Physiological implications of microbial digestion in the large intestine of mammals: 1 relation to dietary factors E. Stevens,2
D. V.M.,
Ph.D.
ABSTRACT and
The
pony
acids
correlated
(VFA),
anions
the
present
rate with
end
in
large
of digesta the
products
marker
relative
of microbial
intestinal
contents
of VFA
are
important
of the
large
to the nutrition intestine
of some
of most
Early studies by Bancroft et al. (1), Elsden et al. (2), and Phillipson (3) demonstrated concentrations of volatile fatty acids (VFAs) in the large intestinal contents of the ox, sheep, horse, pig, rabbit, rat, and dog equivalent to those found in the ruminant forestomach. Fresh human feces have subsequently been reported to contain VFAs at similar concentrations and ratios (4, 5). Since these are the principal end products from microbial digestion of carbohydrate in the rumen, the pertinent features of this extensively studied process are an appropriate introduction to the following discussion. Studies of digestion within the rumen are the subject of a number of reviews (6, 7). Rumen microbes are capable of synthesizing all the B vitamins required by the animal and converting nonprotein nitrogenous compounds into high-quality microbial protein. They also digest starches and sugars as well as a wide range of other polysaccharides (ccllulose, hemicelluloses, pectins) associated with the fibrous portion of plants. Under normal conditions the principal end products are three volatile fatty acids (acetate, propionate, and butyrate) plus CO2 and CH4. Only small amounts of lactic acid, other shortchain fatty acids, and H2 are found. The VFA concentration of rumen contents varies between 60 and 120 mEq/hiter. The relative proportion of the individual VFA varies somewhat with diet; e.g., an increase in the Journal
of Clinical
Nutrition
31: OCTOBER
through
degree
digestion of
mucosa functions
American
and
by the feeding of high- versus low-fiber of all three species. Results of comparative
affected
The
passage
length
the
of all forms
all three
large
of sacculation species.
Total
mammals
and Am.
VFA
Nutr.
secretory 31: 5161-5168,
dog,
Volatile were
concentration
transported production
to the normal J. C/in.
of the
colon.
of carbohydrate,
diets. VFA were rapidly studies indicate that
mammals.
intestine of the
the was
pig, fatty major little
across colonic and absorption and
absorptive 1978.
ratio of plant fiber versus starch results in an increase in the acetate/propionate ratio. The pH of rumen contents varies inversely with VFA production but is normally maintained within a range of 5.5 to 7.0 by means of buffers secreted by the salivary glands and rumen mucosa. Since the VFAs have a pKA of4.8, they are present predominantly in their dissociated form. Yet VFA is rapidly absorbed directly from the rumen, providing a major source of the animal’s basal energy requirement. Several years ago, we initiated comparative studies of large intestinal structure and function to obtain information on the composition of digesta at various times after feeding, the structural and functional mechanisms controlling the rate of digesta passage, and the mechanisms by which VFAs and inorganic electrolytes are absorbed. The gastrointestinal tracts from a large number of species were dissected free (with minimal stretching), laid out in a similar pattern, and drawn to scale-with special attention to gross structural characteristics of the stomach and large intestine. Whenever possible, animals were sacrificed 4 hr after meals. Figure 1 compares the gastrointestinal tract of four species,
1 From the Department of Physiology, Biochemistry, and Pharmacy, New York State College of Veterinary Medicine, Cornell University, Ithaca, New York 14850. 2Profer and Chairman.
1978,
pp.
S161-S168.
Printed
in
U.S.A.
5161
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
Charles
S162
STEVENS Dog (Canis Body
familiaris)
Pig (Sea scrofa) Body Length: 125 cm
Length:9Ocm
cm 20
#{243}cin
Rabbit (Oryctolagus cuniculus) Body Length:48cm
0
cm
10
Pony (Equus caballus) Body Length:164cm
0
cm
20
FIG . I . Gastrointestinal tracts of a carnivore, omnivore, and two simple-stomached herbivores. Note differences in the relative volume of the large intestine, cecal capacity, and the degree of colonic sacculation (haustration) associated with the presence of longitudinal bands of muscle. From Stevens (8), used with permission of Cornell University Press. which represent a range of variation in large intestinal structure from the simple, nonvoluminous large intestine of the dog to the extremely voluminous, sacculated, compartmentalized large intestine of the pony. Comparative studies included measurements of digesta passage as well as the vol-
ume, osmolality, pH, and the concentrations of VFA, lactic acid, and inorganic ions in various segments of the gastrointestinal tract from several species at specific times after feeding. Animals were placed on a defmed diet and fed at 12-hr intervals for a 4- to 6week period before they were killed to allow
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0
MICROBIAL
DIGESTION
LARGE
INTESTINE
S163
on a grain diet, and ponies on a low-fiber pelleted diet were equal to or greater than those found in the ruminant forestomach (Fig. 2). The feeding of diets higher in plant fiber resulted in no significant changes in VFA in any of these species. The pH of large intestinal digesta was similar in the three species, with VFA as the main anion and Na as the main cation (12). Measurements made 2, 4, 8, and 12 hr following feeding showed a cyclic, inverse relationship between VFA and pH, as in the rumen. The study conducted on pigs (10) also demonstrated that acetate/propionate/butyrate ratios of large intestinal contents were similar to those of rumen con-
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FIG. 2. Mean (± SE) of values for VFA and pH concentrations in the dog, pig, and pony gastrointestinal tracts. All animals were fed at 12-hr intervals over a 6-week period prior to the experiment. Dogs were fed a meat diet, pigs a high-concentrate diet, and ponies a oonventional, pelleted horse diet. Each value represents the average from 12 animals, killed in groups of three animals each at 2, 4, 8, and 12 hr after feeding. Sections of tract examined are: oral (5,) and aboral (52) halves of the stomach; two or three equal segments of the small intestine, SI,, 512, and 513; cecum, Ce; and two or three segments of colon, C,, C2, and C3. The pig colon was divided into proximal (C,), centripetal (C2), and centrifugal plus terminal (C3) segments. The pony colon was divided into ventral large colon (C,), dorsal large colon (C2) and small colon (C3). Modified from Argenzio et al. (10), Argenzio and Southworth (11), and Banta (personal communication).
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
necessary adjustment of large intestinal microflora (9). They were administered fluid (PEG or 51Cr EDTA) and particulate markers by stomach tube, before and at the time of the morning meal, and then killed in groups of three at 2, 4, 8, and 12 hr after the meal. The gastrointestinal tract was divided by sutures into segments, pH of the digesta was immediately measured anaerobically (by insertion of a unipolar electrode through a stab wound into the axial center of each segment), and segmental contents were then collected and analyzed. The mean concentrations of VFA in the large intestines of dogs on a meat diet, pigs
IN
STEVENS
S 164
TABLE I Transport of short-circuited
VFA
across epithelium”
isolated,
Vola tile fatty
acid
(j.tmoles/cm’
x hr)
Tissue Ox
Rumen Cecum Proximal
lon Distal
2.4 co-
colon
±
Pony
Pig
Dog
1.6 ± 0.2 1.8 ± 0.2
4.3 ± 0.6 3.7 ± 0.6
4.2
±
1.4 ± 0.1
3.1 ± 0.2
4.8
± 0.4
0.4 0.2
; Values are means ± SE for the rate of VFA transport (lumen to blood) obtained during 2.5-hr experimental periods. Ringer solution, containing a 90-mM equimolar mixture of acetate, propionate, and butyrate, was used to bathe lumen surface of tissue, and normal Ringer was used to bathe blood surface; both solutions were buffered at pH 7.4 with bicarbonate. Transmucosal electrical PD was clamped at zero. Modified from Stevens and Stettler (13), Argenzio et al. (10), Argenzio and Southworth (11), with permission of the Am. J. Physiol., and Herschel, (personal communication).
the rabbit and many rodents, a voluminous cecum is the major site for prolonged retention of digesta. However, in many other species digesta appear to be retained within the sacculated. or haustrated segments of the colon. This retention is essential for the relatively slow microbial digestive process. Elliott and Barclay-Smith (16) conducted an early comparative study of digesta passage through the large intestine of mammals. They concluded that the extensive saccuhation of the human colon is more like that of an herbivore than an omnivore. Studies of the pony large intestine (10, 17, 18) provide a picture of the cyclic events associated with the microbial digestive process (Fig. 5). The volume of large intestinal contents underwent marked periodic changes during the 12-hr period between meals. Measurements of the rate of digesta passage between segments (Fig. 4) allowed calculation of net secretion and absorption of water as well as net production and disappearance of VFA and protein within each segment. The colonic contents underwent marked cycles of net VFA production and absorption, which tended to correlate with the cycles of net water secretion and absorption as well as net protein synthesis and digestion. Minimal estimates showed that 12 liters of water were secreted into and 1.5 M of VFA (25% of the animal’s basal energy requirement) were absorbed from the large intestine per day. Net disappearance of protein was associated with a net disappearance of N indicating that much of this was absorbed. The N could be largely absorbed as NH3, but the studies of Slade et al. (19, 20) provide good evidence that both essential and nonessential amino acids are absorbed from the large intestine of the horse. In vivo perfusion of the goat (21), pig (Crump, personal communication) and pony
(22)
colon
also
demonstrated
rapid
absorp-
tion of VFA. The absorption of YFA and Na could account for isosmotic absorption of the water lost from the perfusate. Furthermore, replacement of VFA in these solutions with Cl markedly inhibited the absorption of both Na and H20, indicating an interdependence between VFA and Na absorption. The in vivo perfusion plus in vitro studies of pony large intestine, by Argenzio and colleagues,
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
tents, and an increase in the dietary ratio of fiber to starch resulted in a similar increase in the acetate/propionate ratio. Furthermore, in vitro studies of VFA transport across the isolated large intestinal mucosa of these three species (Table 1) showed that VFAs were transported at rates approximately equal to or greater than that previously determined for isolated rumen epithelium. A comparison of fluid and particulate marker passage through the gastrointestinal tracts of the dog, pig, and rabbit (Fig. 3) demonstrated marked species differences in: 1) the general rate of marker passage; 2) the relative rates of fluid versus particulate marker passage; and 3) the major sites of marker retention in the large gut. A more extensive and direct study of digesta marker passage through the various segments of the pony large intestine (Fig. 4) showed prolonged retention of fluid and particles in the proximal (ventral and dorsal) colon. Although the results from the rabbit support the conventional wisdom that the cecum is the primary site of fluid (and undoubtedly small particle) retention, this does not appear to hold for the other species. Colonic retention time was prolonged in the pig, which has a colon haustrated throughout its length in a manner similar to that of man. Orad propulsive motor activity of the proximal colon contributes to the retention of microbes and fiber. In some species, such as
MICROBIAL
DIGESTION
IN LARGE
INTESTINE
S165
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
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I
0 0 -U
-
E 3 V
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I
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0 U uJ
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2
3
4
5
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7
8
TIME(days)
9
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1
2
3
4
5
6
7
8
9
10
TIME(days)
FIG. 4. Retention of liquid and particulate markers by the pony large intestine and their excretion in feces. Curve for dorsal colon represents dorsal plus small colon, since it was derived as difference between dose and sum of the other curves. Fecal excretion curves represent experimental data from series of experiments in which feces were collected for a 10-day period following a cecal dose of markers. For these experiments average variation was ± 18% (SE). From Argenzio et al. (10) with permission of Am. J. Physiol.
also indicated that VFA absorption can stimulate secretion of Na and HCO3. The low levels of HCO3 normally present in the digesta are attributable to rapid titration to CO2 in the presence of these organic acids. They proposed a model for VFA transport similar to that proposed earlier for VFA transport across rumen epithelium (23). Comparative studies of a carnivore, omniyore, and herbivore indicated that microbial digestion of carbohydrate within the large intestine resulted in high concentrations of VFA throughout the 12-hr period between meals. Studies of dogs on a meat diet and of pigs on a high-concentrate diet showed that high levels of VFA were present, even on diets in which plant fiber was low or absent. Under these conditions the VFA could origmate from the relatively small amounts of ct-amylase-indigestible polysaccharide in the meat (protein-linked carbohydrates) or grain of the diet and that present in the wall of desquamated gastrointestinal cells. The polysaccharide of mucous (approximately 80%) could provide a substantial source. The decrease in acetate-propionate ratio in pigs on a low-fiber diet suggested the escape of greater amounts of starch and sugar into the large intestine than that previously assumed. Regardless of the origin of VFAs, their presence as the major anion in the large intestine of three mammalian species, which
represent a wide range of variation in both normal diet and large intestinal structure, provides strong circumstantial evidence that the same situation applies to man and other mammals. The demonstration that VFAs are rapidly absorbed from the large intestine of the dog, pig, and pony indicates that they may provide a significant source of energy in some species. Regardless ofthe nutrient value of VFAs or microbial protein, however, the VFAs appear to play a major role in the normal secretory and absorptive functions of the large intestine. A diet low in fiber and high in starch or sugar could produce diseases of the large intestine if substantial amounts of the latter were to escape into the large intestine. When such diets are fed to ruminants there is a rapid production of VFA and lactic acid in the rumen. The osmotic effect results in loss of extracellular fluid into the rumen. In addition, the pH ofrumen contents drops markedly, and rapid absorption of these organic acids is accompanied by atony of rumen muscle, massive ulceration of the rumen epithelium, and systemic acidosis. Allison et al. (24) have recently found that the cecal contents of cattle and sheep fed these high concentrate diets showed a similar increase in lactobacilli and lactic acid concentration and an associated decrease in pH to 4.7. These effects could account for a number of large intestinal diseases in addition to the
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
0
0
MICROBIAL
DIGESTION
IN LARGE CON
INTESTINE
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FIG. 5. Volume, net transmucosal exchange of water, net production, and absorption of VFA and net synthesis and digestion of protein as a function of time. Positive values in the bar graphs represent net secretion on production during the time interval measured and negative values represent absorption or digestion. Standard error of the mean is indicated in brackets. Those for water exchange, VFA production, and protein nitrogen are cumulative estimates which include correction for digesta passage between segments. From Argenzio et al. ( 10), Argenzio and Southworth (1 1), Wootton and Argen.zio (18), with permission of Am. J. Physiol. diarrhea associated with carbohydrate malabsorption syndromes; e.g., lactase deficiency. Therefore the need for determining the optimal dietary combination of starches and sugars versus digestible fiber for normal large intestinal function in man deserves considerable investigation. El
SHALL
3.
A. T.
AND
digestion Biol. 22:
tract
H.
BANCROFT,
LIPSON.
tary Biol. 2.
tract 20:
ELSDEN,
T., R. A. Absorption
MCANALLY
of volatile
of the sheep 120, 1944. S. R., M. W.
and
AND acid
other
from
animals.
A. T. the
6. S. HITCHCOCK,
R. A.
MAR-
J.
I. A
and
comparison
intravenous
of therapy
the
R. E. Ruminal
HUNGATE,
book
ORTIZ.
of
Physiology,
edited
stool
of fatty Exptl.
acids Biol.
0. M. of
Clin.
Studies
the
effects upon the and urine.
in 23:
WRONG.
stool
anal-
Sci. 37:
C. L.
LAVASTIDA,
AND
tion and volume of Invest. 45: 469, 1966.
J. Exptl.
production the dog.
R., M. A.
RODRIGUEZ
feeding
PHIL-
alimen-
Volatile acid in the animals. J. Exptl.
R., A. V. HOWARD AND In vivo dialysis of faeces as a method ysis. IV. The organic anion component. 549, 1969.
diarrhea. 1.
of
other
RUBINSTEIN,
5. TORRES-PINEDO,
References
and
A. T. The
PHILLIPSON,
the alimentary 346, 1947. 4.
PHILLIPSON.
of ruminants 191, 1946.
RIVERA,
on
infant
of milk composiJ. CIin.
fermentation. In: Handby C. F. Code and W.
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
I
S 168
STEVENS
Heidel. Society,
D.C.: American Physiological V, p. 2527. 7. PHILLIPSON, A. T. Ruminant digestion. In: Dukes’ Physiology of Domestic Animals, (9th ed), edited by M. J. Swenson. Ithaca: Cornell University Press, 1977, p. 250. 8. STEVENS, C. E. Comparative physiology ofthe digestive
imals,
system.
(9th
University
9. CRANWELL, alimentary 721, 1968. 10. ARGENZIO, VENS.
In:
ed),
Sites
P. tract
Dukes’
of
Domestic
by M. J. Swenson. Press, 1977, p. 216. D. Microbial fermentation of the pig. Nutr. Abstr.
R. A., M. of
Physiology
edited
organic
SOUTHWORTH acid
production
AND
An-
Ithaca: in Rev.
the 38:
C. E. Si-aand
absorp-
tion in the equine gastrointestinal tract. Am. J. Physiol. 226: 1043, 1974. 1 1. ARGENZIO, R. A., AND M. SOUTHWORTH. Sites of organic acid production and absorption in gastrointestinal tract of the pig. Am. J. Physiol. 228: 454, 1975. 12. ARGENZIO, R. A., AND C. E. STEVENS. Cyclic changes in ionic composition of digesta in the equine intestinal tract. Am. J. Physiol. 228: 1224, 1975. 13. STEVENS, C. E., AND B. K. STETTLER. Transport of fatty acid mixtures across rumen epithelium. Am. J. Physiol. 211: R264, 1966. 14. CLEMENS, E. T., C. E. STEVENS AND M. SouTHWORTH. Sites of organic acid production and pattern of digesta movement in the gastrointestinal tract of swine. J. Nutr. 105: 759, 1975. 15. PICKARD, D. W., AND C. E. STEVENS. Digesta flow through the rabbit large intestine. Am. J. Physiol. 222: 1161, 1972. 16. ELLIOTT, T. R., AND E. BARCLAY-SMITH. Antiperis-
17.
R. A., J. E.
ARGENZIO,
C. E.
muscular 1904.
STEVENS.
Digesta
activities
of the
D. W.
LOWE, passage
and
PICKARD
water
J.
colon. AND
exchange
in the equine large intestine. Am. J. Physiol. 226: 1035, 1974. 18. WooTTort, J. F., AND R. A. ARGENZIO. Nitrogen utilization within equine large intestine. Am. J. Physiol. 229: 1062, 1975. 19. SLADE, L. M., R. BISHOP, J. G. MORRIS AND D. W. ROBINSON. Digestion and absorption of ‘5N-labelled microbial protein in the large intestine of the horse. Brit. Vet. J. 127: xi, 1971. 20. SLADE, L. M., D. W. RoBINSoN AND K. E. CASEY. Nitrogen metabolism in nonruminant herbivores. I. The influence of nonprotein nitrogen and protein quality on the nitrogen retention of adult mares. J. Animal Sci. 30: 753, 1970. 21. ARGENZIO, R. A., N. MILLER AND W. v. ENGELHARDT.
Effect
of
ion absorption 229: 997, 1975. 22.
23.
24.
volatile
from
the
R. A., J. E.
ARGENZIO,
fatty
goat LOWE
acids
colon. AND
on
water
Am.
and
J. Physiol.
C. E.
STEVENS.
Interrelationship of Na, HCO3 and volatile fatty acid transport across equine large intestinal mucosa. Gastroenterology 70: A-2/860, 1976. STEVENS, C. E. Transport across rumen epithelium. In: Transport Mechanisms in Epithelia, Proceedings of Alfred Benzon Symposium V, edited by H. H. Ussing and N. A. Thorn. Copenhagen: Munksgaard, 1973, p. 404.
M. J., I. M.
ALLISON, AND sheep:
and
J. A. changes
rumen.
BUCKLIN.
RoBINSoN, Grain
R. W. overload
in microbial populations Am. J. Vet. Res. 36: 181,
DOUGHERTY in
cattle
in the
1975.
and cecum
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
Cornell
talsis and other Physiol. 31: 272,
Washington, 1968, vol.
D. V.M.,
Ph.D.
ABSTRACT and
The
pony
acids
correlated
(VFA),
anions
the
present
rate with
end
in
large
of digesta the
products
marker
relative
of microbial
intestinal
contents
of VFA
are
important
of the
large
to the nutrition intestine
of some
of most
Early studies by Bancroft et al. (1), Elsden et al. (2), and Phillipson (3) demonstrated concentrations of volatile fatty acids (VFAs) in the large intestinal contents of the ox, sheep, horse, pig, rabbit, rat, and dog equivalent to those found in the ruminant forestomach. Fresh human feces have subsequently been reported to contain VFAs at similar concentrations and ratios (4, 5). Since these are the principal end products from microbial digestion of carbohydrate in the rumen, the pertinent features of this extensively studied process are an appropriate introduction to the following discussion. Studies of digestion within the rumen are the subject of a number of reviews (6, 7). Rumen microbes are capable of synthesizing all the B vitamins required by the animal and converting nonprotein nitrogenous compounds into high-quality microbial protein. They also digest starches and sugars as well as a wide range of other polysaccharides (ccllulose, hemicelluloses, pectins) associated with the fibrous portion of plants. Under normal conditions the principal end products are three volatile fatty acids (acetate, propionate, and butyrate) plus CO2 and CH4. Only small amounts of lactic acid, other shortchain fatty acids, and H2 are found. The VFA concentration of rumen contents varies between 60 and 120 mEq/hiter. The relative proportion of the individual VFA varies somewhat with diet; e.g., an increase in the Journal
of Clinical
Nutrition
31: OCTOBER
through
degree
digestion of
mucosa functions
American
and
by the feeding of high- versus low-fiber of all three species. Results of comparative
affected
The
passage
length
the
of all forms
all three
large
of sacculation species.
Total
mammals
and Am.
VFA
Nutr.
secretory 31: 5161-5168,
dog,
Volatile were
concentration
transported production
to the normal J. C/in.
of the
colon.
of carbohydrate,
diets. VFA were rapidly studies indicate that
mammals.
intestine of the
the was
pig, fatty major little
across colonic and absorption and
absorptive 1978.
ratio of plant fiber versus starch results in an increase in the acetate/propionate ratio. The pH of rumen contents varies inversely with VFA production but is normally maintained within a range of 5.5 to 7.0 by means of buffers secreted by the salivary glands and rumen mucosa. Since the VFAs have a pKA of4.8, they are present predominantly in their dissociated form. Yet VFA is rapidly absorbed directly from the rumen, providing a major source of the animal’s basal energy requirement. Several years ago, we initiated comparative studies of large intestinal structure and function to obtain information on the composition of digesta at various times after feeding, the structural and functional mechanisms controlling the rate of digesta passage, and the mechanisms by which VFAs and inorganic electrolytes are absorbed. The gastrointestinal tracts from a large number of species were dissected free (with minimal stretching), laid out in a similar pattern, and drawn to scale-with special attention to gross structural characteristics of the stomach and large intestine. Whenever possible, animals were sacrificed 4 hr after meals. Figure 1 compares the gastrointestinal tract of four species,
1 From the Department of Physiology, Biochemistry, and Pharmacy, New York State College of Veterinary Medicine, Cornell University, Ithaca, New York 14850. 2Profer and Chairman.
1978,
pp.
S161-S168.
Printed
in
U.S.A.
5161
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
Charles
S162
STEVENS Dog (Canis Body
familiaris)
Pig (Sea scrofa) Body Length: 125 cm
Length:9Ocm
cm 20
#{243}cin
Rabbit (Oryctolagus cuniculus) Body Length:48cm
0
cm
10
Pony (Equus caballus) Body Length:164cm
0
cm
20
FIG . I . Gastrointestinal tracts of a carnivore, omnivore, and two simple-stomached herbivores. Note differences in the relative volume of the large intestine, cecal capacity, and the degree of colonic sacculation (haustration) associated with the presence of longitudinal bands of muscle. From Stevens (8), used with permission of Cornell University Press. which represent a range of variation in large intestinal structure from the simple, nonvoluminous large intestine of the dog to the extremely voluminous, sacculated, compartmentalized large intestine of the pony. Comparative studies included measurements of digesta passage as well as the vol-
ume, osmolality, pH, and the concentrations of VFA, lactic acid, and inorganic ions in various segments of the gastrointestinal tract from several species at specific times after feeding. Animals were placed on a defmed diet and fed at 12-hr intervals for a 4- to 6week period before they were killed to allow
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
0
MICROBIAL
DIGESTION
LARGE
INTESTINE
S163
on a grain diet, and ponies on a low-fiber pelleted diet were equal to or greater than those found in the ruminant forestomach (Fig. 2). The feeding of diets higher in plant fiber resulted in no significant changes in VFA in any of these species. The pH of large intestinal digesta was similar in the three species, with VFA as the main anion and Na as the main cation (12). Measurements made 2, 4, 8, and 12 hr following feeding showed a cyclic, inverse relationship between VFA and pH, as in the rumen. The study conducted on pigs (10) also demonstrated that acetate/propionate/butyrate ratios of large intestinal contents were similar to those of rumen con-
240 2 #{149} dog A pig #{149} pony
-J ...
in 0)
0
E E U-
>
I
#{149}clog
0.
A pig #{149} pony
i
SECTION
OF
..e
TRACT
FIG. 2. Mean (± SE) of values for VFA and pH concentrations in the dog, pig, and pony gastrointestinal tracts. All animals were fed at 12-hr intervals over a 6-week period prior to the experiment. Dogs were fed a meat diet, pigs a high-concentrate diet, and ponies a oonventional, pelleted horse diet. Each value represents the average from 12 animals, killed in groups of three animals each at 2, 4, 8, and 12 hr after feeding. Sections of tract examined are: oral (5,) and aboral (52) halves of the stomach; two or three equal segments of the small intestine, SI,, 512, and 513; cecum, Ce; and two or three segments of colon, C,, C2, and C3. The pig colon was divided into proximal (C,), centripetal (C2), and centrifugal plus terminal (C3) segments. The pony colon was divided into ventral large colon (C,), dorsal large colon (C2) and small colon (C3). Modified from Argenzio et al. (10), Argenzio and Southworth (11), and Banta (personal communication).
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necessary adjustment of large intestinal microflora (9). They were administered fluid (PEG or 51Cr EDTA) and particulate markers by stomach tube, before and at the time of the morning meal, and then killed in groups of three at 2, 4, 8, and 12 hr after the meal. The gastrointestinal tract was divided by sutures into segments, pH of the digesta was immediately measured anaerobically (by insertion of a unipolar electrode through a stab wound into the axial center of each segment), and segmental contents were then collected and analyzed. The mean concentrations of VFA in the large intestines of dogs on a meat diet, pigs
IN
STEVENS
S 164
TABLE I Transport of short-circuited
VFA
across epithelium”
isolated,
Vola tile fatty
acid
(j.tmoles/cm’
x hr)
Tissue Ox
Rumen Cecum Proximal
lon Distal
2.4 co-
colon
±
Pony
Pig
Dog
1.6 ± 0.2 1.8 ± 0.2
4.3 ± 0.6 3.7 ± 0.6
4.2
±
1.4 ± 0.1
3.1 ± 0.2
4.8
± 0.4
0.4 0.2
; Values are means ± SE for the rate of VFA transport (lumen to blood) obtained during 2.5-hr experimental periods. Ringer solution, containing a 90-mM equimolar mixture of acetate, propionate, and butyrate, was used to bathe lumen surface of tissue, and normal Ringer was used to bathe blood surface; both solutions were buffered at pH 7.4 with bicarbonate. Transmucosal electrical PD was clamped at zero. Modified from Stevens and Stettler (13), Argenzio et al. (10), Argenzio and Southworth (11), with permission of the Am. J. Physiol., and Herschel, (personal communication).
the rabbit and many rodents, a voluminous cecum is the major site for prolonged retention of digesta. However, in many other species digesta appear to be retained within the sacculated. or haustrated segments of the colon. This retention is essential for the relatively slow microbial digestive process. Elliott and Barclay-Smith (16) conducted an early comparative study of digesta passage through the large intestine of mammals. They concluded that the extensive saccuhation of the human colon is more like that of an herbivore than an omnivore. Studies of the pony large intestine (10, 17, 18) provide a picture of the cyclic events associated with the microbial digestive process (Fig. 5). The volume of large intestinal contents underwent marked periodic changes during the 12-hr period between meals. Measurements of the rate of digesta passage between segments (Fig. 4) allowed calculation of net secretion and absorption of water as well as net production and disappearance of VFA and protein within each segment. The colonic contents underwent marked cycles of net VFA production and absorption, which tended to correlate with the cycles of net water secretion and absorption as well as net protein synthesis and digestion. Minimal estimates showed that 12 liters of water were secreted into and 1.5 M of VFA (25% of the animal’s basal energy requirement) were absorbed from the large intestine per day. Net disappearance of protein was associated with a net disappearance of N indicating that much of this was absorbed. The N could be largely absorbed as NH3, but the studies of Slade et al. (19, 20) provide good evidence that both essential and nonessential amino acids are absorbed from the large intestine of the horse. In vivo perfusion of the goat (21), pig (Crump, personal communication) and pony
(22)
colon
also
demonstrated
rapid
absorp-
tion of VFA. The absorption of YFA and Na could account for isosmotic absorption of the water lost from the perfusate. Furthermore, replacement of VFA in these solutions with Cl markedly inhibited the absorption of both Na and H20, indicating an interdependence between VFA and Na absorption. The in vivo perfusion plus in vitro studies of pony large intestine, by Argenzio and colleagues,
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tents, and an increase in the dietary ratio of fiber to starch resulted in a similar increase in the acetate/propionate ratio. Furthermore, in vitro studies of VFA transport across the isolated large intestinal mucosa of these three species (Table 1) showed that VFAs were transported at rates approximately equal to or greater than that previously determined for isolated rumen epithelium. A comparison of fluid and particulate marker passage through the gastrointestinal tracts of the dog, pig, and rabbit (Fig. 3) demonstrated marked species differences in: 1) the general rate of marker passage; 2) the relative rates of fluid versus particulate marker passage; and 3) the major sites of marker retention in the large gut. A more extensive and direct study of digesta marker passage through the various segments of the pony large intestine (Fig. 4) showed prolonged retention of fluid and particles in the proximal (ventral and dorsal) colon. Although the results from the rabbit support the conventional wisdom that the cecum is the primary site of fluid (and undoubtedly small particle) retention, this does not appear to hold for the other species. Colonic retention time was prolonged in the pig, which has a colon haustrated throughout its length in a manner similar to that of man. Orad propulsive motor activity of the proximal colon contributes to the retention of microbes and fiber. In some species, such as
MICROBIAL
DIGESTION
IN LARGE
INTESTINE
S165
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__________________
V
I
0 0 -U
-
E 3 V
C,
U
-ci
-C C-,
I
I
I
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-
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1VIOI
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S166
uJ
4 0 w
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0 U uJ
z
uJ
U uJ
0. 1
2
3
4
5
6
7
8
TIME(days)
9
10
1
2
3
4
5
6
7
8
9
10
TIME(days)
FIG. 4. Retention of liquid and particulate markers by the pony large intestine and their excretion in feces. Curve for dorsal colon represents dorsal plus small colon, since it was derived as difference between dose and sum of the other curves. Fecal excretion curves represent experimental data from series of experiments in which feces were collected for a 10-day period following a cecal dose of markers. For these experiments average variation was ± 18% (SE). From Argenzio et al. (10) with permission of Am. J. Physiol.
also indicated that VFA absorption can stimulate secretion of Na and HCO3. The low levels of HCO3 normally present in the digesta are attributable to rapid titration to CO2 in the presence of these organic acids. They proposed a model for VFA transport similar to that proposed earlier for VFA transport across rumen epithelium (23). Comparative studies of a carnivore, omniyore, and herbivore indicated that microbial digestion of carbohydrate within the large intestine resulted in high concentrations of VFA throughout the 12-hr period between meals. Studies of dogs on a meat diet and of pigs on a high-concentrate diet showed that high levels of VFA were present, even on diets in which plant fiber was low or absent. Under these conditions the VFA could origmate from the relatively small amounts of ct-amylase-indigestible polysaccharide in the meat (protein-linked carbohydrates) or grain of the diet and that present in the wall of desquamated gastrointestinal cells. The polysaccharide of mucous (approximately 80%) could provide a substantial source. The decrease in acetate-propionate ratio in pigs on a low-fiber diet suggested the escape of greater amounts of starch and sugar into the large intestine than that previously assumed. Regardless of the origin of VFAs, their presence as the major anion in the large intestine of three mammalian species, which
represent a wide range of variation in both normal diet and large intestinal structure, provides strong circumstantial evidence that the same situation applies to man and other mammals. The demonstration that VFAs are rapidly absorbed from the large intestine of the dog, pig, and pony indicates that they may provide a significant source of energy in some species. Regardless ofthe nutrient value of VFAs or microbial protein, however, the VFAs appear to play a major role in the normal secretory and absorptive functions of the large intestine. A diet low in fiber and high in starch or sugar could produce diseases of the large intestine if substantial amounts of the latter were to escape into the large intestine. When such diets are fed to ruminants there is a rapid production of VFA and lactic acid in the rumen. The osmotic effect results in loss of extracellular fluid into the rumen. In addition, the pH ofrumen contents drops markedly, and rapid absorption of these organic acids is accompanied by atony of rumen muscle, massive ulceration of the rumen epithelium, and systemic acidosis. Allison et al. (24) have recently found that the cecal contents of cattle and sheep fed these high concentrate diets showed a similar increase in lactobacilli and lactic acid concentration and an associated decrease in pH to 4.7. These effects could account for a number of large intestinal diseases in addition to the
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0
0
MICROBIAL
DIGESTION
IN LARGE CON
INTESTINE
Sl67
TROL
z lot
2
i
at
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2t
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FIG. 5. Volume, net transmucosal exchange of water, net production, and absorption of VFA and net synthesis and digestion of protein as a function of time. Positive values in the bar graphs represent net secretion on production during the time interval measured and negative values represent absorption or digestion. Standard error of the mean is indicated in brackets. Those for water exchange, VFA production, and protein nitrogen are cumulative estimates which include correction for digesta passage between segments. From Argenzio et al. ( 10), Argenzio and Southworth (1 1), Wootton and Argen.zio (18), with permission of Am. J. Physiol. diarrhea associated with carbohydrate malabsorption syndromes; e.g., lactase deficiency. Therefore the need for determining the optimal dietary combination of starches and sugars versus digestible fiber for normal large intestinal function in man deserves considerable investigation. El
SHALL
3.
A. T.
AND
digestion Biol. 22:
tract
H.
BANCROFT,
LIPSON.
tary Biol. 2.
tract 20:
ELSDEN,
T., R. A. Absorption
MCANALLY
of volatile
of the sheep 120, 1944. S. R., M. W.
and
AND acid
other
from
animals.
A. T. the
6. S. HITCHCOCK,
R. A.
MAR-
J.
I. A
and
comparison
intravenous
of therapy
the
R. E. Ruminal
HUNGATE,
book
ORTIZ.
of
Physiology,
edited
stool
of fatty Exptl.
acids Biol.
0. M. of
Clin.
Studies
the
effects upon the and urine.
in 23:
WRONG.
stool
anal-
Sci. 37:
C. L.
LAVASTIDA,
AND
tion and volume of Invest. 45: 469, 1966.
J. Exptl.
production the dog.
R., M. A.
RODRIGUEZ
feeding
PHIL-
alimen-
Volatile acid in the animals. J. Exptl.
R., A. V. HOWARD AND In vivo dialysis of faeces as a method ysis. IV. The organic anion component. 549, 1969.
diarrhea. 1.
of
other
RUBINSTEIN,
5. TORRES-PINEDO,
References
and
A. T. The
PHILLIPSON,
the alimentary 346, 1947. 4.
PHILLIPSON.
of ruminants 191, 1946.
RIVERA,
on
infant
of milk composiJ. CIin.
fermentation. In: Handby C. F. Code and W.
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
I
S 168
STEVENS
Heidel. Society,
D.C.: American Physiological V, p. 2527. 7. PHILLIPSON, A. T. Ruminant digestion. In: Dukes’ Physiology of Domestic Animals, (9th ed), edited by M. J. Swenson. Ithaca: Cornell University Press, 1977, p. 250. 8. STEVENS, C. E. Comparative physiology ofthe digestive
imals,
system.
(9th
University
9. CRANWELL, alimentary 721, 1968. 10. ARGENZIO, VENS.
In:
ed),
Sites
P. tract
Dukes’
of
Domestic
by M. J. Swenson. Press, 1977, p. 216. D. Microbial fermentation of the pig. Nutr. Abstr.
R. A., M. of
Physiology
edited
organic
SOUTHWORTH acid
production
AND
An-
Ithaca: in Rev.
the 38:
C. E. Si-aand
absorp-
tion in the equine gastrointestinal tract. Am. J. Physiol. 226: 1043, 1974. 1 1. ARGENZIO, R. A., AND M. SOUTHWORTH. Sites of organic acid production and absorption in gastrointestinal tract of the pig. Am. J. Physiol. 228: 454, 1975. 12. ARGENZIO, R. A., AND C. E. STEVENS. Cyclic changes in ionic composition of digesta in the equine intestinal tract. Am. J. Physiol. 228: 1224, 1975. 13. STEVENS, C. E., AND B. K. STETTLER. Transport of fatty acid mixtures across rumen epithelium. Am. J. Physiol. 211: R264, 1966. 14. CLEMENS, E. T., C. E. STEVENS AND M. SouTHWORTH. Sites of organic acid production and pattern of digesta movement in the gastrointestinal tract of swine. J. Nutr. 105: 759, 1975. 15. PICKARD, D. W., AND C. E. STEVENS. Digesta flow through the rabbit large intestine. Am. J. Physiol. 222: 1161, 1972. 16. ELLIOTT, T. R., AND E. BARCLAY-SMITH. Antiperis-
17.
R. A., J. E.
ARGENZIO,
C. E.
muscular 1904.
STEVENS.
Digesta
activities
of the
D. W.
LOWE, passage
and
PICKARD
water
J.
colon. AND
exchange
in the equine large intestine. Am. J. Physiol. 226: 1035, 1974. 18. WooTTort, J. F., AND R. A. ARGENZIO. Nitrogen utilization within equine large intestine. Am. J. Physiol. 229: 1062, 1975. 19. SLADE, L. M., R. BISHOP, J. G. MORRIS AND D. W. ROBINSON. Digestion and absorption of ‘5N-labelled microbial protein in the large intestine of the horse. Brit. Vet. J. 127: xi, 1971. 20. SLADE, L. M., D. W. RoBINSoN AND K. E. CASEY. Nitrogen metabolism in nonruminant herbivores. I. The influence of nonprotein nitrogen and protein quality on the nitrogen retention of adult mares. J. Animal Sci. 30: 753, 1970. 21. ARGENZIO, R. A., N. MILLER AND W. v. ENGELHARDT.
Effect
of
ion absorption 229: 997, 1975. 22.
23.
24.
volatile
from
the
R. A., J. E.
ARGENZIO,
fatty
goat LOWE
acids
colon. AND
on
water
Am.
and
J. Physiol.
C. E.
STEVENS.
Interrelationship of Na, HCO3 and volatile fatty acid transport across equine large intestinal mucosa. Gastroenterology 70: A-2/860, 1976. STEVENS, C. E. Transport across rumen epithelium. In: Transport Mechanisms in Epithelia, Proceedings of Alfred Benzon Symposium V, edited by H. H. Ussing and N. A. Thorn. Copenhagen: Munksgaard, 1973, p. 404.
M. J., I. M.
ALLISON, AND sheep:
and
J. A. changes
rumen.
BUCKLIN.
RoBINSoN, Grain
R. W. overload
in microbial populations Am. J. Vet. Res. 36: 181,
DOUGHERTY in
cattle
in the
1975.
and cecum
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S161/4656087 by guest on 14 February 2019
Cornell
talsis and other Physiol. 31: 272,
Washington, 1968, vol.
