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Zinc deficiency conditions food aversions in rats
Physiology &Behavior, Vol. 42, pp. 245-247. Pergamon Press plc, 1988. Printed in the U.S.A.
0031-9384/88 $3.00 + .DO
Zinc Deficiency Conditions Food Aversions in Rats D A L E S. C A N N O N , * ¢
I S A A C L . C R A W F O R D * ¢ A N D L A U R A E. C A R R E L L *
*Dallas Veterans Administration Medical Center and +Departments of Psychiatry and Neurology University of Texas Health Science Center at Dallas, Dallas, TX 75216 R e c e i v e d 28 A u g u s t 1987 CANNON, D. S., I. L. CRAWFORD AND L. E. CARRELL. Zinc deficiency conditions .fbod aversions in rats. PHYSIOL BEHAV 42(3) 245-247, 1988.--Zinc (Zn) deficiency is shown to condition aversion to the Zn-deficient diet. After development of a Zn deficiency syndrome during which consumption of the deficient diet decreased, rats readily consumed a familiar Zn-normal diet. After Zn repletion, the previously deficient animals continued to avoid the Zndeficient diet. These results would not be predicted by the competing hypothesis that Zn-deficiency is anorexigenic. Zinc deficiency
Conditioned food aversion
Anorexia nervosa
IT is well documented that zinc (Zn) deficiency produces decreased food intake and loss of body weight in rats [6,18]. It has been suggested that this decreased food consumption is the result of anorexia, i.e., a general loss of appetite [5,7]. A number of pathophysiological states [8, 9, 18] and hypogeusia [4] have been proposed as mechanisms responsible for this putative anorexia. Clinical interest in Zn deficiency is heightened by suggestions that it might be involved in the etiology of anorexia nervosa ([2], cf., [16]). Low Zn levels have been observed in anorexic patients [4], and there have been reports that oral Zn supplementation results in weight gain in these patients [14,15]. It has been clearly demonstrated that decreased food intake following other mineral and vitamin deficiencies is the result of conditioned food aversions rather than a general loss of appetite [10--13]. Thus, an alternative explanation of decreased food intake in Zn-deficient animals is that it is the result of aversion conditioned to foods paired with the Zn deficiency. Conditioned food aversions are robust phenomena that have been observed with a variety of unconditioned stimuli including toxins, psychoactive drugs, neoplastic tumors, and irradiation [3]. Two specific predictions derived from the learned aversion model of the effect of nutritional deficiency on consumption developed by Rozin and associates [10-13] are tested in the present experiment. The first is that, since learned aversions would be specific to foods paired with Zn deficiency, Zn deficient animals will readily accept foods not previously paired with the deficiency. The anorexia model, on the other hand, would predict that deficient animals would not consume non-paired foods any more readily than they would paired foods. Further, the learned aversion model, but not the anorexia model, would predict avoidance of the deficiency-paired food after Zn has been repleted.
METHOD
Subjects Thirty naive male Sprague-Dawley rats weighing 83-108 g (M=91 g) at the beginning of the study served as subjects.
Apparatus Rats were individually housed in 18x 18z24 cm stainless steel cages. Water was presented in glass bottles with silicone stoppers and stainless steel spouts, and the Zn deficient diet was presented in acrylic containers. Other foods were presented in ceramic containers. Plasma Zn concentration was quantified on a Perkin Elmer 372 atomic absorption spectrophotometer using an air-acetylene flame and a hollow cathode lamp at the designated wavelength and current (Zn:213.9 nm, 15 mA). Spectral band width was 0.7 nm standard slit, and readings were taken in the peak height mode using a 2 second integration time and no background correction. Sensitivity of the assay procedure was 0.8 ~zg/ml for 0.2 absorbance (A). The instrument was calibrated for each assay day on the basis of triplicate determinations performed on 4 different concentrations of Zn standard solutions containing 0.25, 0.5, 1.0, and 1.5 mg Zn/ml. The plasma sample concentrations were determined from appropriate standard curves computed by the instrument microprocessor.
Procedure Rats were fed a pelleted, nutritionally complete diet (Tekland Rodent Chow) ad lib for 2 weeks prior to the initiation of the study. Food consumption, food spillage, and body weight were determined daily throughout the study. During the Zn depletion phase of the study, drinking water was
~This research was supported by the Veterans Administration. 2Requests for reprints should be addressed to Dr. Dale Cannon, Chief, Psychology Service (I 16B), Veterans Administration Medical Center, 4500 S. Lancaster Rd., Dallas, TX 75216.
246
CANNON, CRAWFORD AND CARRELL
deionized and distilled twice in a quartz distiller. Rats were randomly assigned to three groups. On experimental Days 1-20, one group (ZD) was fed ad lib a powdered, biotin-enriched Zn deficient diet containing 20% spray-dried egg white and <1 ppm Zn [17]. Two control groups were fed a similar powdered diet containing 25 ppm Zn. One control group (AL) was fed ad lib and the second group (PF) was pair fed the mean consumption of ZD rats the previous day. Pilot work indicated no difference in the palatability of the two powdered diets. Since the pelleted and powdered diets are made of different ingredients, it may be assumed they differ in taste as well as texture. On Day 20, 200-500/xl blood samples were drawn from the tail to assay plasma Zn. Blood was collected in microcapillary tubes and centrifuged at 5,000 rpm for 10 min. An aliquot (0.2 to 0.5 ml) of plasma was placed in plastic labware and diluted 1:10 with deionized double quartz-distilled water, a concentration which produced an absorbance signal in the linear detection range (e.g., 0.20-0.30 A). On Days 21-35, all rats were given the Zn-normal, pelleted diet to replete Zn in Group ZD. In addition to the daily determination of food consumption, intake was measured 1 hr after presentation of the pelleted diet on Day 21. Blood samples were drawn from the tail again on Day 35. On Day 36, half the subjects in each group were given the powdered Zn-deficient diet and the other half were given the powdered Zn-normal diet. Since there was no difference in consumption of these diets, all rats were given the powdered Zn-normal diet on Days 37-38. RESULTS
Group ZD developed a Zn deficiency syndrome during the Zn depletion phase (i.e., Days 1-20). As can be seen in Fig. 1, Group ZD ate significantly less than did Group AL on Day 5 and on Days 7-20, ts>4.6, ps<0.001. Group ZD weighed significantly less than did Group AL from Day 4 on, ts>4.0, ps<0.001. On Day 20, the mean body weights of Groups ZD, PF, and AL, respectively, were 101, 138, and 215 g. Other symptoms of Zn deficiency noted in Group ZD though casual observation included edema and dermatitis of the paws, hair loss, testicular atrophy, parakeratosis, and increased handling-elicited vocalization and aggression. There were also behavioral changes characterized by cyclic patterns of food and water consumption [6]. Analysis of plasma Zn levels on Day 20 confirmed the Zn deficiency of Group ZD at the end of the Zn depletion phase. The mean plasma Zn levels of Groups ZD, PF and AL, respectively, were 0.46, 1.14, and 1.18/zg/ml, F(2,25)=4t.6, p<0,001. The mean food consumption of Groups ZD, PF and AL, respectively, during the first hour the pelleted lab chow was presented on Day 21 were 6.8, 9.2, and 1.8 g, F(2,27)=46.2, p<0.001; and the means of total intake on Day 21 were 16.8, 26.0, and 21.8 g, F(2,27)= 13.9, p<0,001. To control for the effect of group differences in body weight, Day 21 consumption was reanalyzed using analysis of covariance with body weight as the covariate. The residual mean intakes of Groups ZD, PF, and AL, respectively, were 24.1, 28.4, and 12.1 g, F(2,26)= 14.9, p<0.001. As an additional test of whether the decreased food intake of Group ZD while Zn deprived was due to anorexia or to food aversion, consumption on Days 20 and 21 was compared across Groups ZD and AL. There was a significant Group by Day interaction, F(1,18)=20.9, p<&001. Analyses of simple main effects indicated there
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10 12 14 16 '18 20 22 24 2 6 28 3 0 32 34 56 3 8 DAYS
FIG. 1 Mean daily food intake ~g) per group. Group ZD was given ad lib a powdered, zinc-deficient diet on Days 1-20. while Groups AL and PF were given a powdered, zinc-sufficientdiet. On Days 1-20. Group AL was fed ad lib. and Group PF was given the mean amount eaten by Group ZD the previous day. All groups were given ad lib a pelleted, zinc-sufficient diet on Days 21-35. The powdered diets were presented again ad lib on Days 36-38.
was a significant increase in food intake on Day 21 relative to Day 20 for Group ZD, F(1,9)=69,0, p<0.O01, but there was no significant difference across days for Group AL. There was not a statistically significant difference between groups in food intake on Days 25-35. After 15 days on the Zn-normal diet, plasma Zn in Group ZD had returned to normal levels. Mean plasma Zn levels of Groups ZD. PF. and AL, respectively, o n D a y 35 were 0.99, 1.02. and 1.15/zg/ml. F(2,22)=0.59, p =0.56. On Day 36. when half of each group was offered the powdered Zn-deficient diet and the other half was offered the powdered Zn-normal diet, there was no significant difference in consumption due to either the type of diet offered or to a group by type of diet interaction. Mean consumption per group (g) of the powdered Zn-deficient and Zn-normal diets, respectively, on Day 36 was as follows: Group ZD, 4.9 and 5.8; Group PF. 19.4 and 19. i : Group AL. 20.1 and 21.3. The slight difference between the diets in Group ZD was no~ statistically significant. As can be seen in Fig. 1. Group ZD consumed significantly less powdered diet than either of the other groups on Day 36. F(2,27)=47.3, p<0.001. Even with body weight as the covariate. Group ZD consumed significantly less than either of the other groups on Day 36. Group ZD ate significantly less again on Day 37. F(2,27)=6.64, p<0.005, but did not differ from either of the other groups on Day 38. DISCUSSION
The two predictions of the learned aversion interpretation of decreased food intake resulting from Zn deficiency are supported by the results of this study. First, Zn deprived rats did not manifest a general loss of appetite. Although Group ZD rats decreased food consumption during Zn depletion relative to Group AL, they ate more than Group AL did during the first hour they were offered a familiar, ~'safe'" diet. I f differences in body weight are controlled statistically, the 24-hour food intake of Group ZD was significantly greater the first day they were offered the familiar diet (i.e.. on Day 21) than that of Group AL. Further, the intake of the familiar diet by Group ZD o n D a y 21 was substantially greater than their consumption of the Zn-deficient diet the
ZINC D E F I C I E N C Y A N D F O O D A V E R S I O N
247
previous day had been. There was no such increase in consumption by Group A L on Day 21. The marked increase in food consumption by Group ZD from Day 20 to Day 21 cannot be attributed to Zn repletion as it takes longer than 24 hours on a Zn-normal diet for plasma Zn level to increase [1]. The second prediction of the learned aversion model, i.e., that animals with a history of Zn deficiency will avoid foods previously paired with Zn deficiency, was also confirmed. Group ZD ate substantially less powdered diet the first day it was offered after Zn repletion (i.e., on Day 36) than did the other two groups. The lack of a difference by Group ZD in consumption of the two powdered diets on Day 36 suggests the aversion was discriminated on the basis of either the taste or texture of the diet rather than on the absence of Zn. The absence of a difference by the other two groups in consumption of the two powdered diets on Day 36 replicates our pilot work and indicates the two diets are equally palatable. The increase in food consumption by Group ZD across Days 36--38 may reflect the extinction of conditioned aversion or it may simply be the consequence of increased hunger resulting from low food intake on Days 36-37. These results extend to Zn deficiency the account of the decreased food intake of nutritionally deficient animals first
proposed by Rozin and associates [10-13]. The study does not rule out the possibility that Zn deficiency may produce anorexia, although it does not support such a possibility. The data do suggest, though, that conditioned food aversions must be considered in analyses of decreased food consumption in Zn deficient rats. The zinc deficiency observed in anorexia nervosa patients [4] has led to the suggestion it is involved in the etiology of the disorder [2], but the more parsimonious explanation of low Zn in anorexics is that it is the result of poor diet [16]. The present results suggest a way in which low Zn may affect eating behavior in anorexics: perhaps Zn deficiency resulting from poor diet conditions aversions to some foods that are eaten, thereby further reducing food intake. This possibility is, of course, quite speculative without further supporting data. ACKNOWLEDGEMENTS The authors acknowledge the assistance of Amy Adams and Deborah Berger with data collection. They also gratefully acknowledge Dr. James Wallwork and David P. Beguin of the Tufts University Human Nutrition Research Center on Aging for preparing the powdered diets.
REFERENCES 1. Apgar, J. Effect of zinc repletion late in gestation on parturition in the zinc-deficient rat. J Nutr 103: 973-981, 1973. 2. Bakan, R. The role of zinc in anorexia nervosa: etiology and treatment. Med ttypotheses 5:731-736, 1979. 3. Braveman, N. S. and P. Bronstein. Experimental assessments and clinical applications of conditioned food aversions. Ann N Y Acad Sci 443: 1-441, 1985. 4. Casper, R. C., B. Kirschner, H. H. Sandstead, R. A. Jacob and J. M. Davis. An evaluation of trace metals, vitamins, and taste function in anorexia nervosa. Am J Clin Nutr 33: 1801-1808, 1980. 5. Chafetz, M. D. Anorexia: a micronutrient model. South Psychol 2: 3%47, 1984. 6. Chesters, J. K. and J. Quaterman. Effects of zinc deficiency on food intake and feeding patterns of rats. Br J Nutr 24: 10611069, 1970. 7. Essatara, M'B., A. S. Levine, J. E. Morley and C. J. McClain. Zinc deficiency and anorexia in rats: Normal feeding patterns and stress induced feeding. Physiol Behav 32: 46%474, 1984. 8. Essatara, M'B., C. J. McClain, A. S. Levine and J. E. Morley. Zinc deficiency and anorexia in rats: The effect of central administration of norepinephrine, muscimol and bromerogocryptine. Physiol Behav 32: 47%482, 1984.
9. Essatara, M'B., J. E. Morley, A. S. Levine, M. K. Elson, R. B. Shafer and C. J. McClain. The role of endogenous opiates in zinc deficiency anorexia. Physiol Behav 32: 474-478, 1984. 10. Rodgers, W. L. Specificity of specific hungers..1 Comp Physiol Psychol 64: 3%58, 1967. 1I. Rozin, P. Thiamine specific hunger. In: Handbook o f Physiology, vol 1, section 6. Washington, DC: American Physiological Society, 1967. 12. Rozin, P. Specific aversions as a component of specific hungers. J Comp Physiol Psychol 64: 237-242, 1967. 13. Rozin, P. and J. W. Kalat. Specific hungers and poison and avoidance as adaptive specializations of learning. Psychol Rev 78: 45%486. 1971. 14. Safai-Kutti, S. and J. Kutti. Zinc supplementation in anorexia nervosa. Am J Clin Nutr 44: 581-582, 1986. 15. Safai-Kutti, S. and J. Kutti. Zinc therapy and anorexia nervosa. Am J Psychiatry 143: 1059, 1986. 16. Sandstead, H. H. Reply to letter by Safai-Kutti and Kutti. Am J Clin Nutr 44: 582, 1986. 17. Wallwork, J. C. and I. L. Crawford. Effect of zinc nutriture on amygdala kindling in the adult rat. Fed Proc 46: 884, 1987. 18. Wallwork, J. C., G. J. Fosmire and H. H. Sandstead. Effect of zinc deficiency on appetite and plasma amino acid concentrations in the rat. Br J Nutr 45: 127-136, 1981.
0031-9384/88 $3.00 + .DO
Zinc Deficiency Conditions Food Aversions in Rats D A L E S. C A N N O N , * ¢
I S A A C L . C R A W F O R D * ¢ A N D L A U R A E. C A R R E L L *
*Dallas Veterans Administration Medical Center and +Departments of Psychiatry and Neurology University of Texas Health Science Center at Dallas, Dallas, TX 75216 R e c e i v e d 28 A u g u s t 1987 CANNON, D. S., I. L. CRAWFORD AND L. E. CARRELL. Zinc deficiency conditions .fbod aversions in rats. PHYSIOL BEHAV 42(3) 245-247, 1988.--Zinc (Zn) deficiency is shown to condition aversion to the Zn-deficient diet. After development of a Zn deficiency syndrome during which consumption of the deficient diet decreased, rats readily consumed a familiar Zn-normal diet. After Zn repletion, the previously deficient animals continued to avoid the Zndeficient diet. These results would not be predicted by the competing hypothesis that Zn-deficiency is anorexigenic. Zinc deficiency
Conditioned food aversion
Anorexia nervosa
IT is well documented that zinc (Zn) deficiency produces decreased food intake and loss of body weight in rats [6,18]. It has been suggested that this decreased food consumption is the result of anorexia, i.e., a general loss of appetite [5,7]. A number of pathophysiological states [8, 9, 18] and hypogeusia [4] have been proposed as mechanisms responsible for this putative anorexia. Clinical interest in Zn deficiency is heightened by suggestions that it might be involved in the etiology of anorexia nervosa ([2], cf., [16]). Low Zn levels have been observed in anorexic patients [4], and there have been reports that oral Zn supplementation results in weight gain in these patients [14,15]. It has been clearly demonstrated that decreased food intake following other mineral and vitamin deficiencies is the result of conditioned food aversions rather than a general loss of appetite [10--13]. Thus, an alternative explanation of decreased food intake in Zn-deficient animals is that it is the result of aversion conditioned to foods paired with the Zn deficiency. Conditioned food aversions are robust phenomena that have been observed with a variety of unconditioned stimuli including toxins, psychoactive drugs, neoplastic tumors, and irradiation [3]. Two specific predictions derived from the learned aversion model of the effect of nutritional deficiency on consumption developed by Rozin and associates [10-13] are tested in the present experiment. The first is that, since learned aversions would be specific to foods paired with Zn deficiency, Zn deficient animals will readily accept foods not previously paired with the deficiency. The anorexia model, on the other hand, would predict that deficient animals would not consume non-paired foods any more readily than they would paired foods. Further, the learned aversion model, but not the anorexia model, would predict avoidance of the deficiency-paired food after Zn has been repleted.
METHOD
Subjects Thirty naive male Sprague-Dawley rats weighing 83-108 g (M=91 g) at the beginning of the study served as subjects.
Apparatus Rats were individually housed in 18x 18z24 cm stainless steel cages. Water was presented in glass bottles with silicone stoppers and stainless steel spouts, and the Zn deficient diet was presented in acrylic containers. Other foods were presented in ceramic containers. Plasma Zn concentration was quantified on a Perkin Elmer 372 atomic absorption spectrophotometer using an air-acetylene flame and a hollow cathode lamp at the designated wavelength and current (Zn:213.9 nm, 15 mA). Spectral band width was 0.7 nm standard slit, and readings were taken in the peak height mode using a 2 second integration time and no background correction. Sensitivity of the assay procedure was 0.8 ~zg/ml for 0.2 absorbance (A). The instrument was calibrated for each assay day on the basis of triplicate determinations performed on 4 different concentrations of Zn standard solutions containing 0.25, 0.5, 1.0, and 1.5 mg Zn/ml. The plasma sample concentrations were determined from appropriate standard curves computed by the instrument microprocessor.
Procedure Rats were fed a pelleted, nutritionally complete diet (Tekland Rodent Chow) ad lib for 2 weeks prior to the initiation of the study. Food consumption, food spillage, and body weight were determined daily throughout the study. During the Zn depletion phase of the study, drinking water was
~This research was supported by the Veterans Administration. 2Requests for reprints should be addressed to Dr. Dale Cannon, Chief, Psychology Service (I 16B), Veterans Administration Medical Center, 4500 S. Lancaster Rd., Dallas, TX 75216.
246
CANNON, CRAWFORD AND CARRELL
deionized and distilled twice in a quartz distiller. Rats were randomly assigned to three groups. On experimental Days 1-20, one group (ZD) was fed ad lib a powdered, biotin-enriched Zn deficient diet containing 20% spray-dried egg white and <1 ppm Zn [17]. Two control groups were fed a similar powdered diet containing 25 ppm Zn. One control group (AL) was fed ad lib and the second group (PF) was pair fed the mean consumption of ZD rats the previous day. Pilot work indicated no difference in the palatability of the two powdered diets. Since the pelleted and powdered diets are made of different ingredients, it may be assumed they differ in taste as well as texture. On Day 20, 200-500/xl blood samples were drawn from the tail to assay plasma Zn. Blood was collected in microcapillary tubes and centrifuged at 5,000 rpm for 10 min. An aliquot (0.2 to 0.5 ml) of plasma was placed in plastic labware and diluted 1:10 with deionized double quartz-distilled water, a concentration which produced an absorbance signal in the linear detection range (e.g., 0.20-0.30 A). On Days 21-35, all rats were given the Zn-normal, pelleted diet to replete Zn in Group ZD. In addition to the daily determination of food consumption, intake was measured 1 hr after presentation of the pelleted diet on Day 21. Blood samples were drawn from the tail again on Day 35. On Day 36, half the subjects in each group were given the powdered Zn-deficient diet and the other half were given the powdered Zn-normal diet. Since there was no difference in consumption of these diets, all rats were given the powdered Zn-normal diet on Days 37-38. RESULTS
Group ZD developed a Zn deficiency syndrome during the Zn depletion phase (i.e., Days 1-20). As can be seen in Fig. 1, Group ZD ate significantly less than did Group AL on Day 5 and on Days 7-20, ts>4.6, ps<0.001. Group ZD weighed significantly less than did Group AL from Day 4 on, ts>4.0, ps<0.001. On Day 20, the mean body weights of Groups ZD, PF, and AL, respectively, were 101, 138, and 215 g. Other symptoms of Zn deficiency noted in Group ZD though casual observation included edema and dermatitis of the paws, hair loss, testicular atrophy, parakeratosis, and increased handling-elicited vocalization and aggression. There were also behavioral changes characterized by cyclic patterns of food and water consumption [6]. Analysis of plasma Zn levels on Day 20 confirmed the Zn deficiency of Group ZD at the end of the Zn depletion phase. The mean plasma Zn levels of Groups ZD, PF and AL, respectively, were 0.46, 1.14, and 1.18/zg/ml, F(2,25)=4t.6, p<0,001. The mean food consumption of Groups ZD, PF and AL, respectively, during the first hour the pelleted lab chow was presented on Day 21 were 6.8, 9.2, and 1.8 g, F(2,27)=46.2, p<0.001; and the means of total intake on Day 21 were 16.8, 26.0, and 21.8 g, F(2,27)= 13.9, p<0,001. To control for the effect of group differences in body weight, Day 21 consumption was reanalyzed using analysis of covariance with body weight as the covariate. The residual mean intakes of Groups ZD, PF, and AL, respectively, were 24.1, 28.4, and 12.1 g, F(2,26)= 14.9, p<0.001. As an additional test of whether the decreased food intake of Group ZD while Zn deprived was due to anorexia or to food aversion, consumption on Days 20 and 21 was compared across Groups ZD and AL. There was a significant Group by Day interaction, F(1,18)=20.9, p<&001. Analyses of simple main effects indicated there
40 r
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10 12 14 16 '18 20 22 24 2 6 28 3 0 32 34 56 3 8 DAYS
FIG. 1 Mean daily food intake ~g) per group. Group ZD was given ad lib a powdered, zinc-deficient diet on Days 1-20. while Groups AL and PF were given a powdered, zinc-sufficientdiet. On Days 1-20. Group AL was fed ad lib. and Group PF was given the mean amount eaten by Group ZD the previous day. All groups were given ad lib a pelleted, zinc-sufficient diet on Days 21-35. The powdered diets were presented again ad lib on Days 36-38.
was a significant increase in food intake on Day 21 relative to Day 20 for Group ZD, F(1,9)=69,0, p<0.O01, but there was no significant difference across days for Group AL. There was not a statistically significant difference between groups in food intake on Days 25-35. After 15 days on the Zn-normal diet, plasma Zn in Group ZD had returned to normal levels. Mean plasma Zn levels of Groups ZD. PF. and AL, respectively, o n D a y 35 were 0.99, 1.02. and 1.15/zg/ml. F(2,22)=0.59, p =0.56. On Day 36. when half of each group was offered the powdered Zn-deficient diet and the other half was offered the powdered Zn-normal diet, there was no significant difference in consumption due to either the type of diet offered or to a group by type of diet interaction. Mean consumption per group (g) of the powdered Zn-deficient and Zn-normal diets, respectively, on Day 36 was as follows: Group ZD, 4.9 and 5.8; Group PF. 19.4 and 19. i : Group AL. 20.1 and 21.3. The slight difference between the diets in Group ZD was no~ statistically significant. As can be seen in Fig. 1. Group ZD consumed significantly less powdered diet than either of the other groups on Day 36. F(2,27)=47.3, p<0.001. Even with body weight as the covariate. Group ZD consumed significantly less than either of the other groups on Day 36. Group ZD ate significantly less again on Day 37. F(2,27)=6.64, p<0.005, but did not differ from either of the other groups on Day 38. DISCUSSION
The two predictions of the learned aversion interpretation of decreased food intake resulting from Zn deficiency are supported by the results of this study. First, Zn deprived rats did not manifest a general loss of appetite. Although Group ZD rats decreased food consumption during Zn depletion relative to Group AL, they ate more than Group AL did during the first hour they were offered a familiar, ~'safe'" diet. I f differences in body weight are controlled statistically, the 24-hour food intake of Group ZD was significantly greater the first day they were offered the familiar diet (i.e.. on Day 21) than that of Group AL. Further, the intake of the familiar diet by Group ZD o n D a y 21 was substantially greater than their consumption of the Zn-deficient diet the
ZINC D E F I C I E N C Y A N D F O O D A V E R S I O N
247
previous day had been. There was no such increase in consumption by Group A L on Day 21. The marked increase in food consumption by Group ZD from Day 20 to Day 21 cannot be attributed to Zn repletion as it takes longer than 24 hours on a Zn-normal diet for plasma Zn level to increase [1]. The second prediction of the learned aversion model, i.e., that animals with a history of Zn deficiency will avoid foods previously paired with Zn deficiency, was also confirmed. Group ZD ate substantially less powdered diet the first day it was offered after Zn repletion (i.e., on Day 36) than did the other two groups. The lack of a difference by Group ZD in consumption of the two powdered diets on Day 36 suggests the aversion was discriminated on the basis of either the taste or texture of the diet rather than on the absence of Zn. The absence of a difference by the other two groups in consumption of the two powdered diets on Day 36 replicates our pilot work and indicates the two diets are equally palatable. The increase in food consumption by Group ZD across Days 36--38 may reflect the extinction of conditioned aversion or it may simply be the consequence of increased hunger resulting from low food intake on Days 36-37. These results extend to Zn deficiency the account of the decreased food intake of nutritionally deficient animals first
proposed by Rozin and associates [10-13]. The study does not rule out the possibility that Zn deficiency may produce anorexia, although it does not support such a possibility. The data do suggest, though, that conditioned food aversions must be considered in analyses of decreased food consumption in Zn deficient rats. The zinc deficiency observed in anorexia nervosa patients [4] has led to the suggestion it is involved in the etiology of the disorder [2], but the more parsimonious explanation of low Zn in anorexics is that it is the result of poor diet [16]. The present results suggest a way in which low Zn may affect eating behavior in anorexics: perhaps Zn deficiency resulting from poor diet conditions aversions to some foods that are eaten, thereby further reducing food intake. This possibility is, of course, quite speculative without further supporting data. ACKNOWLEDGEMENTS The authors acknowledge the assistance of Amy Adams and Deborah Berger with data collection. They also gratefully acknowledge Dr. James Wallwork and David P. Beguin of the Tufts University Human Nutrition Research Center on Aging for preparing the powdered diets.
REFERENCES 1. Apgar, J. Effect of zinc repletion late in gestation on parturition in the zinc-deficient rat. J Nutr 103: 973-981, 1973. 2. Bakan, R. The role of zinc in anorexia nervosa: etiology and treatment. Med ttypotheses 5:731-736, 1979. 3. Braveman, N. S. and P. Bronstein. Experimental assessments and clinical applications of conditioned food aversions. Ann N Y Acad Sci 443: 1-441, 1985. 4. Casper, R. C., B. Kirschner, H. H. Sandstead, R. A. Jacob and J. M. Davis. An evaluation of trace metals, vitamins, and taste function in anorexia nervosa. Am J Clin Nutr 33: 1801-1808, 1980. 5. Chafetz, M. D. Anorexia: a micronutrient model. South Psychol 2: 3%47, 1984. 6. Chesters, J. K. and J. Quaterman. Effects of zinc deficiency on food intake and feeding patterns of rats. Br J Nutr 24: 10611069, 1970. 7. Essatara, M'B., A. S. Levine, J. E. Morley and C. J. McClain. Zinc deficiency and anorexia in rats: Normal feeding patterns and stress induced feeding. Physiol Behav 32: 46%474, 1984. 8. Essatara, M'B., C. J. McClain, A. S. Levine and J. E. Morley. Zinc deficiency and anorexia in rats: The effect of central administration of norepinephrine, muscimol and bromerogocryptine. Physiol Behav 32: 47%482, 1984.
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