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Plasma Amino Acids and Excretion of Protein End
Products by Mice Fed 10 or 40% Soybean Protein
Diets with or without Dietary 2-Acetylaminofluorene or A/,/V-Dimtrosopiperazme1
PATRICIO HEVIA,2 C. RICHARD TRUEX,
PETER B. IMREY, STEVEN K. CLINTON,
HEATHER J. MANGIAN AND
WILLARD J. VISEK3
University of Illinois College of Medicine at
Urbana-Champaign, Urbana, IL 61801
Downloaded from jn.nutrition.org by guest on June 9, 2014
ABSTRACT
Studies were conducted with 5- to 8-week-old male and female
BeCaFi mice to determine the influence of two carcinogens, 2-acetylaminofluorene
(AAF) and N,]V-dinitrosopiperazine (DNP), on plasma amino acid concentrations and
on the excretion of lipids and nitrogenous metabolites. The carcinogens, AAF and
DNP, were fed at concentrations of 0.25 and 0.05 g/kg of purified diet, respectively.
Soybean protein constituted either 10 or 40% of the diet. Nutritional balances were
measured over a 7-day period, after 7 days of acclimatization. Females ate less feed,
gained less weight during acclimatization and excreted less fecal lipid as a percentage
of intake than males. On the average, animals fed 40% protein consumed less total
feed than those fed 10% protein. During acclimatization, DNP-fed animals ate and
gained significantly less than controls. During week 2 DNP-fed animals gained signifi
cantly less than controls, although their feed intake was not significantly different.
Fecal lipid excretion as a percentage of intake was significantly lower with carcino
gens in the diet. The 40% protein diets increased lipid excretion in total and as a
percentage of intake. With the exception of decreasing fecal lipid, AAF caused no
consistent changes in feed intake, body weight, nitrogen (N) retention or N excretion.
Neither carcinogen significantly influenced total fecal or urinary N, or the relative
concentrations of the different forms of urinary N, when expressed as a percentage
of N intake Plasma ammonia rose with AAF feeding and plasma histidine rose with
DNP feeding. Plasma concentrations of other amino acids were not changed consis
tently by either carcinogen. Feeding 40% protein caused a significant rise in plasma
branched-chain amino acids, glycine and phenylalanine, and a significant decline in
aspartate, threonine, serine, proline, citrulline, lysine and arginine.
J. Nutr. 114:
555-564, 1984.
INDEXING KEY WORDS
plasma amino acids •mice
•2-acetylaminofluorene
•N,Af-dinitrosopiperazine
•soybean protein
A large proportion of human cancers are at any of several steps (2).
related to environmental factors (1). The
Most experimental carcinogenesis models
current outlook views cancer as resulting _
from
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OÕ multiple
genetic
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©19S4 American Institute of Nutrition. Received for publication
chemical
2s,Ju1*'
1983
substances, radiation, infectious agents, and
diet.
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'Current address: Oepto. Procesos Biológicosy BioquÃ-micos,Universidad SimónBolÃ-var,Caracas, Apartado Postal 80659, Venezuela.
555
ib whom
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556
HEVIA ET AL.
MATERIALS AND METHODS
B6C3Fi 5- to 6-week-old male and female
mice, weighing from 17 to 23 g and 18 to 26
g, respectively, were randomly assigned4 to
individual hanging wire cages so that ani
mals of opposite sex occupied alternate posi
tions in the rack. They were fed ad libitum
a 20% soybean protein diet containing the
American Institute of Nutrition recommended
vitamin and mineral mixes (5) (table 1) for
5 days while adapting to their environment.
The animals were then transferred to two
1Diet
TABLE
compositionIngredientsIsolated
diet20.00.315.050.05.05.03.51.00.2100.0
of
protein'DL-MethionineCornstarchSucroseFiber2Corn
soybean
oilMineral
mix3Vitamin
mix4Choline
bitartrateTotal%
'Supro 620, Ralston Purina, St. Louis, MO.
2Solka
Floe, Brown Co., Berlin, NH.
'Mineral mixture
(grams/kilogram mixture): calcium phosphate, dibasic,
500.0; sodium chloride, 74.0; potassium citrate, monohydrate, 220.0; potassium sulfate, 52.0; magnesium ox
ide, 24.0; manganous carbonate, 3.5; ferric citrate, 6.0;
zinc carbonate, 1.6; cupric carbonate, 0.3; potassium
iodate, 0.01; sodium selenite, 0.01; chromium potassium
sulfate, 0.55; sucrose, powdered, 118.03.
'Vitamin
mixture (in milligrams/kilogram mixture except as noted):
thiamin •
HC1, 600; riboflavin, 600; pyridoxine •
HC1,
700; niacin, 3 g; calcium pantothenate, 1.6 g; folie acid,
200; biotin, 20; cyanocobalamin, 1; retinyl palmitate
(500,000 lU/g), 800; d/-a-tocopheryl acetate (250 lU/g),
20 g; cholecalciferol (40,000,000 lU/g), 2.5; menaquinone, 5.0; sucrose, finely powdered, 972.9 g.
racks of 24 identically arranged metabolism
cages (No. AC5462, Acme Metal Products,
Inc., Chicago, IL [name changed to Hazeltine]) with four males and four females
assigned to each of the following six treat
ments: 10% soybean protein, 10% soybean
protein + AAF, 10% soybean protein + DNP,
40% soybean protein, 40% soybean protein
+ AAF, 40% soybean protein + DNP. The
10 and 40% soybean protein diets were iden
tical to the basal diet, except that the pro
tein was exchanged for sucrose on an equal
weight basis. AAF (A-410-9, Aldrich Chemi
cal Co., Milwaukee, WI) or DNP (1033,
Frinton Laboratories, Vineland, NJ) was
mixed into the complete diets by successive
dilutions, to final concentrations of 0.25 and
0.05 g/kg, respectively. Dietary carcinogen
concentrations (6; Perkins, E. G., Truex,
C. R. & Visek, W. J., manuscript in prep
aration), total N (7) and crude lipids (8) of
*A formal sex-stratified randomized scheme, generated using the Statis
tical Analysis System procedure PLAN (16), was employed in the initial
caging of animals and maintained (with necessary adjustments due to
transfers to and from metabolism cages) throughout the experiment.
Downloaded from jn.nutrition.org by guest on June 9, 2014
utilize a small number of exposures to high
doses of an initiating agent. Laboratory
procedures, facilities and safety precautions
necessary to chronically expose animals to
low dietary doses of carcinogenic agents are
expensive and time consuming. However,
models utilizing such long-term low dose
feeding of chemical carcinogens are much
more analogous to human exposure and
offer certain advantages for examining nu
tritional influences on carcinogenesis.
To understand how dietary protein may
influence early stages of chemical carcino
genesis, we are investigating the effects on
nitrogen (N) metabolism of diets containing
carcinogenic agents. JV,N-dinitrosopiperazine (DNP) and 2-acetylaminofluorene (AAF)
represent two major classes of environmental
carcinogens, the nitrosamines and aromatic
amines, respectively. These carcinogens were
fed for 2 weeks, at concentrations known to
be carcinogenic (3, 4), to groups of male and
female adult mice consuming diets differing
in the level of protein. Plasma amino acids,
fecal nitrogen excretion, and urinary urea,
ammonia, creatinine, uric acid and allantoin
were measured.
The present studies were conducted to
obtain information about the effects of car
cinogenic agents on nutritional status and
the influence of specific dietary treatments
on the ability of mice to withstand these
chemical insults. BeCsFi mice, used in these
studies, show a low incidence of spontane
ous tumors, are extensively employed for
testing potential carcinogens and have been
suggested as highly desirable animals for
nutrition and cancer investigations. This
report also includes some base-line nutri
tional data on this mouse strain.
CARCINOGENS AND NITROGEN METABOLISM
tiple covariance analysis (15). The three
major replicates were treated as statistical
blocks, as were the two racks within each
replicate Covariates representing horizontal
and vertical positions within a rack were also
incorporated into the analysis. This was
done to account for environmentally related
differences associated with cage position
that were not fully equalized across diets by
the experimental plan or were later unbal
anced by death of mice during the experi
ment. Main effects and all interactions of
carcinogen, protein level and sex were in
cluded in the statistical models used, as were
interactions of carcinogen with block.
The validity of each statistical analysis
was examined by residual plotting. For each
variable, a few outlying observations were
corrected or removed as most reasonable.
Residual plots strongly suggested that statis
tical analysis of the amino acids and six of
the nitrogen-related variables be conducted
on the log scale, primarily to equalize vari
ances, so this was done All significance test
ing of main effects utilized the 5% level, but
testing of interactions employed the 1%
level to control the effects of testing multiple
hypotheses about the same variable Statisti
cal computing was accomplished with the
Statistical Analysis System procedure GLM
(16), employing Types I and III sums of
squares.
RESULTS
Observed means for each sex by diet com
bination over all three experimental blocks
are shown in table 2 for food, N, carcinogen
and lipid intakes, body weight changes,
carcass N, N output, overall N recovery and
fecal lipid output. Table 2 also summarizes
statistically significant main effects of sex
and dietary factors. For each experimental
factor, the magnitude of a significant effect
is reported as a relative difference, obtained
by dividing the adjusted mean difference
between relevant groups by the adjusted
mean of a reference group [eg., (adjusted
female mean minus adjusted male mean)/
(adjusted male mean) x 100% for effect of
sex]. For instance, females averaged 19%
less carcass N than males, although N intake
for females averaged only 5% less. Summa
rizing sex effects in table 2, female mice con
sumed less food and protein, gained less
Downloaded from jn.nutrition.org by guest on June 9, 2014
the experimental diets were determined be
fore initiation of feeding. Protein (6.25%
X Kjeldahl N) ranged between 8.7 and
8.9% for the 10% protein diets and between
35.1 and 35.8% for the 40% protein diets.
After the 5-day adaptation period, mice
were assigned to the experimental diets and
placed in metabolism cages for 14 days. The
metabolism cages were modified from the
standard design to minimize loss of feed and
mixing of excreta with food. Food consump
tion and weight change were recorded for
each week of this period. After 6 days the
mice were placed in their conventional cages
for 24 hours while the metabolism cages
were cleaned. The mice were then returned
to the metabolism cages for a 7-day collec
tion period. Urine and feces were collected
on days 9, 12 and 14 and subsequently pooled
for analysis. To prevent microbial growth,
urine was collected in vessels containing 0.7
ml of l M H2SO4. On day 14, the mice were
transferred to their conventional cages, fasted
for 9 hours, lightly anesthetized with ether,
and blood samples were collected by cardiac
puncture. Plasma from heparinized blood
was immediately deproteinized with sulfosalicylic acid precipitation (9). The depro
teinized plasma was quickly frozen in liquid
N and stored at -70°C (Beckman applica
tion DS 561) until analysis (Beckman 119CL
Amino Acid Analyzer, Beckman Instruments,
Palo Alto, CA). The mice were killed by
cervical fracture, and the intestinal contents
were removed. The carcasses were analyzed
for N content.
Fecal and urinary collections were stored
at - 18°C.Feces were freeze-dried, weighed,
ground and analyzed for total N (7) and
crude lipids (8). Urine total N (7), urea (10),
creatinine (11), ammonia (12), uric acid (13)
and allantoin (14) were determined. The
ratio of N recovered in the urine to the total
in urine and cage washings was used to esti
mate losses during the collection process.
The entire experiment as described above
was replicated three times in sequence, with
48 animals used each time, for a total of 144
animals. The three replicates differed in
arrangements of diets across cages, which
were chosen so as to minimize the influence
of any environmental effects related to cage
position. Some adjustments to the feeder
were also made during the first run. The
observations were interpreted by using mul
557
558
HEVIA ET AL.
TABLE 2
Mean intakes, weight changes, carcass nitrogen, nitrogen balance measures, fecal weight and fecal lipid output
measures, by sei and treatment, with statistically significant main effects of sex, protein and carcinogen'
Males fed
40% protein +
10% protein
Measure
Week 1
Food intake, g
Carcinogen intake,7 mg
Body wt change, g
Week 2
Food intake, g
N intake,'
Body wt change, g
Carcass N, mg
Carcass N, %
N gain,9 mg
N balance
Total N output,8 mg
27.3
1.39
26.4
363
1527
1.10
752
3.01
31.6
16.2
361
26.3
5.7
1.41
25.9
1.5
1.25
25.2
361
25.6
352
1505
5.5
0.82
749
2.96
23.4
31.1
330
1485
1.5
1.01
689
2.85
23.9
30.2
322
AAF
24.9
1.80
24.5
5.4
1.56
DNP
22.5
1.3
-0.90
25.2
1426
25.7
1433
24.8
1406
1565
1680
5.7
1.32
807
3.25
42.7
200.1
1232
1674
1.4
1.00
694
2.87
28.4
184.0
1245
1.27
746
3.08
35.5
152.8
1269
Overall
%Fecal N recovery,"
gFecalwt,
outputFecal
lipid
lipid/lipid Intakes x 1Q-3942.19564922.18858922.18357893.0206132892.8200123882.9192115
'Values are means. Abbreviations: C.V., coefficient of variation; AAF, 2-acetylaminofluorene; DNP, N,Ndinitrosopiperazine; Prot, protein.
'Levels of statistical significance: 'P < 0.001; bP < 0.05; "P < 0.01; NS,
P > 0.05.
3For variables analyzed on the logarithmic scale, this "% difference" characterizes the relative ef
fect of the involved factor at all levels of the other experimental variables when there is no interaction. For variables
not transformed prior to analysis, it is the absolute rather than the relative difference which is constant across
levels of the other factors, and relative variation may be higher or lower than reported at particular factor levels
associated with low and high average responses, respectively. The reported percent difference is, however, essen
tially an average (the harmonic mean) of the true percent differences at the various levels of the other factors,
and hence provides a reasonable measure of relative effect.
4Female mice relative to males.
540% soybean
protein diet relative to 10% soybean protein diet.
"Carcinogen present relative to no carcinogen.
weight, showed less total N output and car
cass N, and excreted less fecal dry matter,
less fecal crude lipids and less fecal lipid per
lipid intake than their male counterparts.
Mice fed 40% protein diets consumed less
food during both experimental weeks, and
gained 28% less weight during week 1 than
mice fed the 10% protein diets, while con
suming almost four times the protein. They
also excreted almost four times the N, 37%
more fecal dry matter and lipid, and more
fecal lipid per lipid intake during the collec
tion period (week 2). Average food intake
was depressed during the first week in all
groups fed DNP, with weight gain depressed
by 86% in the first week of carcinogen feed
ing and 18% during the collection week. No
significant interactions were observed involv
ing the above variables. A statistically signifi
cant interaction of sex and AAF, relative to
N gain, stems from an estimated overall
15% greater N gain associated with AAF in
females, but not in males. However, this
difference between males and females was
not observed consistently across blocks or
protein levels and appears to be a statistical
artifact. Mice fed AAF or DNP had less
fecal lipid per lipid intake. A significance
test suggests that AAF affects N balance,
but unusual patterns of variation in the data
indicate that the test is unreliable and that
the results must be viewed as inconclusive.
Downloaded from jn.nutrition.org by guest on June 9, 2014
Lipid intake7
Carcinogen intake,7 mg
DNP
AAF
559
CARCINOGENS AND NITROGEN METABOLISM
Females fed
40% protein
10% protein
AAF
24.5
0.85
23.5
5.2
0.66
DNP
23.6
1.4
0.41
AAF
22.7
0.73
22.6
5.0
0.88
Overall
mean
DNP
20.6
1.2
0.01
24.1
0.84
Significant effects2
% difference1
C.V.
11.3
187
Sex4
Prot5
AAF"
DNP6
-9a
-9-
NS
-8'
-48b
-28e
NS
-86e
25.53551480__1.086002.7927.832.9322912.1835624.735414725.41.136022.8730.048.0306862.0735024.934514361.51.075872.7725.934.8310891.96647
L15261.116672.9230.5101.77366.35
Downloaded from jn.nutrition.org by guest on June 9, 2014
L892.4132.986.410.3—1.711.3_6112.910.96.35.4—3.217.813.317.214.3-6e-5eNS-1
Proportional to food intake and dose (carcinogen intake) or proportional to food intake and dietary lipids (lipid
intake).
"L denotes mean natural logarithm, for variables analyzed on that scale. Reported group means for
nitrogen intake, output, balance and recovery occasionally appear inconsistent due to partial missing data for
particular animals. Each variable was analyzed appropriately by using all reliable data pertaining to it.
"N
gain represents the change in weight observed during week 2 of the experiment converted to N using the carcass
N determined after death. Significant interaction (P < 0.01) of sex and AAF. See text.
'"Although a statistical
ly significant main effect of AAF was found through formal hypothesis testing, discrepancies between blocks and
unusual patterns of within-group variation preclude meaningful inference of an effect of AAF on this variable.
"Nitrogen recovery = (Total N output/N intake) x 100.
No other significant effects of AAF or DNP
were found for this set of variables.
Table 3 summarized significant main
effects of experimental factors on fecal and
urinary N, and on five forms of urine N.
Excretion variables were analyzed on two
scales: total measured N and fraction of N
intake. Use of the latter would be expected
to eliminate differences between groups due
solely to variations in N intake, if excretion
were roughly proportional to intake over the
range of intake studied. Female mice excreted
significantly more ammonia and uric acid N
but less urea N than male counterparts,
after adjustment for intake Significant re
ductions of total N output, urinary N and
urinary creatinine N in females did not per
sist after correction for intake. Total and
urinary N excretion rose as N intake increased
from 10 to 40% soybean protein diets. How
ever, the distribution of excretion products
varied with protein level. Urea and ammonia
N excretion increased for 40% protein-fed
animals more than did N intake, and the
increase in urea remained significant after
correction for intake. Based on either scale
of analysis, urinary allantoin, uric acid,
creatinine and fecal N outputs increased
significantly less than intake Whereas AAF
produced small reductions in total urea and
allantoin N and DNP reduced total urinary
N, allantoin, creatinine, fecal and total N
excretion, none of these differences remained
significant when corrected for intake No
560
HEVIA ET AL.
TABLES
Statistically significant main effects of sex, diet and carcinogen on total nitrogen (N) output, total urinary N,
colorimetrie N, and fecal N, with and without adjustment for intake, expressed as % différences1
Excretion/ intake
Excretion
Parameter
measured
Sex"
Significant effects
Prot3 AAF4 DNP4
Mean5
C.V.
Significant effects
Sex2 Prot3 AAF4 DNP4
mg/wk
Mean5
C.V.
mg excreted/
mg intake
%
Urinary N
TotalUrea
L514.05.97
NAmmonia
L18.42.61
NAllantoin
L14.30.62.098.04.41
NUric
NCreatinine
acid
NFecal
KA-72-24NSNSNSNSNSNSNSNSNSNSNSNSNSNS0.7630.5920.0220.0210.0010.004-
L—3.5—3.0—11.515.419.915.3—5.6NS-834NS10NSNSNS16"NS-54_
'See table 2, footnotes l, 3.
!Female mice relative to males. Total urinary N, uric acid and unadjusted ammonia effects are
significant at P < 0.05; total N output, creatinine N, and adjusted urea and ammonia N effects are significant at P < 0.01, and
other effects at P < 0.001.
3Dietary 40% soybean protein relative to 10% soybean protein. Creatinine and adjusted urea effects
are significant at P < 0.01; other effects are significant at P < 0.001. The adjusted urea N effect is the average of effects which
differed significantly (P < 0.01) between experimental blocks.
'Carcinogen present relative to no carcinogen. AAF effects are
significant at P < 0.05, as are DNP effects on total urinary N, total N output and fecal N. DNP effects on creatinine N and allantoin N are significant at P < 0.01 and P < 0.001, respectively.
5L denotes mean natural logarithm, for variables analyzed
on that scale.
"Reported effect averaged over significantly different blocks.
significant interactions were observed.
Observed pooled means of plasma amino
acids are shown in table 4 along with statis
tically significant main effects of sex and
diet. All plasma amino acid concentrations
were statistically analyzed after logarithmic
transformation. Females showed higher plas
ma concentrations of citrulline, tryptophan
and lysine but lower concentrations of aspar
tate, threonine, serine, asparagine, gluta
mate, glycine, valine, leucine, tyrosine and
histidine than males. Mice fed 40% protein
diets had higher concentrations of glycine,
valine, isoleucine, leucine, tyrosine and
phenylalanine and lower concentrations of
aspartate, threonine, serine, proline, citrul
line, lysine and arginine than those fed 10%
protein diets. The reduction in lysine associ
ated with the 40% protein diet was signifi
cantly greater for females (32%) than for
males (15%).
Dietary AAF was associated with a sig
nificant (39%) increase in plasma ammonia
concentrations, and the sex difference in
aspartate concentration when AAF was fed
was slightly greater than in control animals.
No other effects of dietary AAF on plasma
amino acid concentrations were statistically
significant.
Dietary DNP increased plasma histidine
by 13%, decreased tryptophan by 32% in
males and increased tryptophan by 16% in
females. DNP was associated with an in
crease in plasma phosphoserine of about
60% in females fed 10% protein diets, al
though other sex by diet groups showed
DNP-linked reductions averaging 23%. This
finding was not consistent across blocks, and
may be artifactual. Another three-way inter
action, involving DNP and phenylalanine,
was significant due to large but opposite
effects in different blocks, and no reliable
conclusion is apparent. The occasional occur
rence of statistically significant but possibly
random complex interactions is to be expected
in a study involving this large a number of
variables, in which several types of interac
tion are separately tested in relation to each
variable.
DISCUSSION
The mouse and rat are competing animal
models for use in long-term cancer studies.
Downloaded from jn.nutrition.org by guest on June 9, 2014
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5.825
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CARCINOGENS
AND NITROGEN
METABOLISM
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562
HEVIA ET AL.
examined in both human and animal studies
(22-26). However, much less consideration
has been given to the role of dietary protein
despite epidemiological and experimental
evidence (27-29). The observation of greater
fecal lipids with high protein diets suggests
a possible interaction of protein and fat on
intestinal carcinogenesis.
The effects of the initial actions of chemi
cal carcinogens on protein and purine metab
olism are unknown. Other studies have
shown elevated uric acid and allantoin ex
cretion by rats following irradiation or in
jection of alkylating agents such as dimethylnitrosoamine (30-33), suggesting some al
teration of nucleic acid metabolism during
the initiation phase of carcinogenesis. Al
though adding DNP to the diets reduced
food intake, weight gain and N output, the
effects of DNP or AAF on the N in the ex
cretory products measured in the present
studies were minimal and generally not sig
nificant when expressed as a function of N
intake This suggests that the depressions in
the urinary output of urea and allantoin
associated with dietary AAF or of total N,
allantoin, creatinine, fecal N and total N
output associated with DNP resulted from
differences in N intake and were unrelated
to direct toxic effects. Even though these
concentrations of AAF and DNP are carcino
genic in long-term studies, short-term feed
ing caused no significant changes in excre
tion of protein or purine metabolites.
The levels of plasma amino acids in BeCaFi
mice used in this study compared well with
values reported for other mouse strains (3436). The higher level of lysine in the plasma
of female mice compared to male mice has
also been well documented by previous re
searchers (34). The high concentrations of
citrulline, tryptophan and lower levels of
threonine, serine, glutamate, glycine, valine,
leucine, tyrosine and histidine found in
female mice in our study have not been
noted previously. The large number of in
dividual samples, more modern equipment
and procedures, precise control of dietary
intake, and powerful statistical analysis
undoubtedly increase the sensitivity of the
presently reported experiment.
There are few studies reporting plasma
amino acid levels in mice fed diets differing
in protein concentration. The present data
provides base-line information on plasma
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Since mice are smaller and require 75 to
80% less feed than rats, their use offers dis
tinct economic advantages. However, although
dietary requirements of the two species are
known to be qualitatively similar (17, 18),
more nutritional data and studies of carcino
gen metabolism are available for rats than
for mice. Also, mice tend to scatter food,
requiring special precautions to adequately
measure food intake. Since the volume of
excretory products produced by mice is low,
small losses of urine or feces introduce pro
portionately larger errors into metabolic
studies of mice than of rats.
The special feeder employed in this study
minimized spillage and urination into the
feed. Recovery of N in urine and feces av
eraged 89% of N intake, estimated from
ration weights. Several factors presumably
contributed to the departure from total
recovery. Skin and hair losses were not esti
mated. Subject animals, which gained weight,
undoubtedly accumulated N over the study
period. This accumulation was estimated,
from weight change and carcass N data, as
about 30 mg/week. Food scattering and
excretory collection errors also contributed.
As expected, increasing dietary protein by
a factor of four increased excretion of pro
tein end products. However, while there was
approximately a fourfold increase in the
excretion of urea and ammonia associated
with increasing protein from 10 to 40% of
the diet, there was only a doubling of the
purine end products, uric acid and allantoin.
Increases in uric acid excretion with increased
protein intake have also been shown in hu
mans (19). On the other hand, others have
seen no difference in urinary allantoin and
uric acid in rats fed 8 or 21% protein (20).
A doubling of fecal crude lipid excretion
was associated with the increase in dietary
soybean protein from 10 to 40% (table 2).
Since the ratio of lipid excretion to lipid
intake was also higher, the higher fecal lipid
output was clearly due to the increased in
take of soybean protein. The increase in
fecal lipids resulting from increased protein
intake may influence intestinal carcinogenesis. We have previously reported increased
1,2-dimethylhydrazine (DMH)-induced colon
carcinogenesis and blood cholesterol in rats
fed diets high in protein (21). The association
among dietary lipids, fecal lipids, fecal
steroid excretion and colon cancer has been
CARCINOGENS AND NITROGEN METABOLISM
food intake when high protein or amino
acid-deficient or imbalanced diets are fed
(41, 43).
Other than the elevation in serum am
monia concentrations by AAF and the eleva
tion of histidine by DNP, these carcinogens
had no consistent effects on plasma amino
acid concentrations.
ACKNOWLEDGMENTS
The authors are grateful to Joan M. Alster,
M.S., for data management and statistical
programming support, as well as editorial
assistance; Pamela A. Anderson, Ph.D. and
Sherry Long, M.S., for assistance with tech
nical procedures.
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