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British
Journal
of
Nutrition
(1995),
73,
773-781
773
Caecal and faecal short-chain fatty acids and stool output in
rats fed on diets containing non-starch polysaccharides
BY
C.
A. EDWARDS* AND
M.
A. EASTWOOD
Gastrointestinal Laboratory, Edinburgh University, Western General Hospital, Edinburgh EH4
2XU
(Received
23
March
1994
-
Revised
I8
July
1994
-
Accepted
17
August
1994)
The exact mechanisms by which non-starch polysaccharides increase stool output are unknown. In the
present study the hypothesis that the
site
of fermentation and short-chain fatty acid (SCFA)
accumulation
is
related to the action of non-starch polysaccharides
(NSP)
on stool output was tested.
The basal diet
(45
g NSP/kg) of forty-three male Wistar rats was supplemented with
50
g/kg of either
guar, karaya, tragacanth, gellan, xanthan
or
ispaghula for 28d. A further twenty-three rats were
maintained on the basal diet for the same time period. Faeces were then collected over
2
d and caecal
contents obtained post-mortem. Caecal and faecal wet
and
dry weights and SCFA were measured. Each
supplement had a different effect on the caecal and faecal contents but they appeared
to
fall into three
groups when compared with the basal diet.
In
group
1,
guar gum affected only caecal SCFA. It had no
effect on stool output or faecal SCFA. In group
2,
karaya increased caecal SCFA and tragacanth,
karaya and xanthan increased faecal SCFA and faecal water. In group 3, ispaghula
and
geUan
had
no
consistent effect on caecal
or
faecal SCFA concentrations but increased total faecal SCFA output and
increased faecal wet and dry weight. Although the knowledge that SCFA are rapidly absorbed in the
large intestine has led us to believe that they play no role
in
determining faecal output, these results
suggest that
in
some cases where NSP are slowly fermented, and increase faecal SCFA, the role of the
SCFA may need to be reassessed.
Non-starch polysaccharides: Short-chain fatty acids: Caecal fermentation: Stool output
The mechanisms by which non-starch polysaccharides
(NSP)
increase faecal output, and in
particular faecal water, are not fully understood.
NSP
which are resistant to fermentation
by bacteria in the large intestine are the most effective faecal bulkers, probably because they
retain their water-holding capacity (WHC). The extent to which a
NSP
is fermented in the
large intestine
of
an animal is dependent not only on the ease of fermentation but also on
the time it remains within the large intestine (Van Soest
et
al.
1982). The factors which
determine colonic transit time are not clear but propulsion may be increased by distension
(Chauve
et
al.
1976), bile acids (Kirwan
et
al.
1975)
or
by the stimulation of the mucosa
with the edges of particulate matter (Tomlin
&
Read, 1988). The effect of fermentation
products on motility has not been investigated thoroughly and short-chain fatty acids
(SCFA) have been shown to inhibit motility in the caecum
of
the sheep (Svendsen, 1972)
and the isolated rat colon (Squires
et
al.
1992), but to stimulate contractions in rat mid- and
distal colonic strips (Yajima, 1985). The osmotic activity of the
SCFA
has been largely
discounted as it has been shown that they are rapidly absorbed (McNeil
et
al.
1978). The
major sites of fermentation are thought to be the caecum and proximal colon but it may
be possible, if propulsion is stimulated before fermentation is complete, for fermentation
of
a
NSP
to continue further along the large intestine. If this occurs, the SCFA produced
at distal sites may have a greater effect on stool output as the water remaining in the gut
*
Present address: Department
of
Human Nutrition, University
of
Glasgow,
Yorkhills
Hospitals, Glasgow
G3
8SJ.
774
C.
A.
EDWARDS AND M. A. EASTWOOD
Table
1.
In vitro
fermentability and residual water-holding capacity
(
WHC)
of
non-starch
polysaccharides
(NSP)
used
in
the present study*
Fermentabilityt
Residual (mmol SCFA/I
WHCt produced
in
vitro
NSP
(g/&
after 24
h)
Guar 1.87 71.4
Tragacanth 2.13 61.4
Karaya 4.65 24.2
Xanthan 2.15 63.4
Ispaghula
ND
40.0
Gellan 3.08 31.2
Control 0.9
1
155
ND,
not determined; SCFA, short-chain fatty acids.
*
Values from Adiotomre
et
al.
(1990) and Edwards
&
Eastwood (1992).
t
After 24 h incubation with human faeces.
is related to the balance between
SCFA
production and absorption. In addition, as SCFA
have been shown to influence the metabolism and turnover of colonic enterocytes, higher
concentrations of SCFA at more distal sites, where disease is more common, may have an
important role in pathology and treatment.
The aim of the present study was to test the hypothesis that the site of SCFA
accumulation is related to the action of
NSP
on stool output. Further, that the
fermentability of
NSP
is related to the site of SCFA accumulation and
so
influences faecal
output. We therefore compared the caecal and faecal
SCFA
of rats fed on a variety of
NSP
with a range
of
in
vitro
fermentabilities (Adiotomre
et al.
1990;
Edwards
&
Eastwood,
1992;
Table
1)
and have related these to the effects of the
NSP
on faecal output and faecal
water.
MATERIALS AND METHODS
Animals
A
total of sixty-six male Wistar rats, initial weight approximately
150
g, were fed on a basal
diet containing (g/kg):
NSP
(measured by the Englyst method, Englyst
&
Cummings,
1984)
45,
digestible fat
29-9,
digestible protein
129,
starch
629.5,
sugar
22-8
(Special Diet Services
Ltd., Witham, Essex) for
4
weeks before administration of the test diet. They were housed
together in groups
of
up to five and maintained within the facilities of the Animal Unit,
Western General Hospital, Edinburgh. The room was regulated to a 12 h light
-
12 h dark
schedule.
Diets
Rats were maintained on the basal diet for
4
weeks. Six groups of seven or eight rats were
studied for the test
NSP.
As many rats were being studied and the number of metabolism
cages was limited, the groups were staggered, with two groups being studied at any one
time, and a control group at the beginning and end of the study giving a total of twenty-
three control animals. After
4
weeks each group was maintained on the basal diet
supplemented with
50
g/kg of either guar gum (molecular weight
(M,)
0.25
x
lo6,
Sigma
Chemical Co. Ltd., Dorset), xanthan (Keltrol
T;
M,
2
x
los,
Kelco Inc., San Diego, CA,
USA),
karaya
(M,
4.7
x
lo6,
Norgine Ltd., London), tragacanth
(M,
0.5-1
x
lo6,
D.
M.
W.
Anderson, Department of Chemistry, University of Edinburgh), gellan
(M,
0.5-1
x
lo6,
Kelco Inc.) or ispaghula (Richardson and Vicks Ltd, Egham, Surrey) for a further
4
weeks.
NSP, FERMENTATION AND
STOOL
OUTPUT
775
Animals were allowed to feed and drink
ad
lib.
Weight gain was monitored weekly. For
3
d
at the end of the test diet period the animals were housed separately in metabolism cages.
Sample collection and analysis
While the animals were housed in the metabolism cages, food intake was measured. Faeces
were collected daily. In a previous study (Edwards
et al.
1992)
we have shown that for
faecal pellets over 1.5
g
there
is
only
8
YO
water loss over
24
h in our faecal collection system
independent of diet. Values for wet weight are calculated allowing for this water loss.
After
3
d the animals were killed by cervical dislocation. The caecum and complete large
intestine were removed. The two were separated and the contents carefully removed. The
weight of the caecal contents was measured and the contents treated in an identical manner
to the faecal material. The caecal and large intestinal tissue were carefully dissected free
from fat and mesentery and washed with saline
(9
g
NaCl/l). They were then blotted and
weighed. After weighing, the pH of faeces and caecal contents were adjusted to
>
pH
9
before freeze-drying.
SCFA were analysed by gas chromatography of diethyl-ether extracts (Spiller
et al.
1980).
Faecal samples from the last
2
d in the metabolism cages were pooled and used for
all analyses. Collections from the first day were not analysed to allow
for
the adaptation
of the animals to the cages. Animals were placed in the metabolism cages in the morning.
The faeces were collected before removing the animals on the final day. Any faeces passed
during handling on the final day after removal from the cages were regarded as colonic
contents. Previous studies have indicated that
2
d collections of faeces give values for stool
output that are not significantly different from values obtained over longer collection
periods
(28
d; Edwards
et al.
1992).
Statistical analysis
Results from rats given
NSP
were compared with rats given the basal diet by one-way
analysis of variance followed by Student’s
t
test using the pooled estimate of standard
deviation.
RESULTS
Food intake, body and tissue weights
Rats fed on guar gum and gellan ingested significantly more food, calculated without the
contribution of the added NSP, than animals fed on the basal diet (Table
2).
There was no
significant difference for the other diets (Table
2).
There was no difference in the final
weight of any group of rats when compared with the rats fed on the basal diet. Ispaghula-
fed rats, however, had a higher final weight than some of the other groups of rats. This was
due to a higher gain in body weight in the run-in period on the basal diet. The reason for
this is unclear. Each analysis of the results is a comparison of each test group with the rats
maintained on the basal diet and
so
this does not affect the overall conclusions (Table
2).
Ingestion of all the
NSP
was associated with an increase in caecal tissue weight although
this was most marked with guar and karaya. Only guar, tragacanth and ispaghula
significantly increased colonic tissue weight. Guar and ispaghula increased colonic length
(Table
2).
Caecal contents
The wet and dry weights of the contents of the caecum were increased by guar, tragacanth,
karaya, and ispaghula: xanthan and gellan both decreased caecal content dry weight.
Xanthan also decreased caecal content wet weight (Table
3).
Guar gum increased the concentration of caecal
SCFA as
a component of both dry and
776
C.
A. EDWARDS AND
M.
A. EASTWOOD
Table
2.
Mean food intake and caecal and colonic tissue variables of rats fed
on
a basal diet
(45
g
non-starch polysaccharides
(NSP)/kg)
supplemented with
50
g
NSPIkg
from various
sources,
for
28
8
Diet (g dry wt/4
(g)
(g)
(ma
(g)
Food intake$ Caecal tissue wt Colon tissue wt Colon length Final rat wt
Basal
(n
23)
Guar
(n
7)
Tragacanth
(n
7)
Karaya
(n
7)
Xanthan
(n
7)
Gellan
(n
7)
Ispaghula
(n
8)
Pooled
SD
228§
24-8*
24.2
22.2
23.6
26.9***
228
1-9
077
1*12***
1.02*
**
1.08***
1.0***
0.97***
0*97***
0.1
1
1.50
1%6*
1.73*
1.47
1.46
1-48
2.27***
033
190
223***
206
190
204
191
219**
20
390.0
353.8
349.1
382.6
387-1
3497
41 1.8
39.6
Mean values were significantly different from those of the basal diet:
*P
<
005,
**P
<
0.01,
***
P
<
0.001
t
For details of
diets
and procedures,
see
Table
1
and pp.
774-775.
$
Excluding supplemented
NSP.
8
n
18.
(Student’s
t
test after ANOVA).
Table
3.
Short-chain fatty acid
(SCFA)
content in the caecum of rats fed
on
a
basal diet
(45
g
non-starch polysaccharides
(NSP)/kg)
supplemented with
50
g iVSP/kg
from various sources,
for
28
df
Total SCFA Wet wt
of
Dry wt
of
Pmollg
,umol/g PmWg caecal caecal Caecal
Diet dry wt wet
wt
in caecum contents contents PH
Basal
(n
23)
Guar
(n
7)
Tragacanth
(n
7)
Karaya
(n
7)
Xanthan
(n
7)
Gellan
(n
7)
Ispaghula
(n
7)
Pooled
SD
46
1.7
699.3*
451.8
467.1
448.2
585.3*
4314
96.8
87.6
113.7**
78.0
89.7
67.5.
62.5**
67.3.
18.3
350.3
871.5***
4397
61 1.2***
179.0**
267.8
464.9*
136.4
4.00
7*58***
5.63***
6.91***
2.73**
4.24
6.91***
0.90
0.75
1.25***
0.97**
1.31***
0.40***
0.45**
*
1.1***
0.18
6.561
6.58
6.57
6.7
1
6.6
7.08
*
*
*
6.38
0.27
Mean values were significantly different from those for
the
basal diet:
*
P
<
0.05,
**
P
<
0.01,
***
P
<
0,001
t
For details
of
diets and procedures, see Table
1
and pp.
774-775.
1
n
18.
(Student’s
t
test after
ANOVA).
wet weights (Table
3).
Guar also increased the total amount
of
SCFA
in the caecum. Gellan
increased
SCFA
measured as a component
of
dry
weight but reduced
SCFA
concentration
per g wet weight, and had no effect
on
total amount
of
SCFA
in the caecum. Xanthan
decreased the
SCFA
concentration per g wet weight and the total amount
of
SCFA
in the
caecum. Ispaghula also decreased
SCFA
concentration per g wet weight probably reflecting
the increased proportion
of
water in the caecum. Tragacanth had no significant effect on
amount or concentration
of
SCFA.
Tragacanth significantly reduced, whereas gellan increased, the molar proportions
of
acetic acid in the caecum
(see
Table
5).
Ispaghula increased the proportion
of
propionic
acid and decreased the proportion
of
butyric acid whereas gellan had the opposite effect.
NSP, FERMENTATION AND
STOOL
OUTPUT
777
Table
4.
Mean faecal output and faecal short-chain fatty acid
(SCFA)
content of rats fed
on
a basal diet
(45
g
non-starch polysaccharides
(NSP)/kg)
supplemented with
50
g
NSPlkg
from
various sources, for
28
dt
Total faecal SCFA
Faecal output (g/d) Faecal Faecal
Wet wt
Dry
wt (g/kg) (g)
dry
wt wet wtz prnolld
pH
water water
,umol/g
pmol/g Faecal
Basal
(n
23) 3.26 1.58 5345 1.70 86.3 403 124.1 7.27
11
Guar
(n
7) 3.85 1.61 569 2.23 91.6 39.5 153.9 6.97
Tragacanth
(n
7) 4,65** 2.01* 564 2.64** 125.5 53.9* 248.2** 6.89*
Karaya
(n
7) 4.92*** 1.95 603** 297*** 139-5* 551 2730** ND
Xanthan
(n
7) 4*73** 1.86 602 2.87*** 183.5*** 71.3* 351.8*** 6*43***
Gellan
(n
7) 6.11*** 2.68*** 563 3.44*** 72.7 31.7 191.3 7.04
Ispaghula
(n
8) 5.87*** 2.56*** 652 3.68*** 108.7 39.9 238.1** 6.69***
Pooled
SD
1.10 0.48 38 074 39.6 13.6 87.3 0.36
ND,
not determined.
Mean values were significantly different from those for the basal diet:
*
P
<
0.05,
**
P
<
0.01,
***P
<
0001
(Student’s
t
test after ANOVA).
t
For details
of
diets and procedures,
see
Table
1
and pp. 774-775.
1
Assuming 8% water
loss.
0
n
22.
I/
n
18.
Table
5.
Molar proportions
of
short-chain fatty acids
(SCFA)
in caecal contents and faeces
of rats fed on a basal diet
(45
g
non-starch polysaccharides
(NSP)/kg)
supplemented with
50
g NSPIkg
from various sources for
28
dl.
Caecal SCFA Faecal SCFA
Basal
(n
23)
Guar
(n
7)
Tragacanth
(n
7)
Karaya
(n
7)
Xanthan
(n
7)
Gellan
(n
7)
Ispaghda
(n
8)
Pooled
SD
Acetic
66.2
64.8
61.5*
679
66.1
71.3*
66
1
4.9
Propionic N-butyric
17.3 13.0
16.3 150
19.7 14.0
15.7 12.2
15.6 13.6
I1.5***
19.0***
22.5** 94*
33 3.6
Acetic
80.0
85.1*
850*
76.2
74.1*
71*3***
78.6
5.3
Propionic
9.7
9.3
9.6
13.9.
13.3.
11.5
14.1*
4.0
N-butyric
5.0
0.95***
1.7**
3.3
6.1
6.0
5.0
2.3
Mean values were significantly different from those for the basal diet:
*
P
<
0.05,
**
P
<
0.01,
***
P
<
0.001
t
For details
of
diets and procedures,
see
Table
1
and pp. 774-775.
(Student’s
t
test after ANOVA).
There was no significant effect of any of the other
NSP
on the molar proportions of the
SCFA.
Gellan increased caecal pH (Table
3);
none
of
the other
NSP
had any
effect.
Faecal content
Guar gum had no effect on faeces apart from causing a slight increase in water content.
Tragacanth, karaya and xanthan increased faecal wet weight but not dry weight. They
increased faecal
SCFA
concentration (although this did not achieve statistical significance
for tragacanth), increased daily
SCFA
output and decreased faecal pH (Table
4).
Gellan
778
C.
A. EDWARDS AND
M.
A. EASTWOOD
and ispaghula increased faecal wet and dry weight and ispaghula increased total SCFA
output, but neither NSP had a significant effect on faecal SCFA concentrations.
Guar and tragacanth significantly increased the molar proportion of acetic acid in faeces
(Table
5)
whereas xanthan and gellan had the opposite effect. Karaya, xanthan and
ispaghula all significantly increased the proportion of faecal propionic acid. Guar and
tragacanth significantly decreased the proportion of butyric acid. None of the NSP tested
significantly increased the molar proportion of butyric acid.
DISCUSSION
The NSP in the present study showed a range of effects on caecal and faecal SCFA and
stool output that probably relate to their fermentability. The effects span a continuum
which can be approximately divided into three groups. The first group, containing guar
gum, was rapidly fermented, had the greatest effects on caecal SCFA and had little effect
on faeces. Tragacanth, karaya and xanthan form the second group. These NSP had
moderate effects on caecal SCFA, increased faecal SCFA concentration and increased
faecal water. The final group, consisting of gellan and ispaghula, had little effect on the
caecal SCFA. They had less effect on the total faecal SCFA than the second of group NSP
and did not increase faecal SCFA concentration. However, they had the greatest effect on
stool output increasing both wet and dry weights, probably in part due to some remaining
physical structure.
The increase in stool water caused by the NSP in group
2
may be related to a slower
fermentation at a more distal site, resulting in the increase in faecal SCFA concentration.
This group had the highest faecal SCFA output. The role of the SCFA in this increase in
faecal water is unclear. It was originally thought that SCFA in the colon caused increased
stool water by osmotic activity (Forsythe
et
al.
1978).
However, it has been shown that
SCFA are rapidly absorbed from the colon (McNeil
et
al.
1978)
and theoretically should
not have a significant osmotic action.
In
the present study the increased concentrations of
faecal SCFA indicate that this absorption capacity is not sufficient
to
maintain constant
concentrations of SCFA and an increased osmotic pressure may still be achieved. The
movement of the faecal stream may be related to the balance between production and
absorption of SCFA.
When the increase in faecal SCFA concentration is plotted against the SCFA produced
by each NSP
in vitro
(Table
1,
Adiotomre
et
al.
1990; Edwards
&
Eastwood, 1992) an n
shaped curve is produced (Fig.
1).
This supports the hypothesis that fermentability is
related to the site of SCFA accumulation, rapidly fermented NSP producing no change in
faecal SCFA, less rapidly fermented NSP increasing faecal SCFA, and NSP with low
fermentability having little effect on faecal SCFA. Karaya does not fit into this n shaped
plot due to the low fermentability
in vitro
compared with an apparent effect on faecal SCFA
in the rat
in vivo.
Karaya was anomalous in the predictive index of SCFA stool output in
the study of Adiotomre
et
al.
(1990) with little effect on stool output in man despite a low
fermentability and a high residual WHC. We have since carried out four further
in vitro
fermentations with karaya using different human faecal donors and found very little
evidence of fermentation. The fermentability of karaya in the rat may be higher than that
measured in human faeces. Moreover, the enzymes necessary for karaya fermentation may
need to be induced by prior ingestion of karaya. Karaya data have thus been omitted from
Fig.
1.
Vernia
et
al.
(1988)
showed that in patients with ulcerative colitis, increased faecal lactate
was related to an increase in faecal water. However, this was associated with decreased
faecal SCFA concentrations. Faecal SCFA and lactate did not play a role in Crohn’s
NSP, FERMENTATION AND STOOL OUTPUT
u
3
20-
(I)
-
(I)
0
a
C
2
10-
.-
779
a
u)
(I)
a
C
b
0.
-
X
I
T
G
disease. These patients, however, have inflammatory diarrhoea. They have abnormal
absorption and motility, as well as probable abnormal carbohydrate fermentation, and
their results are not representative
of
normal colonic physiology. The level of lactate in
normal adult faeces is very low. In infants where it is high and in
in
vitro
cultures of adult
faeces, lactate production is associated with rapid fermentation of sugars or oligo-
saccharides. Lactate was not measured in the present study but is unlikely to play a role in
the fermentation of slowly fermentable NSP. Other studies of ulcerative colitis have shown
elevated faecal SCFA (Roediger
et
al.
1982).
There are several other possible mechanisms by which SCFA may affect stool output.
SCFA may have significant actions on motility (Yajima, 1985). It is also possible that, due
to slow fermentation, the WHC of these NSP is maintained for longer and further round
the colon preventing water absorption until the NSP is fermented in a more distal part of
the colon with a consequent increase in SCFA. We have previously shown that ispaghula
did not increase faecal WHC (Edwards
et
al.
1992). The exact site of the loss of WHC of
a NSP may be as important in its action on stool water as a significant increase in the WHC
of faeces.
The contribution of bacterial cell mass to the faecal weight was not measured in the
present study and it may be possible that there is a different role for the bacterial cells in
the action of the different groups of NSP studied (Stephen
&
Cummings, 1980).
None of the NSP decreased caecal pH despite obvious evidence of fermentation. This is
in contrast to previous work in rats (Jacobs
&
Lupton, 1986; Wyatt
et
al.
1988) and may
relate to the lower dose used in the present study. The lower dose may explain why guar
gum increased food intake in the present study in contrast to other reports (Wyatt
et
al.
1988). The physical form of the food may also be relevant. The diet in the present study
was fed in the form of a paste. The physical form of food is not made clear in other studies.
The reason that guar did not increase body weight despite an increased food intake is
unclear but may indicate that
2
d measurement of food intake may not fully represent food
780
C.
A.
EDWARDS
AND
M.
A.
EASTWOOD
intake over the entire 4-week period. There may also be increased energy costs associated
with moving viscous
NSP
along the upper gastrointestinal tract.
Each
NSP
diet was associated with its own pattern of caecal and faecal SCFA. In the
caecum, ispaghula was associated with a higher proportion of propionic acid whereas rats
given gellan had more butyric acid, The reason for this difference in end-products is not
clear. The predominance of acetic acid is generally higher in the faeces than in the caecum
and reflects either a change in fermentation pattern in more distal colon or a difference in
the utilization or absorption of the SCFA. However, those fibres which had the greatest
effects on faecal water had the least predominance of acetic acid in faeces and in most cases
a higher predominance of propionic acid, indicating the possibility of sustained
fermentative activity in more distal parts of the colon.
In summary, the rate of fermentation
of
a dietary fibre and its subsequent effects on
caecal and faecal SCFA appears to be an important factor in determining its effect on stool
water and stool output. Fibres identified as group
2
in the present study appear to be
fermented at a distal site in the colon producing the highest faecal SCFA concentrations
and increased stool water. The mechanism for this action is not clear. It may be related to
action of the SCFA on motility or
a
delay in the loss of WHC to a more distal site but the
osmotic activity of
SCFA
produced in the distal colon should not be discounted.
We would like to thank the Proctor and Gamble Company for financial support.
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