (3). Since the various approaches to the analysis of dietary fiber have been discussed in earlier papers and at length in recent reviews, they will not be repeated here. There are, however, some important points to be made about the analysis of dietary fiber. The original dietary fiber hypothesis relating diverticular disease to low fiber intakes (4) has been enlarged to include a wide range
The American
food
Ph.D.
ABSTRACT
intake
and
fractionation
chemistry
Nutr.
of all
of the 31: 5107-S
of
and
some characterization role. The composition
contributions
J. Clin.
the
properties
major
the
their
the
various analytical
polysacchanide
of dietary of the dietary food
groups
fiber fiber to the
1 10, 1978.
of other diseases in Western civilization (5). This expansion is related to several different types of physiological effects. It is unreasonable to expect all these effects to be caused by all the components of dietary fiber; indeed it is already apparent that the capacity of fiber to increase fecal bulk and reduce transit time is unrelated to any effect on serum cholesterol concentrations (6). Analogously, polysaccharides that can be shown to reduce serum cholesterol concentrations do not necessarily affect fecal bulk (7). It therefore seems essential to use methods that characterize the various components making up the total. If it can be shown that individual components are associated with specific physiological effects, it is probable that our aim should be the measurement of specific classes of polysaccharides and not the combined mixture. This would have the advantage of avoiding the confusion associated with the use of the term fiber for nonfibrous polysaccharides (8). It is vital, however, to avoid linking the definition of dietary fiber with any one empirical procedure in the way that crude fiber is linked. At the present time, and probably in the future, there is no one procedure that
‘From the Dunn Nutritional Laboratory, Medical Research Council and University of Cambridge, Milton Road, Cambridge CB4 IXJ, England. 2 Member of Scientific Staff, Medical Research Council. ‘In this context, dietary fiber is defined as the sum of the polysaccharides and lignin that are not digested by the endogenous secretions of the human digestive tract (1).
1978,
pp. S 107-S
110.
Printed
in U.S.A.
S107
SOUTHGATE
S 108
is appropriate for all types of foodstuffs or diets. Ultimately the method chosen will not be the one that is the simplest, nor the most rapid, nor the one that is most repeatable by analysts with a range of skills and experience, but the method that enables the measurement and characterization of dietary fiber in such a way that will predict and explain the physiological effects of the consumption of the mixture in question. Food
Characteristics foods
sources
Quantitatively the major part of dietary fiber is derived from cell wall materials in most diets although few detailed analyses have been made. The major food sources are therefore cereals, vegetables, fruits, and nuts. Calculations based on crude fiber data (9), TABLE Components Structural of the
1 of dietary
Other
fiber Noncellulosic polysaccharides Cellulose Lignin Gums and mucilages Algal polysacchanides Modified celluloses Protein, lipids, waxes, cutin, etc.
components plant cell wall
Nonstructural charides
polysac-
substances
TABLE Structural of dietary
features fiber
of the
fiber
in different
Cereals The dietary fiber content of cereals depends greatly on the degree to which the cereal has been refined. Wheat flour at the extraction rate used in bread making contains about 3 to 4% as measured by the method of Southgate (11, 12). As the extraction rate is increased, the total dietary fiber increases to the levels found in whole grain of between 11 and 14%. Other cereals are not usually consumed at a range of extraction rates, and some typical values are shown in Table 3. Table 3 also shows the composition of the total in terms of noncellulosic pohysaccharides, cellulose, and lignin. The composition of the noncelluhosic fraction varies according to the extraction rate and among the various
components
Noncellulosic
groupings
.
pohysaccharides
Pectin Gums,
mucihages
Algal
polysacchanides
Modified
Principal structural types
Galactunonans Arabinoand ronoxylans
glucu-
Glucoand gahactomannans Anabino galactans $-D-glucan Aromatic polymer
Cellulose Lignins Nonstructunal
of dietary
2
Major
Structural
which may be misleading, suggest that cereals provide about one-third to one-half the total fiber in the United Kingdom. The remainder is provided mainly by vegetables with minor contribution from fruits and nuts. Crude fiber values underestimate the contribution of dietary fiber from all foods but especially from refined cereals (10). We must await further calculations to assess the dietary fiber sources more appropriately.
celluloses
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Galacturonans Great variety including arabino-xylans and glucoand galactomannans Sulphated galactans and gulurono-mannuronans Esters, ethers
Main
.
variations
Methoxy groups; side chains Branched and linear xylan chain; number and distribution of side chains Number and distribution of side chains Branching and side chains Degree of polymerization Type of polymer; functional groups Methoxy group; side chains Branching; side chains
Variations in the composition the back-bone chain Cross-linking
of
DIETARY
cereals. High-extraction rich in the polysaccharides nose and xylose.
FIBER:
wheat and containing
ANALYSIS
AND
rye are arabi-
Vegetables are eaten at high water contents, which effectively dilutes the dietary fiber so that with the exception of seed legumes the values for total dietary fiber in fresh vegetables are often less than 5% (10). All the vegetables we have examined contamed minor amounts of lignin; this is consistent with histological examination of these foods as consumed in the United Kingdom. 3 fiber
in some
SOURCES
Fruits The total appreciably
dietary lower
Total
dietary fibe
Noncellulosic poly. saccharides
of the fiber
Cellulose
Composition n oncellulosic
Lignin
g/10t7g
(72%) (90-95%) flour (100%)
TABLE Dietary
matter 4 fiber
5) is basis.
of fraction
Hexoses
Penioses
Uronic . acids
80 44 38 32 19 62 82 46
11 45 49 57 69 26 9 45
9 11 13 11 12 12 9 9
c
3.45 8.70 13.51
Bran, sieved Bran, coarse Oatmeal, for porridge Rice, long grain Rye, whohemeal product “Dry
fiber in fruits (Table on a fresh weight
cereals C omposition dietary
Wheat products White flour Brown flour Wholemeal
S109
In leafy vegetables collenchyma with nonlignified walls are a major element in maintaining the erect plant in contrast to the monocotyledonous grasses where lignified scherenchyma are important structural elements. Table 4 shows sOme typical values. In leafy vegetables the noncellulosic polysaccharides contain similar proportions of pentoses and uronic acids. Root vegetables on the other hand contain more uronic acids.
Vegetables
TABLE Dietary
FOOD
80 72 74 75 74 82 78 71
30.6 48.0 7.66 2.74 12.7
19 18 20 16 18 12 22 14
1 10 6 9 7 6 Tn 15
basis.
in some
raw
vegetables .
Total
dietary
Fresh weight
Composition . dietary
fiber
Dry . weight
Noncellulosic
.
.
Composition noncellulosic polysacchandes
of the fiber
Cellulose
Lignin
g/JOOg
Leafy vegetables Brocolli tops Brussels sprouts Cabbage, winter Cabbage, white Onions
.
Hexose
Pentose
of the .
Uronic acid
%
3.60 4.22 3.44 2.66 1.30
32.7 35.5 29.4 27.4 18.1
76 72 62 66 74
23 25 25 23 26
1 3 13 11 Tn
15 17 9 7 29
42 48 47 42 26
44 35 43 50 45
7.75 6.28 2.90
37.1 34.1 26.4
69 61 52
27 39 42
2 Tn 7
48 23 20
22 30 42
30 47 39
2.90 2.40 3.41
28.4 22.1 14.1
60 67 71
40 33 29
Tn Tn 1
20 17 80
35 28 ND.
45 55 20
Legumes Peas,
frozen
Peas, canned Runner beans Root vegetables Carrots Swedes
Potato
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SOUTHGATE
SIlO TABLE Dietary
5 fiber
in some
fruits
(edible
Tota
matter
1 dietary
Fresh weight
only) Composition dietary
fiber
Dry weight
Composition noncellulosic polysaccharides
of the fiber
Noncellulosic
Cellulose
66 64 74 78 71 64 54 65 46 47
33 21 20 9 14 9 28 15 16 32
Lignin
of the
Hexose
Peniose
Uronic acid
20 54 25 17 19 21 20 28 22 14
35 19 35 43 37 38 46 46 33 42
44 27 39
s//flog Apple, flesh Banana Cherry” Grapefruit, canned Orange Peaches” Pears Plums” Strawberries Tomato, fresh” “Skin
included
1.42 1.75 1.24 0.44 1.90 2.28 2.44 1.52 2.12 1.40 in edible
9.16 5.97 6.70 2.42 13.7 16.5 14.7 9.56 19.1 21.9
Barbara Bailey and Mrs. the values reported in this
with
physiology 2.
Calculations made from average food intakes and the application of the same methods to mixed diets show that, with the usual diet eaten in the United Kingdom and the United States, the average individual intake is about 20 g/day. Diets rich in whohewheat cereals may contain up to 35 g/day, but these are relatively low levels compared with, for example, the plantain banana diet typical of Uganda where preliminary calculations suggest over 100 g/day. In the diets we have examined, hignin vatues were very low, even with diets that had been cooked and may, therefore, have included hignin artifacts. Few studies have been reported where it is possible to identify the effects of specific polysaccharides, but studies with bran show that fecal excretion of pentose containing noncelluhosic polysaccharides may increase by a factor of3 to 4 (13). Uronic acid concentrations in feces are usually very how, which may explain why pectin does not have any effect on fecal bulk. The understanding of more specific relationships will depend on metabolic studies with defmed dietary fiber sources that may have been carefully characterized.
a
The
author
acknowledges
the
parts
played
by
Mrs.
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S107/4656072 by Queen Mary University of London user on 25 March 2018
Edna paper.
Collinson
in
obtaining
References 1. TROWELL, EVER, A.
of fiber
40 43 41 35 25 45 44
portion.
Lignin values are low except when lignified seeds are eaten or when skins are consumed because cutin has been analyzed with the hignin. Correlation
1 15 6 13 15 27 19 19 38 21
3.
4.
5.
6. 7. 8.
9. 10.
1 1.
12.
13.
H.,
M. S. W0LD. A. JENKIN5. Dietary fibre redefined. Lancet I: 967, 1976. SOUTHGATE, D. A. T. The chemistry ofdietary fiber. In: Fiber in Human Nutrition, Edited by G. A. Spiller and R. J. Amen. New York: Plenum Press, 1976, pp. 3 1-72. SOUTUGATE, D. A. T. The analysis of dietary fiber. In: Fiber in Human Nutrition, edited by G. A. Spiller and R. J. Amen. New York: Plenum Press, 1976, pp. 73-107. PAINTER, N. S., AND D. P. BURKITr. Diverticuhar disease of the colon. A deficiency disease of Western civihisation. Brit. Med. J. 2: 450, 1971. BURKITF, D. P., AND H. C. TROWELL. Refined Carbohydrate Foods and Disease. New York: Academic Press, 1975. TRUSWELL, A. S., AND R. M. KAY. Absence of effect of bran on blood lipids. Lancet I: 922, 1975. TRUSWELL, A. S. Food fibre and blood lipids. Naringsforskning (Suppl. 1420): 5 1, 1976. SPILLER, 0. A., AND R. J. AMEN. Plant fibers in nutrition. Need for better nomenclature. Am. J. Chin. Nutr. 28: 675, 1975. ROBERTSON, J. Changes in fibre content of the British diet. Nature 238: 290, 1972. SOUTHGATE, D. A. T., B. BAILEY, E. C0LLIN50N AND A. F. WALKER. A guide to calculating intakes of dietary fibre. J. Human Nutr. 30: 303, 1976. SOUTHGATE, D. A. T. The determination of carbohydrates in food. II. Unavailable carbohydrates. J. Sci. Food Agric. 20: 331, 1969. SOUTHGATE, D. A. T. Determination of Food Carbohydrates. London: Applied Science Publishers, 1976. SOUTHGATE, D. A. T., W. J. BRANCH, M. J. HILL, B. S. DRASER, R. L. WALTERS, P. 5. DAVIES AND I. R.
D. A. T. SOUTHGATE, M. A. GASSUL
LEEDS,
MCLEAN
BAIRD.
plements
of bran.
Metabolic
Metabolism
responses
T.
AND
to dietary
10: 1129,
1976.
sup-