Physical properties of dietary fiber that influence physiological function: a model for polymers along the gastrointestinal ,2 Martin

A Easiwood

ABSTRACI does sterol

and

The

not recognize metabolism,

Edwin

R Morris

quantitative

measurement

its diverse fermentation

ofdietary

actions on nutrient in the colon, and

fiber

absorption, stool weight.

analysis

or quantitative

measurement

of the

fiber

content

foods does not allow prediction oftheir biological the physiological effects of dietary fiber depend

specific because

of

action pre-

differences in action are more likely due to differences in physical characteristics along the gastrointestinal tract. This paper explores such physical characteristics and attempts to classify dietary fiber in a more physical manner. This approach recognizes the diverse and variant action of each dietary fiber, which may be modified as a result of processing and cooking without

dominantly

changing

of dietary

assemblies,

could

also

the

gas-

sistant to hydration and swelling and to enzymic attack. At the other extreme, polysaccharide chains can exist in solution as fluctuating, disordered coils, interacting with one another only by physical entanglement and are readily accessible to appropriate enzymes. Between these extremes lie the hydrated, swollen networks typical ofplant tissue and ofmany manufactured foods.

These

fiber.

in any

way

The general

the

quantitative

principles

be applied

to other

trointestinal

tract.

developed

polymeric

Am

KEY WORDS

measurement

in this paper

materials

J C/in Nutr

Dietary

passing

along

1992;55:436-42.

fiber, gastrointestinal

tract

on physical

pie or direct

way

to chemical

Polysaccharides, fiber, show nature and treme

which

a wide extent

the

properties

that

do not relate

composition

polymer

chains

such

as cellulose

may

constituents

properties, association

be packed

fibrils,

sim-

(3).

are the principal

spectrum ofphysical of intermolecular

in any

together

which

of dietary

reflecting (4). At one into

are almost

the cx-

ordered totally

re-

Introduction Dietary

fiber

comprising

consists

various

and lignin, and plexes are found

has been

ofplant-cell-wall

amounts

ofcellulose,

is often accompanied in cereals, fruits,

defined

complex

as plant-cell-wall

hemicellulose,

by starch and vegetables.

materials

Hydrated

carbohydrates, (1).

pectin,

Such Dietary

resistant

cornfiber

to the in-

testinal secretions ofthe host (2). Such a definition has the appeal of apparent precision and permits gravimetric and chemical measurements but is complicated by the observation that a proportion

of other

protein,

and

dietary

constituents, namely retrograde starch, along the gastrointestinal tract without absorption or metabolism until they reach the cecum. Furthermore, while this definition of dietary fiber has been suggested to be physiological, it emphasizes negative properties rather than the positive role of fiber in protecting against disease. It has also raised a debate as to whether or not starch should be regarded fat, also pass

as a dietary fiber (3). The purpose of this

principles

may

physiological

be action

article

is to draw

general

Dietary wall

436

material

sequences

mote bilized

of value of other

in

explaining dietary

and and

or in a purified

in its natural or semipurified

that hydration

are disordered, and swelling

by arrays

trostatic

of

as in solution, (5). The ordered

and therefore projunctions are sta-

bonds

bonds,

of noncovalent

(hydrogen

dcc-

dipolar interactions, Van der Waals attractions). Because these bonds are individually weak, the junctions are stable only above a minimum critical length, and their formation and

and

disruption

often

response

to

comparatively

perature

or

solvent

occur

quality

as sharp,

cooperative

small changes (ie, the nature

dissolved solids). Because of the sequence-length for ordered association, the network properties

saccharides

are highly

irregularities

dependent

along

chain

in

ternof

requirement of specific poly-

on the spacing

the polymer

processes

in, for example, and concentration

rather

of minor than

struc-

on overall

tract general

predicting

secreted

the

polymeric

relationships

fiber can be ingested

networks are formed by ordered packing as in insoluble fibers, but have interconnecting

observations

fiber along the gastrointestinal to the diet in general. Such

substances. Structure-function

Polysaccharide chain segments,

tural

from the function ofdietary and to extend such principles

networks

form

as plant-cell-

form.

Chemical

Am J C/in Nuir

l992;55:436-42.

Downloaded from https://academic.oup.com/ajcn/article-abstract/55/2/436/4715317 by University of Cambridge user on 18 March 2018

I

From the Gastrointestinal

of Edinburgh,

Unit, Department

of Medicine,

University

General Hospital, Edinburgh, and the Cranfield Institute ofTechnology, Silsoe College, Silsoe, Bedford, United Kingdom. 2 Address reprint requests to MA Eastwood, Gastro Intestinal Unit, Department of Medicine, University of Edinburgh, Western General Hospital, Edinburgh EH4 2XU, Scotland. ReceivedMarch

Accepted

Printed

Western

18,

1991.

for publication

in USA.

June

27, 1991.

© 1992 American

Society

for Clinical

Nutrition

FIBER: composition proportion

as determined of constituent

by normal sugars.

A MODEL

chemical

FOR

analysis

of the

reaction

products

formed

during

cooking

or processing.

Cell wall structures

integrity

volving

of the

extensive

structures, associated cell wall

cell

be further

wall stabilized

which,

of processing

consequence insolubility

in addition

of chains by the

into

covalent

to in-

ordered bonding

ofthe

function ofthe plant within which nutrients

and

cooking.

extraction

of the complex

Many

of the

hemicelluloses This solubility process

contrasts

polysaccharide

constituents

of

and pectins, unmasked with

are as a

the overall

assemblies

ofthe

in-

cell wall.

Physiological

allowing

digesta to the

from

colligative

properties

of the

water-soluble-fiber

components, the surface properties of the water-insoluble cornponents, and the network properties of the swollen, hydrated components. Such properties include viscosity, water holding, cation exchange, organic acid adsorption, gel filtration, and partide-size distribution. The biological effects of dietary fiber along the intestine and colon may be summarized as modulation of absorption in the foregut, modification ofsterol metabolism, inducement of cecal fermentation, and increase in stool weight.

of absorption

meals

contact

the center

ofthe

Nutrients

in the foregut

reduces

the

postprandial

blood

with

lumen

then

the rnucosa

create

turbulence,

to be transported

have

to diffuse

the thin, relatively unstirred layer offluid immediately to the intestinal mucosa. An increase in the viscosity minal

contents

will obviously

impair

across

adjacent of the lu-

the peristaltic

mixing

pro-

cess. Dissolved polysacchandes may also inhibit diffusion of nutrients across the unstirred layer by presenting a physical obstacle to movement of small species (8), but this is likely to be a sigeffect

only

at very

high

polymer

concentrations

(9).

Hindered mixing oflurninal contents because ofviscosity enhancement by dissolved polysaccharides may also retard transport of digestive enzymes to their substrates (10). For example, wheat

and

more

quickly

maize

(which

as the

not

Digesta

contain

particle

of a larger oats (which

whereas ride) are

little size

soluble

fiber)

is reduced,

are digested

as expected

surface area to the digestive have a high content ofsoluble

from

environment, polysaccha-

(1 1).

viscosity

Soluble

polysaccharides

present

in digesta

as disordered

coils

confer viscosity by overlapping and interpenetrating one another to form an entangled network (12). The viscosity generated is dependent on the number ofcoils present and on their size (hyvolume),

but

10. Other

polymeric

not

on

their

components

gelatinized epithelia

starch) and will contribute

viscosity

is thus

to secretion testinal ical

chemical

tract,

highly making

from

of the

composition.

the concentration by about a factor diet

(eg,

proteins

or of and

mucus glycoproteins liberated from the to viscosity in the same way. Digesta sensitive

or absorption

action

to changes

of aqueous

it extremely

viscosity

in concentration

fluids

difficult

along to predict

measurements

due

the gastroin-

in vitro.

physiologCoil

volume,

and hence viscosity, may ofthe digesta. In particular,

also be altered by other constituents the hydrodynamic volume of charged is decreased by salts and, in the case of poly-

polysaccharides anions

such

of reducing individual

as pectin,

by low

internal

electrostatic

coils

material but

In general the two main effects ofdietary fiber in the foregut are to prolong gastric emptying time and to retard absorption of nutrients. Both are dependent on the physical form of the fiber, and in particular, on its influence on digesta viscosity. The inclusion of viscous polysaccharides (notably guar gum) in carbohydrate

into

contractions

epithelium.

to contract

present

plant-cell-wall

Modulation

nutrients

Intestinal

As a rough rule of thumb, doubling either the molecular weight will increase viscosity

properties

and

bring

intestine.

drodynarnic

Dietary fiber may be likened to a water-laden sponge passing along the intestine. The physical properties that influence function along the gastrointestinal tract are a combination of the rheological

mechanisms

of the small

exposure

Rates of release are influenced by factors such of tissue histology, degree of ripeness, and the

the plant cell wall, specifically soluble in water after extraction.

tact

(1),

packing

with lignification. An important is to provide an insoluble matrix

may be trapped. as the intactness effects

plant

organization the physical propofdietary fiber are dependent on

noncovalent

may

of nutrients

nificant

At a higher level ofstructural erties and physiological action the

Transport

close

437

POLYMERS

Two

For charged polysaccharides, such as pectin, thejunctions may be stabilized by incorporation ofarrays ofsite-bond counterions and are critically sensitive to pH and ionic environment, so that network structure may be lost or reformed under the varying conditions prevailing along the gastrointestinal tract (4). Network properties in vivo may also be influenced by other factors, such as the presence of bile acids, coacervation with proteins, or Mallard

DIETARY

in chyrne,

case

the

imation,

directly

ofdigesta sensitive

occupied to changes

An overall function

such

viscosity

ofwhich

and

compact

as insoluble contribute

enhancement to the fraction

by the particles (13) in the water content

of dietary

have

repulsions

will also

proportional

view

both

to a more

assemblies,

in this

pH,

fiber

the

effect

allowing

the

form.

Particulate

fiber

or hydrated

to overall

viscosity,

is, to a first

approx-

ofthe

total

and is therefore of the digesta.

in small

volume far less

intestinal

glucose

in humans (6, 7). This effect was originally cxplained as a result ofa delay in the delivery ofthe viscous material from the stomach into the small bowel. However, there appears to be no correlation between the rate of gastric emptying and postprandial concentration of blood glucose. There is also little evidence to suggest that viscous polysaccharides inhibit transport across the small intestinal epithelia.

Dietary

concentration

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ical

fiber can act in the small

forms:

as soluble

macromolecular

polymer

assemblies,

intestine

chains

in three

in solution,

and as swollen,

hydrated,

like networks. Although certain fiber components lulose are inherently insoluble, others may change form

with

tered

physical

time,

mechanical

conditions

agitation

along

by peristalsis,

the gastrointestinal

main

phys-

as insoluble

sponge-

such as ccltheir physical and/or

tract.

al-

In gen-

438

EASTWOOD

AND

eral, such changes will be in the direction ofincreased solubility; the fiber is first insoluble, then becomes swollen and hydrated, and is then fully dissolved. However, certain components, particularly charged polysaccharides such as pectin, may encounter conditions within the lumen that halt or reverse the progress of hydration (eg, interchain association promoted by specific countenons, changes in pH, reduction in solvent quality by small cosolutes, or coacervation with proteins). To a first approximation, the soluble fiber components can be regarded as forming a continuous sol phase within which the insoluble and hydrated components are dispersed as a disconparticulate

tinuous

chemical

phase.

composition

More

and/or

rigorously, physical

form

MORRIS

In addition to their contribution to the viscosity ofthe luminal contents, however, fiber particles may also reduce rates of absorption by an entirely different mechanism: by physical trapping of nutrients within the fiber matrix. Chyme may be considered a two-phase system with a discontinuous particulate phase dispersed in a continuous liquid phase. Nutrients trapped within the particles must first be released into the continuous solution phase before they can be absorbed through the gut wall. The rate of release (R) must clearly be proportional to the total surface area (A) of the particles (ie, the total surface over which transport can occur) and also proportional to the difference

particles

of different

in concentration

should

be regarded

continuous

between

(solution)

as constituting separate discontinuous phases. Similarly, other dietary components that do not form part of the homogeneous continuous phase (eg, unmicellized fat) can be treated as separate

specific nutrient is c, and is f, then its concentration

as before,

t/,

phases.

particles)

and

In such

two-phase or multiphase systems, such as density, obey a simple rule

properties,

P where

P,,

=

P is the overall

+ P242

physical

.

.

.

property

certain physical of mixing:

ofthe

entire

system;

P,,

physical property in the are the phase volumes

impossible

ofthe

=

to predict

from

the behavior

such as viscosity (12), and may be difficult or individual

salts

(and

perhaps

enzymes)

to specific

fiber

components

and inhibition

of diffusion across the unstirred layer (14). Because of their more compact physical form, fiber particles make a far smaller contribution to digesta viscosity (weight-forweight) than do dissolved polysaccharides. The overall viscosity, however, is influenced by both (and by other macromolecular species such as proteins and starch) and is also critically dependent on water content. The rate of release of nutrients from fibrous particles into the surrounding intestinal fluid is inversely proportional to particle size

and

is directly

proportional

the fraction

(particulate)

of the

trapped total

the concentration

within

phase

volume

in the

and

concentration

in the particulate

fraction

of a

the particles

is cf/

(where,

occupied

solution

by the

phase

is c(l

4). Thus,

-

R

=

kAc[f/4

(1

-

-

f)/(l

-

4)1

to solute

gradient.

where k is a rate constant for the release process, dependent on, for example, the molecular weight ofthe nutrient and the internal structure and surface properties of the particles. When the concentration within the particles is very much higher than that in the luminal fluid (eg, before any significant release has occurred), the expression simplifies to R

phases

in isolation. The principal physiological effect ofdietary fiber in the small intestine is to reduce the rate (and in some cases the extent) of release of nutrients. The dominant factors involved are 1) physical trapping ofnutrients within structured assemblies such as plant tissue, and 2) enhanced viscosity restricting the peristaltic mixing process that promotes transport of enzymes to their substrates, bile salts to unmicellized fat, and soluble nutrients to the gut wall. Subsidiary factors may include binding ofbile

0/(1

discontinuous

If the overall

+

P2, P represent the corresponding individual phases; and 4, , 42 (/) (with , + 4’2 + + n 1). However, other physical properties, combine in a much more complex way ‘

-

is the

the

states.

It is also

by, for example, the physical state of the solute (eg, whether it is present in solid form or is already dissolved in water trapped within the particle); the physical structure ofthe particle (eg, whether it is readily deformable, like a sponge, so that dissolved solids can be squeezed out by peristaltic contractions, or rigid, so that solutes must diffuse out); and the surface properties ofthe particle (eg, surface-tension effects). The concentration of nutrients within the continuous aqueous phase is constantly depleted by enteric absorption and replenished, as outlined above, by release of material from food particles. The progress of these sequential release processes is, ofcourse, also influenced by transit time (ie, the duration ofexposure to a particular absorptive surface or digestive environment). affected

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If the

particulate

mensions

(eg,

phase

width)

=

kAcf/q

consists

w, then

total volume (ie, ‘/0is proportional plification to R

of n particles

A is proportional

=

to nw3,

of linear to nw2

giving

di-

and

a further

the sirn-

kcf/w

(with the change in the constant of proportionality taking into account the geometric relationships between linear size, surface area, and volume). This final expression is, of course, simply a formalization of the intuitively obvious conclusion that the rate of release of nutrients from cohesive particles increases in direct proportion to the concentration within the particle (cf) and decreases with increasing

particle

Modification

size

(w).

of sterol

metabolism

Dietary fiber has been shown to have an effect on sterol metabolism (15). This effect is not simple because it is possible that dietary fiber displaces fat from the diet (16) or that polyunsaturated fatty acids frequently eaten in conjunction with the fiber may also be important (17). The effect of fiber on sterol metabolism may be through one of several mechanisms (1 5): altered lipid testine,

absorption, altered

reduced bile

acid

bile

acid

absorption

absorption in the

in the cecum,

small

in-

or indirectly

via short-chain fatty acids, especially propionic acid, resulting from fiber fermentation. Dietary fiber may retard absorption oflipids by the processes outlined above (enhancement of digesta viscosity or physical trapping within particles) or by sequestering either the bile acids necessary for formation of lipid micelles, or the micelles them-

selves.

Adsorption

acids

in the

to dietary

stool

cholestyramine in influencing

fiber

(18). However, sterol metabolism

Alternatively, acid

dietary

in the

loss (21),

of bile quantity in the

increase

fecal

loss of bile

circulation

akin

to that

conservation

of

that the physiological

effect

vein,

may

influence

the

metabolism

of

several

might

be a change

in the end

modulate

the catabolism

cecal

bacterial

activity.

However,

it has

product

of choles-

been

synthesis

and

may

be an indirect

catabolism

through

fatty acids, with the possibility may influence sterol synthesis

effect

offiber

fermentation

that propionic and catabolism

on sterol

to short-chain

acid in particular (24).

not

alter

in the colon may be summarized in terms of its susceptibility to bacterial fermentation, its ability to increase bacterial mass, its ability to increase bacterial saccharolytic enzyme activity, and the water-holding capacity of the fiber residue after fermentation (25).

fiber

Bacteria/fermentation The process whereby a compound is bacterially dissimilated in the cecum under ananerobic conditions is complex and varied, leading

to partial

or complete

ucts being

(26)absorbed

absorbed

and

reexcreted

decomposition

from the colon

with

to be utilized

in the enterohepatic

the end

prod-

as nutrients,

circulation,

or cx-

creted in stool. Many compounds are variably and simultaneously decomposed in the mixture, and apparently unrelated compounds may well influence the metabolism of each other. It has now been shown that a proportion of dietary starch may be resistant to pancreatic enzymes and passes to be fermented in the cecurn (27). Mucoprotein and biliary excretion compounds are also metabolized in the cecum but are quite separate entities from fiber. There is no simple way to predict the biological activity of complex carbohydrates in the colon (3). The disparate actions of cecal bacterial with each dietary fiber source can be defined only

by complex

and

time-consuming

experiments

(20).

Water soluble conjugates arriving in the cecum, usually biliary excretion products that have not been absorbed in the intestine (eg, bile acids and bilirubin), are modified by bacteria to cornpounds of lower solubility. Such modified metabolites may be absorbed to bacteria or fiber or be absorbed into the enterohepatic

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contributing

to the

cornis of

and composition

bacterial

flora

is dependent

for nutrition.

There

on dietary

are variations

and

endog-

in the amounts

(26).

Metabolic

products

Dietary acids

fiber

and

may

other

be extensively

metabolic

degraded

products,

to volatile

including

fatty

methane,

hydro-

gen, and carbon dioxide (29, 30). The degree, to which this occurs depends on the type of fiber and also on the amount of fiber. The duration of fiber feeding is important. Experiments feeding arabic

or carrot

to develop

evidence

in contrast

with

diet

show

that

a period

of fermentation

acute

ofpolysaccharide short-chain fatty

the

of dietary

thus

and possibly other (26). This process

ofsubstances passing through the intestine from the ileum, with an inverse relationship between cecal bacterial metabolism and upper intestinal nutrient absorption. Dietary fiber has an influence on bacterial mass and enzyme activity (28). The consensus is that while the cecal bacterial mass may increase as a result of an increased fiber content in the diet, the types of bacteria do

creased

fermentation

The effects

mass

sources

with Cecal

mucosa,

of bile acids metabolism

importance.

The cecal

gum there

body

of intermediate

enous

estimated

that 0.8-1. 1 imol/kg of deoxycholic acid and lithocholic acid are produced each day by cecal metabolism in the colon. This represents 25% ofcholic acid and 50% ofchenodeoxycholic acid passing through the cecurn either to be absorbed or excreted in feces (23). Alternatively,

the colonic

in the

physiological

terol to bile acids (22). The precise relationship between ileal and cecal absorption of bile acids is difficult to estimate. Particularly as there may be bacterial colonization of the ileum simulating

through

Bacterial

may

439

POLYMERS

is

in the cecum (22). A change in the acid absorbed from the colon, returning well

DIETARY

pounds

the fibers that are most effective (eg, pectin) (19) are fermented

through

or there

acid metabolism or type ofbile portal

FOR

mechanisms. Binding to transport of bile acids from the ileum to the there may be an alteration in the amount of out of the body in the stool and an increase

cecum

fiber may enhance cecum; thereafter, bile acids passing in steroid

fiber

A MODEL

somewhat

in the colon (20), so it is unlikely due totally to adsorption. bile

may

by a mechanism

FIBER:

exposure,

when

fermentation acids (acetate,

relative

amounts

is incorporated

arabic

and

is required

fibers may

(3 1). This

proportion ofthe and propionate)

butyrate

diet

major varies

results in an inin the rat colon; produced

depend

fed and also on whether

in an elemental

is

be no evidence

Gum arabic fatty acids

of acetate

of gum

there

(32). The butyrate,

and other conditions. production of short-chain

on the amount

ofingestion

ofthese

the gum

or in a standard

rat

pellet

the colon and utilized by the colonic mucosa and more in the body. It has been suggested that in human on a control diet, only 20% of the overall ingested fiber is recovered in the stools. Apdiet

(33).

The

proximately

metabolic

products

20 g of cell-wall

may

from remotely

be absorbed

polysaccharides

and

other

carbo-

hydrates are fermented in the human colon each day. Approximately 200 rnmol of short-chain fatty acids are produced, yet only 7-20 mmol/d are excreted in the stool. There is substantial absorption

of short-chain

fatty

acids

from

the

colon

and

sub-

that fermentable fibers have the potential to provide -4.2-8.4 Id (1-2 kcal)/g fermentable fiber to the system. The estimation ofthis calorie provision is complicated since there is often an associated increase in fecal fat with fermentable fibers, resulting in an increase of short-chain fatty acids to be absorbed and a fecal loss of fatty sequent

acids;

metabolism

the overall

30, 34). Hydrogen,

(30),

which

colonic

suggests

balance

methane,

carbon

stitute flatus. The volume estimated to range from

is difficult

dioxide,

to calculate

and swallowed

of flatus passed per rectum 200 to 2400 mL/d. Certain

(20,

26,

air conhas

been

foods are well-known to produce flatulence, particularly beans. Hydrogen production in the colon is dependent on the delivery of ingested, nonabsorbable, fermentable substrates to a high concentration of bacteria. Usually these substrates consist of nonabsorbable oligosaccharides

more

complex

such

as stacchyose

carbohydrates

such

and

raffinose,

as starch

(26).

and

possibly

440

EASTWOOD

TABLE

1

Effects

AND

MORRIS

Water-holding

of various

dietary

fibers on stool

weight

3.4

16

3.9

20

water is distributed in three ways: free water that from the colon, intracellular water within the fecal bacterial mass, and water that is bound by residual unfermented fiber (36). The degree to which free water is absorbed from the colon will be affected by many factors, which are poorly understood. In a comparison ofcecal and fecal contents in rats, it was shown that the fermentation of some

Wet weight Fiber

supplement

in stool

Reference

g

Fruit,

vegetables,

bread

Wheat bran Wheat

bran

In the colon, can be absorbed colonic and/or

3.9

38

complex

Gumtragacanth

6.3

20

effect

Gum arabic Gum karaya

0.6 0.4

20 20

Potato

fiber

1 .9

20

Rawcarrot Fruit and vegetables

6.0

39

1.8

38

Citruspectin

0.3

38

Stool

weight

Fecal

mass

Feces tribute

are complex

of 75% water; bacteria conthe residue consisting of unferfiber and excreted compounds, including bile excretion (eg, bile acids and bilirubin metabolites) and bacterial

largely

mented products cell-wall

to its dry

and

and

consist

weight,

fermentation

products

(eg,

long-

and

short-chain

fatty

acids). There is a wide range in individual and mean stool weights. In a study in Edinburgh, the variation in stool weight among apparently normal individuals was between 19 and 280 g over 24 h. For an individual there was considerable variation over the week of study. Fecal constituents (bile acids, sterols, fat, electrolytes)correlated strongly with fecal mass. Ofthe dietary constituents, only dietary fiber influenced stool weight (34). However, fibers differ in their ability to alter stool weight: wheat bran is predictable unpredictable in

and their

effective action

whereas fruit and vegetables are (20, 35). The most important

mechanism

whereby dietary fiber increases stool weight is the water-holding capacity of unfermented fiber, eg, wheat bran (36). The greater the water-holding capacity of the bran, the greater the effect on stool weight (37). Fiber may influence fecal output by another mechanism. Coionic microbial growth may be stimulated by the ingestion of such fermentable fiber sources as apple, guar, or pectin (35). However, there is not always an increase in stool weight as a result ofeating these fibers (Table 1)There may also be an added osmotic effect ofbacterial-fermentation products on stool mass, though this is not yet well-defined (38). Although animal cxperiments suggest that there is substantial absorption of shortchain fatty acids from the colon, the role of short-chain fatty acids on fecal weight and transit time may nevertheless be important (40). through

Fecal

mass

All but

and transit ‘-l2

time

time required for food residues to pass tract is spent in the colon. The time taken is called the intestinal transit time and may be measured by using a variety of radiopaque markers. Transit time is curvilinearly related to fecal weight, with an inflexion at 150 g, after which there is no influence ofincreased stool weight on a transit time of -‘-48 h (41). through

capacity

h ofthe

the intestinal

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carbohydrates

(eg, ispaghula

and

gellan)

has a significant

on the content of luminal short-chain fatty acids in the more distal colon. This appears to be related to continued fermentation along the colon (40). An increase in the short-chain fatty acid concentration of feces appears to be related to an increased output of fccal water, which suggests that under some circumstances, the absorption of short-chain fatty acids may be less efficient and may play a part in determining fecal output. This view agrees with that of Hellendoorn (42), who wrote in the l950s. Subsequently, the knowledge that short-chain fatty acids are rapidly absorbed from the colon led to the belief that short-chain fatty acids play no part in determining fecal output (30). However, it appears that there is continued fermentation of complex carbohydrates in the distal colon; under these circumstances, fecal short-chain fatty acids may influence fecal water absorption (40). It is therefore not unreasonable to suppose that bacterial metabolism will create changes in the colonic contents that will alter osmolality and absorption (43). The effect of fiber in the colon may be summarized as shown in Figure 1, or as Stool

weight

=

W,( 1 + H)

+ Wb(l

+ H,,) + Wm(l

+ Hm)

where W, Wb, and Wm are, respectively, the dry weights of fiber remaining after fermentation in the colon, bacteria present in the feces, and osmotically active metabolites and other substances in the colonic contents, that could reduce the amount of free water absorbed; and H, Hb, and Hm denote their respective water-holding capacities (ie, the weight of water resistant to absorption from the colon per unit dry weight of each fecal constituent).

Wider

implications

This description ofthe a fiber sol has implications

immobilization ofwater and solute by for other polymeric substances along

Colon

Foregut

t t t

t t I t Bacteria

FF I I I

I

I

I

1

11

1

1

EE

1

1

1

II

Metabolism fibre Residual

Faeces

}*.

Dfusion FIG

1. Schematic

representation

ofthe

function

ofdietary

fiber

fore-

gut, modulation of intestinal physiology and absorption in the foregut by an intact polymeric mass; colon, as a nutrient for bacteria in the cecum and as a key contributor to fecal constituents and stool weight.

FIBER: the

gastrointestinal

tract.

protein,

whose

cellular

structure

of cooking

physical of the

or

Such

logic

may

characteristics meat

processing.

A

also

may

or plant Similar

MODEL

apply

FOR

source

and

by the

by the

modifications

12.

to ingested

be modified to

13.

intestinal

It is ofcourse

of a particular similation Protein

physical that and

the time scale ofthe

structure

will vary gelatinized

nutrients

will be much

more

and thus The

described

trointestinal Recently,

by fiber

and

persistent

above,

the

biological

modalities

whereby

and

depending

absorption

symbols

ships

between

tance,

as described

polymers,

the aqueous

sorption,

and

small

environment

eventually,

fecal

exact

and

retrograde

intestinal

site

of dietary

affects

of more

and

on the

Such

development

of water

Similarly, analogous

fiber and

gas-

used

to

on the four

metabolism

in

successful when comparing with in vitro methods (44).

here

should

descriptions

molecules

provide of the

of nutritional

of intestinal

and

a basis relationimporab-

15. Eastwood

8.

J Hum Nutr Diet l988;l:77-84. H#{228}glundBO, Elisson M, Sundel#{246}f LO. Diffusion

permeability

in

concentrated polymer solutions. Chemica Scripts 1988:28: 1 29-3 1. 9. Edwards CA, Johnson IT, Read NW. Do viscous polysaccharides reduce absorption by inhibiting diffusion or convection? Eur J Clin Nutr 1988;42:307-l 2. 10. Schneeman BO, Gallacher D. Effects of dietary fiber on digestive enzyme activity and bile acids in the small intestine. Proc Soc Exp Biol Med l985;180:409-14. 1 1. Heaton KW, Marcus SN, Emmett PH, Bolton DH. Particle size of wheat, maize, oat test meals: effects on plasma glucose and insulin responses and rate of starch digestion in vitro. Am J Gin Nutr 1988;47:675-82.

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D, eds.

New York:

25.

1972;25:926-32.

Burley VJ, Leeds AR, Peterson DB. A guar-enriched bread reduces postprandial glucose and insulin responses.

J Clin

Kritchevsky

1. Selvendran

1978; 1:1392-4.

London:

So-

fiber on

K, eds. Dietary

Academic

Press,

Fiber

and

Lipoproteins.

Cah

Nutr

Diet

(in French).

Nutr

In: Nair PP, Kritchevsky

Hermus Ru, et al. The effect lipids, fecal lipids and colonic

1979;32:1881-8.

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AF.

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physiology

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In: Nair

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bran,

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Nutr

Rep Int 198 l;24:1093-8. MI, Horvarth

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PJ, Jeraci JL, van Soest PJ. Effect of in fecal inoculum on the water holding capacity Br J Nutr l985;53: 17-24.

Eastwood

Food,

McBurney

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MA.

drugs,

Engyst

HN,

Trowell

Rowland

IR,

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Mallett

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Southgate

and Wilkins, DAT,

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Walter DJ, Eastwood

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malian gut flora l985;16:31-103.

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fiber and resistant

29.

circulation

on lipid

the anal canal.

28.

and

Press 1973:1-33.

In: Kirsner JB, Shorter 27.

physiology

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1976;21:SOl-34.

WJL,

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Chemistry,

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7. Ellis PR, wholemeal

D, Heaton

olism. Vol 2. New York: Plenum Press, l973;l9l-248. 22. Danielsson H. Mechanisms of bile acid biosynthesis.

24.

Eastwood MA. What does the measurement ofdietary fiber mean? Lancet 1986; 1:1487-8. 4. Morris ER. Physical properties ofdietary fiber in relation to biological function. In: Southgate DAT, Waldron K, Johnson IT, Fenwick R, eds. Dietary fiber chemical and biological aspects. London: Royal Society of Chemistry, 1990:91-102. (Special Publications no. 83.) 5. Rees DA, Morris ER, Thom D, Madden JK. Shapes and interactions of carbohydrate chains. In: Aspinall GO, ed. The polysaccharides. Vol 1 . New York: Academic Press, 1982:195-290. 6. Jenkins DJA, Wolever TMS, Leeds AR, Ct al. Dietary fibers, fiber analogues and glucose tolerance: importance of viscosity. Br Med J

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effects ofdietary

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17. Swain JF, Rouse IL, Curley CB, Sacks F. Comparison of the effect of oat bran and low fiber wheat on serum lipoprotein levels and blood pressure. N Engl J Med l990;322:l47-52. 18. Eastwood MA, Hamilton D. Studies on the adsorption of bile salts to non absorbed components of diet. Biochim Biophys Acta 1968;l52: 165-73. 19. Kritchevsky D, Story JA. Binding ofbile salts in vitro by non nutritive fiber. J Nutr l974;l04:458-62. 20. Eastwood MA, Brydon WG, Anderson DMW. The effect of the polysaccharide composition and structure ofdietary fibers on cecal fermentation and fecal excretion. Am J Gin Nutr l986;44:5l-5. 21. Miettinen TA. Ginical implications ofbile acid metabolism in man.

23.

RR. The plant cell wall as a source of dietary fiber: chemistry and structure. Am J Gin Nutr l984;39:320-7. 2. Trowell H. Ischemic heart disease and dietary fiber. Am J Clin Nutr

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16. Staase-Wolthuis M, Hautvast JGAJ, ofa natural high fiber diet on serum

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MORRIS

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