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Departments of Molecular Biosciences, a Department of Anatomy, Physiology and Cell Biology, School of Veterinary Medicine, b Department of Nutrition, University of California, Davis, Davis, CA 95616 and Department of Psychiatry, Cornell University Medical College, White Plains, NY 10605
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: • lysine • deficiency • food intake • diet selection • amino acid requirements • rats
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). However, we have demonstrated
that rats adapted to a moderately threonine-deficient diet become
extremely sensitive to an exacerbation of the existing deficiency and
will select against a similar diet that contains only 0.009 g/100 g
less threonine (Hrupka et al. 1997
). Under this situation, it would
appear that "protein regulation" per se is not finely controlled,
but that the dietary limiting amino acid
(LAA)5concentration becomes the key dietary stimulus for controlling diet
selection patterns.
Lysine (Lys) is the first limiting amino acid in proteins of feed
grains, and it seems reasonable that lysine-deficient animals should be
able to alter their behavior to avoid further exacerbation of the
existing deficiency and/or alleviate the deficiency. Most of the work
investigating whether lysine-deficient rats can detect the exacerbation
of an existing deficiency has been done with amino acid–imbalanced
diets (Cieslak and Benevenga 1984a and 1984b
, Murphy and King 1989
,
Peng 1979
, Tews et al. 1981b
). Amino acid–imbalanced diets are created
when one or more amino acids, other than the LAA, are added to a low
protein basal diet (Harper et al. 1970
). Because the added amino acids
cause a drop in the plasma LAA concentration and compete with the LAA
for uptake across the blood brain barrier, the brain detects the
imbalanced diet as being more deficient than the previous basal diet
(Tews et al. 1978 and 1981a
). The animal responds by reducing its
intake of the imbalanced diet. Although it is easy to create severe
imbalances for most of the essential amino acids, severe lysine
imbalances are difficult to produce, and large amounts of excess amino
acids (up to 9 g/100 g diet) must be added to cause even a small
reduction in food intake (Cieslak and Benevenga 1984a and 1984b
, Peng 1979
, Tews et al. 1981b
). Results such as these would support the
conclusion that rats are not sensitive to exacerbation of a lysine
deficiency.
In contrast, lysine-deficient rats do seem to be able to make the
appropriate behavioral adjustments to help alleviate a lysine
deficiency. For example, lysine-deficient rats will select a
lysine-containing solution over other amino acid–containing solutions,
whereas nondeficient rats will not make this selection (Mori et al. 1991
, Tabuchi et al. 1991
). Rats will also alter their diet selection
patterns on the basis of lysine concentration when forced to consume
poor quality proteins such as wheat gluten, which is severely limiting
in lysine (Ashley and Anderson 1975
, Peters and Harper 1981
). In these
studies, the average amount of protein selected by rats that were
allowed to choose between two food cups containing a high (45 or 55
g/100 g) and low (5 or 15 g/100 g) wheat gluten diet was significantly
lower when the diets were supplemented with adequate lysine. It appears
that lysine-deficient rats will attempt to replenish their limited
lysine pool and that lysine, rather than protein, is the major stimulus
for the selection behavior.
Although it is clear that growing rats are capable of responding to large changes in dietary lysine, or severe lysine imbalances, it has not been demonstrated that growing rats are actually "sensitive" to small changes in lysine deficiency in the same manner that threonine-deficient rats can detect a difference of 0.009% threonine between two diets. In this paradigm, the existing deficiency is exacerbated by specifically reducing the LAA concentration, and the measured response is a selection against the more deficient diet. The three experiments performed were conducted to determine the following: 1) whether small reductions in dietary lysine concentration can alter dietary selection patterns in lysine-deficient rats fed purified amino acids as the sole protein source; 2) how deficient diets must be before rats detect small changes in dietary lysine concentrations and alter their feeding behavior; and 3) whether dietary selection patterns can be altered in rats fed a lysine-deficient protein supplemented with lysine.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Before the baseline and experimental periods, rats were adapted to a low protein diet (Tables 1–4) in which the protein fraction was composed exclusively of crystalline amino acids (Experiments 1 and 2) or of lysine-supplemented wheat gluten (Experiment 3). Lysine was the growth-limiting amino acid in all diets; it was included at levels above the maintenance requirement of rats so that the diets allowed for slow to maximal rates of growth.
The side preferences demonstrated by rats were assessed by preceding each trial with a baseline period in which rats were offered their adaptation diet (AD) in two food cups on opposite ends of the cage; food intake was recorded. During the experimental period, rats were offered two diets that differed only in the concentration of the limiting amino acid; intake was recorded daily. The left-right position of the cups was determined randomly for each rat. All food cups were washed at the beginning of the baseline and experimental periods. For each experiment, all diets were made on the same day and separated into aliquots. Fresh diet was used at the beginning of the baseline and experimental periods to prevent any contamination that might affect the sensory characteristics of the diet. Food intake was recorded daily and corrected for spillage.
Experiment 1.
Experiment 1 was conducted to examine the rat's sensitivity to changes
in the concentration of the LAA. The growth-limiting amino acid,
lysine, was included in the AD diet at 0.250% (g lysine /100 g diet),
~35% of the requirement for maximal growth for the rat (0.7%; NRC
1978
). This was above the rat's maintenance requirement for lysine
(0.11%; NRC 1978
) and allowed for a slow rate of growth. Forty rats
weighing 213.7 ± 1.6 g (mean ± SEM) were
blocked by body weight and randomly assigned to one of five treatment
groups (Table 1
).Rats were adapted to their AD for 7 d, followed by a 1-d baseline
period. During the experimental period, rats were given a choice
between the 0.250% Lys AD and a similar diet containing one of the
following: treatment (Trt) 1, 0.250% Lys (control); Trt 2, 0.245%
Lys; Trt 3, 0.240% Lys; Trt 4, 0.235% Lys; or Trt 5, 0.230% Lys.
Because lysine · HCl was removed from the diets, it was replaced
with glutamic acid so that all diets were isonitrogenous.
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Experiment 2.
Rats in Experiment 1 that were adapted to a diet containing ~35% of the NRC requirement for lysine selected diets on the basis of their lysine concentration; for this reason, a subsequent experiment was conducted to determine how deficient rats must be before they select on the basis of the LAA. Forty rats (n = 10/group) weighing 135.6 ± 0.7 g were blocked according to body weight and randomly assigned to one of four treatment groups that differed primarily in the level of lysine in the AD (Tables 2 and3).Lys was included in the four dietary treatments at 57, 86, 114 or 143% of the requirement for growth for the rat (0.4, 0.6, 0.8 and 1.0 g/100 g diet, respectively). To ensure that lysine remained the first limiting amino acid, all other essential amino acids were added at 92, 121, 149 or 178% of their requirement for the growing rat with respect to the NRC requirement (i.e., 35% above the level of lysine).
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Rats were allowed to select between the two diets for 9 d, with food intake recorded on d 1, 2, 3, 4, 5 and 9. Rats were fed only their respective AD on d 10 and only the more deficient diet of the pair on d 11. On d 12, rats were again allowed to select between the two diets. Position of the food cups within the cage was not changed throughout the trial, so that rats had the opportunity to associate the postingestive effects of the diet with the food cup and its location.
Experiment 3.
The protein source in Experiments 1 and 2 was comprised exclusively of purified amino acids; therefore, Experiment 3 was conducted to determine whether rats alter their dietary selection patterns in response to differences in the LAA concentration of amino acid–supplemented whole-protein diets. During Trial 1, 20 rats (n = 10/group) weighing 141.0 ± 2.0 g (mean ± SEM) were blocked by body weight and randomly assigned to dietary treatments in which they would select between their AD and a diet containing either 0.12 or 0.09% less supplemental Lys. All diets contained 25% wheat gluten as the protein source (Table 4 ).Rats that selected between a difference of 0.12% dietary Lys were adapted to a diet containing 0.21 g Lys/100 g, whereas rats that selected between diets differing by 0.09% Lys were adapted to a diet supplemented with 0.09 g Lys/100 g (Table 5 ).A 3-d dietary adaptation period was followed by a 2-d baseline period. During the 7-d experimental period, rats that selected between diets differing by 0.12% Lys chose between their AD (0.21 g Lys/100 g) and a diet containing 0.09 g Lys/100 g. Rats that selected between diets differing by 0.09% Lys chose between their AD (0.09 g Lys/100 g) and a diet containing no supplemental Lys. In Trial 2, 20 rats (143.3 ± 1.7 g; mean ± SEM) were blocked by body weight and randomly assigned to dietary treatments in which they would select between their AD and a diet containing either 0.06 or 0.03% less supplemental Lys. Rats that selected between a difference of 0.06% dietary Lys were adapted to a diet containing 0.09 g Lys/100 g, whereas rats that selected between diets differing by 0.03% Lys were adapted to a diet supplemented with 0.03 g Lys/100 g (Table 5) . During the experimental period, rats that selected between diets differing by 0.06% Lys chose between their AD (0.09 g Lys/100 g) and a diet containing 0.03 g Lys/100 g. Rats that selected between diets differing by 0.03% Lys chose between their AD (0.03 g Lys/100) g and a diet containing no supplemental Lys.
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Because rats have to consume both diets before they can make a choice based on a postingestive effect, rats were excluded from analysis if they did not consume at least 2 g from each food cup during one of the first three trial days. One rat in Experiment 1, two rats in Experiment 2 and no rats in Experiment 3 were excluded according to this criterion. Treatment differences for dietary choice trials were determined by analyzing the percentage of total intake that rats consumed of the AD. The data from Experiments 1 and 2 were not normally distributed; thus they were analyzed using nonparametric ANOVA using Wilcoxon (Rank Sums, PC-SAS, release 6.12, SAS Institute, Cary, NC) scores to determine differences in intake within treatments over time. The effect of diet on total food intake was analyzed by ANOVA using General Linear Models procedures (PC-SAS, release 6.12) appropriate for a randomized complete block design with repeated measures. Results from Experiment 3 were normally distributed after arcsine square root conversion; thus they were first analyzed by repeated measures ANOVA using General Linear Models procedures. When a significant within-subjects (time) effect was observed, results were analyzed by one-way ANOVA, and differences in diet consumption between days determined by Duncan's multiple-range test.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Rats adapted to the 0.250% Lys AD selected this diet over the 0.230%
Lys diet by d 1 of the trial (
2 = 12.3,
P < 0.02; Fig. 1
).Although rats given a choice between the 0.250% Lys AD and a 0.235%
Lys diet made no choice during the 5-d trial, rats allowed to select
between the 0.250% Lys AD and a 0.240% Lys diet selected the AD by d
5 of the trial (2 = 14.4, P < 0.01).
Rats allowed to choose between the 0.250% Lys AD and the 0.245% Lys
or the 0.250% Lys (control) diets did not make a choice by d 5, the
end of the observation period in this experiment. Rats in this
experiment (and also Experiment 2) often made a nearly all-or-none
selection, meaning that once an individual rat apparently detected a
difference, it would consume >90% of its daily intake from its AD. As
the means in Figure 1
(and also Fig. 2
)approach 100% selection for the AD over time, they reflect primarily
individual rats making this all-or-none choice more than the group or
rats slowly shifting their selection patterns.
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Experiment 2.
All groups consumed ~50% of their respective AD from both food cups
during the baseline day. Rats adapted to the 0.40% Lys AD
significantly altered their intake over time (2 = 32.45,
7 df, P < 0.0001) and started to consume more of the
0.40% Lys AD than 0.35% Lys diet by d 3 of the experiment (Fig. 2)
.
By d 12 of the trial, these rats consumed 93% of their daily intake
from the 0.40% Lys AD. Rats adapted to the 0.60% Lys AD also altered
their eating pattern over time (2 = 20.65, 7 df,
P < 0.005) and started to consume more of their AD by
d 3. Selection for the 0.60% Lys AD waned after d 3, so that by d 12,
this group consumed only 70% of its daily intake from the 0.60% Lys
AD. Rats adapted to the 0.80% Lys AD or 1.0% Lys AD diets showed no
change in dietary selection during the trial (P < 0.3
and P < 0.7, respectively).
Total food intake was significantly affected by the treatments (P < 0.0001); rats adapted to the 0.40% Lys AD ate significantly more on all days than rats adapted to diets containing higher levels of lysine. Average daily intake from d 1 to 9 was 21.3 ± 0.5, 17.1 ± 0.6, 16.6 ± 0.5 and 17.0 ± 0.3 g/d for rats fed the 0.40, 0.60, 0.80 and 1.0% Lys AD, respectively. Body weight gain during the same period was not significantly different (P = 0.06), but was lowest in rats fed the 0.60% Lys AD. Average daily body weight gains were 4.9 ± 0.3, 4.3 ± 0.2, 5.7 ± 0.3 and 5.2 ± 0.3 g/d for rats fed the 0.40, 0.60, 0.80 and 1.0% Lys AD, respectively. When food intake was compared between d 10 and 11, when rats had to consume diet from a single food cup (d 10, AD; d 11, more deficient choice), there was no difference in food intake within each treatment (all P > 0.35).
Experiment 3.
Rats in Trial 1 were allowed to choose between their AD and a diet containing 0.12 or 0.09% less supplemental lysine. Both groups significantly altered their dietary selection patterns over time (F8df = 3.78, P < 0.03), so that they were consuming more of their AD by the end of the trial (Fig. 3A ).Initially, both groups selected ~50% of their daily intake from each diet. By d 7, rats selecting between their AD and a diet containing 0.12% less lysine selected 62.1 ± 2.5% of their daily intake from their AD, whereas rats that selected between their AD and a diet containing 0.09% less lysine selected 74.1 ± 5.5% of their daily intake from the AD diet. Rats in Trial 2 were allowed to choose between their AD and a diet containing 0.06 or 0.03% less supplemental lysine. Both groups selected ~50% of their daily intake from each diet, but neither treatment altered their dietary selection patterns to consume more of a specific diet during the trial (P > 0.15, Fig. 3 B). Average daily food intake and body weight gains during the choice period are presented in Table 5 . Food intake was significantly different between treatments in Trial 1 (P < 0.03). However, the lower food intake observed in rats fed the 0.09% Lys AD was not consistent with the food intake of rats fed the same diet in Trial 2.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, Murphy and King 1989
, Newman and Sands 1983
, Tabuchi et al. 1991
, Tews et al. 1981b
), it has not been reported to date that rats are actually
"sensitive" to small changes in dietary lysine concentration. In
Experiment 1, rats were sensitive to a change in dietary lysine
concentration as small as 0.01% lysine when measured as a change in
diet selection between two lysine-deficient diets. This threshold is
very nearly the same as that reported for threonine (0.009%; Hrupka et
al. 1997). One might expect this threshold to be different, given that
Lys is catabolized more slowly than threonine and that Lys imbalances
are much more difficult to produce than threonine imbalances (Cieslak and Benevenga 1984a and 1984b
, Peng 1979
, Tews et al. 1981b
). These
results demonstrate that rats fed a Lys-limiting diet are sensitive to
an exacerbation of the existing deficiency and will alter their dietary
selection patterns to prevent further compromise of the Lys pool.
Rats that were adapted to diets containing 57 or 86% of their
requirement for lysine (0.40 and 0.60% Lys-basal diets; Experiment 2)
readily detected the more deficient diet and selected against it after
2-3 d of diet choice. Rats that were adapted to diets with a lysine
concentration higher than their requirement for growth (i.e., those
adapted to a 0.80 or 1.0% Lys AD) did not alter their dietary
selection when challenged with a diet containing 0.05% less lysine.
The estimated requirement for lysine based on nitrogen retention is
1.1% (g/100 g) of the diet (Gahl et al. 1991
), which is substantially
above the requirement for maximal growth (0.7%, NRC 1978
). If rats in
this experiment had made their selection because of a metabolic need
based on nitrogen retention, rats adapted to the 0.80% Lys AD should
have altered their dietary selection behavior and selected this diet
over the 0.75% Lys diet. The sensory and behavioral mechanisms
responsible for initiating this change in selection do not appear to
function in rats adapted to diets in which lysine is near or above
their requirement for growth. However, it is possible that rats adapted
to the 0.80 and 1.0% Lys AD did not make a distinction between their
AD and their respective more deficient diet because the difference in
lysine concentration was not large enough. For example, birds adapted
to diets containing 100% of their requirement for lysine or valine
will alter their dietary selection patterns if given a choice between
this diet and a diet containing 25% of their requirement for lysine or
valine (Murphy and King 1989
, Newman and Sands 1983
).
After rats in Experiment 2 were given a choice between their AD and a more deficient diet for 9 d, they were then fed only the AD on d 10, followed by only their more deficient diet of the choice on d 11. Food intake was not different for any treatment between d 10 and 11. It is interesting that rats adapted to the 0.40% Lys AD diet did not reduce their intake of the 0.35% Lys diet on d 11. Rats in this treatment had clearly sensed the difference in dietary lysine concentration by d 3, given that they selected against the more deficient 0.35% Lys diet thereafter. Rats given a single food cup containing an amino acid imbalanced diet will reduce their food intake, once they recognize the diet as deficient. The lack of an anorectic response from these rats on d 11 suggests that a depression in food intake may not be a necessary first behavioral component in the rat's response to deficient diets. Furthermore, it suggests that the choice test is a more sensitive measure of amino acid imbalances or deficiencies than the amount of food ingested.
Rats prefed the 0.4% Lys AD in Experiment 2 consumed significantly
more food during the trial than rats fed higher levels of lysine. In
this case, the hyperphagia helped sustain an average daily gain near
that of the rats fed the 0.80 and 1.0% Lys AD. This tendency for
animals fed low levels of lysine (or protein) to overconsume energy per
unit of body weight relative to rats fed adequate lysine has been
observed previously in rats (Cieslak and Benevenga 1984a
, Tanphaichitr et al. 1976
) and pigs (Chen et al. 1995
). Emmans (1981)
suggested that
in such a state of deficiency, animals may eat for the first limiting
nutrient (lysine) rather than for energy. Although this may be true to
a limited extent, animals fed diets low enough in an essential amino
acid or protein do not eat enough to overcome the inadequate dietary
first limiting amino acid levels, as indicated by a reduced growth rate
and protein accretion. Still, this may be an important mechanism for
helping to maintain adequate limiting amino acid intake in the face of
an impending deficiency.
It is clear from Experiment 3 that a selection against a dietary lysine
deficiency can occur in rats fed diets containing whole proteins
supplemented with small amounts of free lysine. However, rats in
Experiment 3 did not alter their dietary selection pattern until the
diets differed by 0.09% lysine, whereas rats adapted to purified diets
(Experiment 1) were able to detect a difference of 0.01–0.02% lysine.
Furthermore, the degree of change in selection is quite different. Rats
in Experiments 1 and 2 often made nearly an all-or-none selection,
meaning that once an individual rat apparently detected a difference,
it would consume >90% of its daily intake from its AD. Rats in
Experiment 3 never showed such a strong selection for their AD. Free
amino acids are absorbed more rapidly after a meal than protein-bound
amino acids (Canolty and Nasset 1975
). The protein source in
Experiments 1 and 2 was exclusively free amino acids, and the more
rapid absorption of amino acids may have provided a stronger more
immediate postingestive feedback signal regarding the diet composition.
It is also possible, but unlikely, that the relative degree of
deficiency influenced the degree of change in diet selection. Rats fed
the 0.40% Lys AD in Experiment 2 grew faster than all groups in
Experiment 3, yet still had a more robust selection response to a
smaller change in dietary Lys than rats in Experiment 3.
In conclusion, we have demonstrated that small changes in dietary lysine concentration are a meaningful stimulus for dietary selection when animals are adapted to lysine-deficient diets. More specifically, we have demonstrated that lysine-deficient rats are exquisitely sensitive to small reductions in dietary lysine concentration, and will select against these diets when given the chance. The biological importance of this mechanism is most likely to help prevent rats that are consuming a deficient diet from selecting an even more deficient diet, thus further comprising their growth and probability of survival. Under these conditions, deficiency is defined as being below the rats lysine requirement for growth because rats that had adapted to diets containing lysine at levels above the requirement for growth but below the requirement for maximal nitrogen retention were not sensitive to a reduction in lysine concentration. Finally, even though lysine imbalances are difficult to produce, lysine deficiency caused by decreasing the concentration of the limiting amino acid can cause changes in dietary selection without causing a decrease in total energy intake.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked ''advertisement'' in accordance with 18 USC section 1734 solely to indicate this fact.
3 Current address: Swiss Federal Institute of
Technology, Institute for Animal Sciences, Schorenstrasse 16, 8603
Schwerzenbach, Switzerland.
4 To whom reprint requests should be addressed.
5 Abbreviations used: AD, adaptation diet; LAA,
dietary limiting amino acid; Trt, treatment.
Manuscript received May 20, 1998. Initial review completed August 7, 1998. Revision accepted November 2, 1998.
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