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OCR for page 46
Digestion and Absorption
in the Small Intestine
NITRO GEN SUPPLY
Because of N transactions in the rumen, intake of pro-
tein (IP) is not an accurate indicator of N flow to the
small intestine (Whitlow and Satter, 1979J. Examina-
tion of data obtained with cows, steers, calves, sheep,
and lambs showed that for both concentrate and forage
diets, duodenal flow (SCP) ranged between 10.5 and
12.5 g NAN (nonammonia nitrogen) per Meal ME con-
sumed (Oldham and Tamminga, 1980~. This illustrates
that energy consumption is the major determinant of the
amount of N entering the small intestine. As discusser] in
other sections, factors that affect microbial protein pro-
duction and ruminal degradation of dietary protein can
be expected to modify N supply tSCP) to the small intes-
tine.
The N entering the duodenum is a combination of mi-
crobial (BCP), undegraded intake (UIP) and endoge-
nous protein. Differences in amino acid composition be-
tween bacteria and protozoa (Chalupa, 1972), IP and
BCP (Smith, 1975; Laughren and Young, 1979), and IP
and UIP (Smith and Mohamed, 1977) imply that the
quantities of BCP and UIP entering the intestine can in-
fluence the supply of absorbable amino acids. Unfortu-
nately, technical problems in partitioning BCP and UIP
and in estimating endogenous secretions have made ac-
curate quantitation difficult. The N entering the cluode-
num from the stomach can range between 0.3 to 1.0
BCP and 0 to 0.70 UIP (Smith, 1975~. Endogenous in-
fluxes can equal the supply from the stomach (Nolan,
1975)
From a nutritional point of view, it is important to
know the chemical composition of intestinal N. The pro-
portionate distribution of N in duodenal contents of
cows (SCP) was described by Oldham and Tamminga
(198(~) as essential amino acids, 0.35; nonessential
46
amino acids, 0.30; amides, 0.04; nucleic acids, 0.11;
ammonia, 0.06; and unknown, 0.14. Increases in the
ratio of essential amino acids to nonessential amino ac-
ids in duodenal digesta as a function of the concentra-
tion of intake protein (IPDM) indicate that the amino
acid composition of IP may influence the balance of
amino acids available for absorption (Laughren and
Young, 1979~. Because it is measured easily, NAN is a
frequently used measure of N entering the small intes-
tine. In data summarized by Stern and Satter (1982),
amino acid N in the duodenum of lactating cows was
0.79 of NAN. In sheep, amino acid N was 0.80 of NAN
(Hogan and Weston, 1970~. Considerable work is
needed before it will be possible to predict the amounts
of specific amino acids presented to the small intestine.
DIGESTION SYSTEM
Digestion of protein in the abomasum and small intes-
tine appears to be the same for ruminants as in nonrumi-
nants except for the slow neutralization of digesta in the
small intestine and the abundance of pancreatic ribQnu-
clease (Armstrong and Hutton, 1975; Bergen, 1978;
Chalupa, 1978~.
Slow neutralization of digesta in the upper small in-
testine of ruminants appears to be related to the low bi-
carbonate content of pancreatic juice (Taylor, 1962~.
This extends the activity of abomasal pepsin but delays
the onset of activities of intestinal enzymes. Thus, con-
siderable proteolysis in the duodenum is due to the gas-
tric protease, pepsin. Optimal activity for trypsin,
chymotrypsin, and carboxypeptidase does not occur
until the middle jejunum, and peak activity of exopep-
tidases and dipeptidases is found in the mid ileum (Ben-
Ghedalia et al., 1974~.
Breakdown of nucleic acids is achieved by DNases,
OCR for page 47
Digestion and Absorption in the Small Intestine 47
RNases, phosphodiesterases, and phosphomonestera~ses
(Bergen, 1978; Roth and Kirchgessner, 1980~. An im-
portant role for abundant pancreatic RNase in the rumi-
nant is release of nucleic acid phosphorus for recycling
to the rumen via saliva (Barnard, 1969~. It appears that
the products of nucleic acid digestion that are absorbed
are nucleotides, nucleosides, and bases (Bergen, 1978;
Smith, 1979).
ABSORPTION MECHANISMS
The mucosa of the small intestine contains uptake sys-
tems for free amino acids, peptides, nucleotides, and
nucleosides (Armstrong and Hutton, 1975; Bergen,
1978; Scharrer and Amann, 1980~.
The most active site for amino acid absorption in
sheep is the mid to lower ileum (Johns and Bergen, 1973;
Phillips et al., 1976), but the highest rate of amino acid
disappearance in situ from the digesta in the small intes-
tine occurs in the mid jejunum (Ben-Ghedalia et al.,
1974~. Johns and Bergen (1973), using jejunal strips,
demonstrated that amino acid uptake in sheep occurs
against a concentration gradient, exhibits saturation ki-
netics, and depends upon metabolic energy. Km and
Vm ~` values for glycine, methionine, and lysine trans-
port in sheep jejunum were similar to values obtained
with rat jejunum. The preferential disappearance of es-
sential amino acids over nonessential amino acids from
digesta flowing through the small intestine has been
demonstrated in sheep (Johns and Bergen, 1973; Phillips
et al., 1976) and in cattle (Van's Klooster and Boekholt,
1972~. Using exteriorized intestinal loops, Williams
(1969) ranked amino acid absorption as follows:
Ile>Arg-Val>Leu>Met>Phe>Lys>Try>
Asp ~ Ser > Ala > Pro > His 2 Thr 2 Glu > Gly.
The order with jejunal strips in vitro was Met > Lys >
Gly (Johns and Bergen, 1973) and with everted sacs in
vitro was Met > Val > Thr (Phillips et al., 1976~. The
overall order of uptake by sheep gut is similar to that
noted in man and rats. A depressing effect of leucine on
lysine uptake has been shown both in vitro Johns ant}
Bergen, 1973) and in viva (Hume et al., 1972~.
It seems likely that as in the nonruminant (Matthews,
1972; Munck, 1976), absorption of peptides is quantita-
tively important in the ruminant. Steps involved in-
clude peptide uptake, peptide hydrolysis, and transport
of amino acids.
Removal of the end products of nucleic acid digestion
from digesta flowing through the small intestine implies
efficient absorption mechanisms (Bergen, 1978~. Nucle-
osides are absorbed from the small intestine by a Na-
dependent saturatable transport process (Scharrer and
Amann, 1980~.
EXTENT OF APPARENT ABSORPTION
Measuring disappearance of N (SCP) or amino (STP)
acids between the duodenum and ileum provides an es-
timate of apparent absorption. Samples from cannulae
inserted into the duodenum prior to the entry of bile and
pancreatic secretions only includes endogenous N from
gastric secretions, whereas samples from cannulae in-
serted posterior to the entry of bile and pancreatic secre-
tions also contain N from pancreatic secretions.
Apparent absorption of NAN and amino acids from
the small intestine of lactating cattle, nonIactating cat-
tle and sheep fed a variety of diets is listed in Appendix
Tables 11, 12, and 13. Table 11 summarizes the results.
Overall, apparent absorption was 0.65 of NAN and 0.68
of amino acids entering the duodenum (Table 11~. Ap-
parent absorption of NAN was similar in the groups
summarized and was less than absorption of amino acids
in lactating cattle and sheep but not in nonIactating cat-
tle. In experiments reviewed by ARC (1980) in which
absorption of both NAN and amino acids were mea-
sured, the values did not differ markedly. However,
Tamminga (1980) conclucled that apparent absorption
of total N is usually 0.05 lower than that of amino acids.
Based upon the foregoing, values suggested for appar-
ent absorption of NAN and amino acids from the small
intestine are 0.65 and 0.70 of amounts entering the duo-
denum.
Apparent absorption of essential amino acids is about
0.05 greater than nonessential amino acids (Armstrong
et al., 1977, Tamminga, 1980~. Apparent absorption of
essential amino acids, as summarized by Tamminga
(19803 suggests that absorption of lysine and arginine is
greater while absorption of threonine, valine, and
phenylalanine is less than the absorption of total essen-
tial amino acids (Table 12). Apparent absorption of
TABLE 11 Summary of Apparent Absorption of
Nonammonia Nitrogen and Amino Acids from the
Small Intestine of Ruminantsa
Measurement
Calculation
nb x
.
SD CV
NAN
Lactating cattle
Nonlactating cattle
Sheep
All
Amino acids
Lactating cattle
Nonlactating cattle
Sheep
All
12
17
29
58
21
11
22
54
0.65
0.66
0.64
0.65
0.69
0.62
0.70
0.68
0.04
0.04
0.06
0.05
0.05
0.06
0.06
0.06
0.07
0.06
0.09
0.08
0.08
0.10
0.09
0.10
a Based upon data in Appendix Tables 11, 12, 13.
bNumber of diets.
OCR for page 48
48 Ruminant Nitrogen Usage
TABLE 12 Proportionate Disappearance of Amino
Acids from the Small Intestinea
Animal and Experiment
Sheep Cow Ib
0.77 0.77
0.70 0.77
0.79 0.82
0.68 0.70
0.72 0.72
0.75 0.75
0.73 0.76
0.74 0.78
0.69 0.68
0.73 + 0.75 +
0.008 0.000
8
Amino Acid
Cow IIb
Cow IIIb
Lysine
Histidine
Arginine
Threonine
Valine
Methionine
Isoleucine
Leucine
Phenylalanine
Total essential amino
acids
No. observations
0.74
0.73
0.79
0.71
0.72
0.61
0.72
0.73
0.71
0.75
0.76
0.79
0.71
0.72
0.66
0.72
0.73
0.72
0.72 + 0.74 +
0.012 0.005
13
72
a Data summarized by Tamminga (1980~.
''Cow I, II, and III are not individual cows but refer to experiments
involving cows.
methionine was quite variable in the cow experiments
and may be a consequence of location of the duodenal
cannula in that only absorption of methionine was
lower in experiments where duodenal samples were col-
lected beyond rather than prior to the pancreatic and
biliary duct. In experiments reviewed by Armstrong et
al. (1977), apparent absorption of methionine was 0.06
+ 0.05 more than apparent absorption of total essential
amino acids. Net disappearance of cystine was only 0.40
to 0.50.
ENDOGENOUS LOSS
Calculation of true absorption requires correction for
the influx of enclogenous nitrogen that is not reabsorbed
from the small intestine.
Endogenous protein enters the small intestine in the
form of enzymes, bile, mucus, serum albumin, lymph,
epithelial cells, and oth~egradable products from the
gastrointestinal lining (Swanson, 1982~. Summation of
the endogenous input to the entire gastrointestinal tract
is large (Phillipson, 1964; Swanson, 1982~. In nonIactat-
ing cattle, it is more than twice the maintenance value
(Swanson, 1982~. In studies where i5N was used to study
N metabolism in sheep, inputs (g/~) of NAN to the small
intestine were: UIP, 6.5; BCP, 10.3; and intestinal se-
cretions, 17.0 (Nolan, 1975~. Thus, enclogenous N was
equivalent to NAN from the stomach. The N in duode-
nal contents from abomasal juice, pancreatic juice, bile,
and epithelial cells was estimated at 0.004 dry matter
consumed (Tamminga et al., 1979~. In lactating cattle,
this was equivalent to 0.10 to 0.15 of N from the stom-
ach, but not included in this estimate are endogenous
inputs posterior to the entry of bile and pancreatic juice.
Total influx of endogenous N is important for an un-
derstanding of the dynamic involvement of intestinal
tissue in N metabolism. However, as indicated previ-
ously, calculation of the true intestinal absorption of N
derived from the stomach requires correction for the en-
dogenous input that is not reabsorbed.
Endogenous losses, as well as true digestibility, can be
estimated as Y = a + bX where Y is disappearance be-
tween two points in the gastrointestinal tract (i.e.,
mouth and anus; proximal duodenum and terminal il-
eum), X is the supply (amount or concentration) to a
point in the gastrointestinal tract (i.e., mouth; proximal
duodenum), a is a negative value and represents the en-
dogenous loss, and b is true digestibility (Van Soest,
1982~.
Applying the regression approach to data obtained
with sheep, Hogan and Weston (1970) calculated the
endogenous loss from the small intestine that appeared
in feces as 0.0016 organic matter entering the duode-
num. The endogenous loss appearing in feces from the
entire gastrointestinal tract was 0.004 organic matter
consumed. This prompted Hogan and Weston (1970)
and Hogan (1975) to conclude that only about one-third
of the N in the classic metabolic fecal fraction is of en-
clogenous origin and the remaining two-thirds is of mi-
crobial origin.
Regression analysis of other data yielded the follow-
ing estimates of endogenous losses from the small intes-
tine (g/day): sheep, 2.2 g NAN; lactating cattle' 56 g
NAN and 250 g amino acids; nonlactating cattle, 0.77 g
NAN and 98 g amino acids (Table 13~. In sheep, Tas et
al. (1981) estimated the mean endogenous loss of amino
acids secreted into the small intestine to be 13 gig. Since
NAN is 0.8 amino acid nitrogen, this is equivalent to 2.6
gNAN/~13 . (0.8 x 6.25~.
The enclogenous losses in Table 13 for sheep and lac-
tating cattle are 0.10 to 0.13 of the N supply to the proxi-
mal duodenum. With nonlactating cattle, the endoge-
nous loss of amino acids was equivalent to 0.16 of the
supply from the stomach. The endogenous loss of NAN,
however, was only 0.01 of duodenal NAN. As shown in
Table 13, estimates for the two data sets were not in
agreement (Zinn and Owens, 1982, 0.01; Sharma et al.,
1974, 0.24~.
EXTENT OF TRUE ABSORPTION
NAN and Amino Acidsfrom the Stomach
Regression analysis used to estimate endogenous losses
from the small intestine also provided estimates of true
absorption (Table 13~. Expressed as a proportion of the
N supply to the proximal duodenum, values obtained
were: sheep, 0.75 NAN; lactating cattle, 0.78 NAN and
OCR for page 49
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OCR for page 50
50 Ruminant Nitrogen Usage
0.82 amino acids, nor~lactating cattle, 0.67 NAN and
0.83 amino acids.
When endogenous loss is expressed as a proportion of
the N supply to the duodenum, true absorption is the
sum of apparent absorption and endogenous loss (i.e.,
apparent absorption = 0.65; endogenous loss = 0.10;
then true absorption = 0.75~. Thus, the low value for
true absorption of duodenal NAN from the small intes-
tir~e of nonlactating cattle is ~ consequence of the small
correction for endoger~ous loss.
Our estimates of true absorption obtained by regres-
sion analysis are in agreement with the few available
published reports (Table 14~. With 23 diets, true ab-
sorption of NAN from the small intestine of sheep was
0.76; true absorption of essential amino acids was 0.80
(Hogan and Weston, 1970~. Tas et al. (1981) obtained a
mean value in sheep of 0.86 for true absorption of amino
acids. Using data from Nolan's (1975) model of N utili-
zation in sheep, true absorption was 0.80 of the NAN
supply to the duodenum. Values suggested for true ab
TABLE 14 Summary of True Absorption From the Small Intestine
sorption of NAN and amino acids from the small intes-
tine are 0.75 and 0.80 of amounts entering the duode
num.
Microbial Protein
Information on the true digestibility of microbial N
(BCP) is summarized in Table 14. In experiments re-
viewed by Bergen (1978), digestion of pure cultures of
rumen bacteria in vitro ranged from 0.44 to 0.93. In
data summarized by Chalupa (1972) and Zinn and
Owens (1982~7 true absorption of rumen bacterial and
protozoa! protein in rats was 0.66 and 0.88, respec-
tively. Labeling ruminal bacteria with 35S yielded val-
ues of 0.74 (Bird, 1972) and 0.85 (Salter and Smith,
1977~. Early studies with i5N-labelecl rumen bacteria
(Smith et al., 1974) gave low and variable estimates of
true absorption (0.41 to 0.70~. In later studies (Salter
and Smith, 1977) a value of 0.79 was obtained. i5N can
label compounds such as DAP to give variable and low
Source of True
Fraction Specie Method Data Absorption
NAN
Amino acids
Microbial N (BCP)
Microbial amino acids (BTP)
Escape N (UIP)
Escape amino acids
Endogenous amino acids
Sheep
Sheep
Sheep
Lactating cattle
Nonlactating cattle
Sheep (EAA)
Sheep
Lactating cattle
Nonlactating cattle
Rat
Rat
Sheep
Sheep
Sheep
Nonlactating cattle
Regression
Regression
15N
Regression
Regression
Regression
Regression
Regression
Regression
Isolated bacteria
Isolated protozoa
35S-bacteria
35S-bacteria
i5N-bacteria
Regression
Sheep Regression
Sheep t5N Leaf protein
Sheep t4C Chloroplast protein
Nonlactating cattle Regression
Sheep
Sheep
Regression
Regression
Table 12
Hogan and Weston (1970)
Nolan (1975)
Table 12
Table 12
X
SD
CV(~)
Hogan and Weston (1970)
Tas et al. (1981)
Table 12
Table 12
X
SD
CV
Chalupa (1972)
Zinn and Owens (1982)
Bird (1972)
Salter and Smith (1977)
Salter and Smith (1977)
Zinn and Owens (1982)
X
SD
CV(~o)
Tas et al. (1981)
Salter and Smith (1977)
Smith et al. (1974)
Zinn and Owens (1982)
X
SC
CV(%)
Tas et al. (1981)
Tas et al. (1981)
0.78
0.76
0.80
0.-8
0.67
0.76
0.05
0.06
0.80
0.86
0.82
0.83
0.83
0.03
0.03
0.66a
0.88a
0.74
0.85
0.79
0.73
0.78
0.05
0.06
0.87
0.85
0.73-0.82
0.68
0.77
0.09
0.11
0.82
0.7&-0.84
UAssuming equal biomasses of bacterial and protozoa! nitrogen.
OCR for page 51
estimates of true absorption. Multiple regression analy-
sis yielder] estimates of 0.87 for microbial amino acids
(Tas et al., 1981) and 0.73 for microbial N (Zinn and
Owens, 1982~. However, in the data of Zinn and Owens
(1982), estimates of endogenous losses were small ant]
true absorption of NAN was almost identical to appar-
ent absorption.
As indicated in a previous section, microbial N is 0.10
to 0.20 nucleic acid N. Data summarized by Bergen
(1978) and Smith (1975) indicate that digestion and ab-
sorption of nucleic acids is an efficient process. In sheep
and cattle, 0.75 to 0.90 of the nucleic acids that enter the
proximal duodenum are removed prior to the terminal
ileum.
Undegraded Intake Protein
Estimates of the true absorption of protein that es-
capes ruminal degradation (UIP) have been obtained by
isotonically labeling plant materials and by regression
analysis (Table 14~. Absorption of leaf protein labeled
with i5N was 0.85 (Salter and Smith, 1977), while 0.73
to 0.82 of i4C-labeled chloroplast protein was absorbed
(Smith et al., 1974~. By regression analysis, the true ab
sorption of escape amino acids in sheep was 0.82 (Tas et
al., 1981~. The true absorption of escape NAN in non-
lactating cattle was, however, only 0.68 (Zinn and
Owens, 1982~.
Endogenous Nitrogen
Information on the true absorption of amino acids in
the endogenous influx to the small intestine is scarce.
Applying regression techniques to data from sheep (Tas
et al., 1981) yielded values of 0.78 to 0.84 (Table 14~.
NITROGEN METABOLISM IN
INTESTINAL TISSUE
A substantial part of most amino acids apparently ab-
sorbed from the small intestine are metabolized in pro-
cesses associated with absorption (Bergman and Heit-
man, 1978; MacRae, 1978; Tamminga and Oldham,
1980).
An estimate of amino acids metabolized by intestinal
tissue can be obtained by comparing amino acids disap-
pearing from the small intestine with those appearing in
portal blood. In sheep fed 800 g/d of a high-protein diet
(19.8 percent) or 650 g/d of a me~lium-protein (15.6 per-
cent) diet, 0.67 to 0.71 percent and 0.55 to 0.57 percent,
respectively, of the amino acids absorbed from the small
intestine were metabolized in the intestinal wall (Tagari
and Bergman, 1978~. No preference appeared for either
essential or nonessential amino acids.
Digestion and Absorption in the Small Intestine 51
Measurements of absorption based upon appearance
of amino acids in blood (Hume et al., 1972, Sniffen and
Jacobson, 1975), therefore, reflect the balance of re-
moval from intestinal contents and metabolism in intes-
tinal tissue.
SYNOPSIS
Summaries of apparent and true absorption of NAN
and amino acids from the small intestine of sheep and
cattle are in Tables 11 and 13. These data suggest that
absorption from the small intestine does not vary
greatly.
True absorption of microbial N (BCP) and N from
dietary protein (IUP) that escaped ruminal degradation
are similar. This is expected on the basis of the constancy
of apparent and true absorption of NAN and amino ac-
ids. Bacterial cells might be less digestible because of
mucopeptides in bacterial cell walls. However, even
though DAP passes quantatively from the small intes-
tine (Mason armful White, 1971), cell walls and cell con-
tents of 35S-labelecl rumen bacteria are digested to the
same extent (Bird, 1972~.
Duodenal N in animals fed purified flints containing
urea or casein is largely microbial (BCP), whereas it is a
mixture of BCP and IUP with purified diets containing
plant proteins and with diets consisting entirely of natu-
ral feed ingredients. In some experiments apparent ab-
sorbability of NAN from the small intestine was similar
in animals fed natural diets or purified flints in which
urea was the sole source of N (Salter and Smith, 1977;
Zinn and Owens, 1982~. In studies summarized by Arm-
strong et al. (1977~7 apparent disappearance of amino
acid nitrogen was 0.70, 0.69, 0.64, and 0.76 for purified
diets containing urea, casein, corn gluten meal, and
field beans. These data imply that escape amino acid N
from corn gluten meal is less digestible, whereas escape
amino acid N from field beans is more digestible than
microbial amino acid N.
Level of feed intake could affect absorption of N from
the small intestine by influencing passage rate of digesta
or by adjusting proportionate amounts of microbial and
undegraded feed N. Absorption of amino acid N in
sheep fed chopped dried grass or pelleted ciried grass at
900 or 1400 g/d was 0.06 + 0.05 greater at the low level
of feed intake (Armstrong et al., 1977~. On the other
hand, Zinn and Owens (1981a,b, 1982) observer] that
increasing intake of a high-grain diet increased both by-
pass of dietary protein and apparent absorption of NAN
*om the small intestine.
Although there are no comparable data with cows,
studies with rats and ewes suggest that absorption may
be a more efficient process in lactation compared with
pregnancy and in pregnancy compared with the non
OCR for page 52
52 Ruminant Nitrogen Usage
pregnant, nonlactating state. This is a consequence of
an enhancer] absorptive area of the gut and an increased
partition of cardiac output to the gut (Oldham, 1981,
1984).
The potential for protecting feed protein from degra-
dation in the rumen was discussed in a previous section.
In many early experiments, lack of improvements in an-
imal performance was often a consequence of overpro-
tection and decreased intestinal digestibility (Chalupa,
1975a, 1984~. In more recent experiments, treatment of
a wide range of feedstuffs with formaldehyde substan
tially increased the amounts of leucine, isoleucine,
valine, histidine, arginine, and phenylalanine absorbed
from the small intestine (Barry, 1976~. Absorption of ly-
sine, threonine, and sulfur-containing amino acids was
increased little or in some experiments decreased.
Based upon data summarizer! in this chapter and by
other investigators, the following values are suggested:
apparent absorption of NAN, 0.65; true absorption of
NAN, 0.75; apparent absorption of amino acids, 0.70;
and true absorption of amino acids, 0.80.
Representative terms from entire chapter:
true absorption
