Absence of a Salt (NaCl) Preference or Appetite in Sodium-Replete or Depleted Kittens

Appetite, 1997, 29, 1–10

Absence of a Salt (NaCl) Preference or Appetite in SodiumReplete or Depleted Kittens†

SHIGUANG YU, QUINTON R. ROGERS and JAMES G. MORRIS Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA 95616, U.S.A.

Many omnivores and herbivores exhibit an appetite for sodium or salt (NaCl) solutions, but a similar sodium appetite has not been demonstrated in carnivores. The choice for or against sodium-adequate diets of sodium-replete and depleted kittens (confirmed by an elevated plasma aldosterone concentration) was examined using a two-bowl choice test. Both bowls contained purified diets, one bowl with one of various levels of sodium (as NaCl) and the other bowl a sodium-deficient diet (0.1 g Na/Kg). Neither sodium-replete nor depleted kittens showed a choice of the diet containing 2 g Na/kg over the deficient diet. Both groups of kittens showed significant aversion to a diet containing 10 g Na/kg diet, with no change in total food intake. Kittens previously exposed to a diet containing 10 g Na/kg diet appeared to have a learned aversion to sodium in subsequent choice tests. We conclude that kittens do not possess an innate sodium appetite and that a sodium appetite is not induced in sodium-depleted kittens.  1997 Academic Press Limited

I Salt (NaCl) or sodium appetite, and salt or sodium hunger have been used to describe the behaviors of animals that seek or choose salt solutions over water, or salted over unsalted food when in need of sodium. In this paper, the term “preference” is used to describe the choice of a higher sodium diet or solution by the sodiumreplete animals, whereas the term “appetite” is used to describe the choice of a higher sodium diet or solution by the sodium-depleted animals. Sodium appetite may be innate or induced (Rowland & Fregly, 1988) and has been reviewed by a number of authors (Rowland, 1990; Rowland & Fregly, 1988; Dethier, 1977; Abraham et al., 1975; Denton, 1982, 1969). Most studies on salt preference and appetite have been on herbivores and omnivores (rats in particular) using salt solutions, and little is known about the salt preference and appetite of carnivores. Fregly (1980) reported that beagles had no salt preference under normal conditions, and no salt appetite when treated with hydrochlorothiazide, ethacrynic acid, or mineralocorticoid. However, a † Parts of these results were presented at the “1996 Purina Forum” in St. Louis, Missouri, U.S.A. This project was supported by a grant from the Center for Companion Animal Health, School of Veterinary Medicine, University of California, Davis. The authors are grateful to Ms. Yuling Wu for taking care of the kittens. Correspondence should be addressed to: J. G. Morris, Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA 95616, U.S.A. 0195–6663/97/040001+10 $25.00/0/ap960088

 1997 Academic Press Limited

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modest salt appetite was induced in dogs under severe experimental conditions (Brown, Thrasher, Keil & Ramsay, 1980; Fitzsimons & Moore-Gillon, 1980). Using a two-bottle test, Carpenter (1956) found that sodium-replete adult cats exhibited no preference for salt (NaCl) solutions over tap water, except when the salt concentration was 0.1  for which cats showed a slight preference. It appears that no study on salt appetite has been performed in sodium-depleted cats. It has been suggested that evolutionary pressure may have played an important role in the development of salt appetite (Denton, 1982; Abraham et al., 1975; Denton, Orchard & Weller, 1969). Herbivores frequently show a salt appetite under natural conditions, presumably because they are most likely to experience a sodium deficiency. On the contrary, carnivores would unlikely experience sodium-deficient diets under normal situations, as all prey contains substantial amounts of sodium, and thus may not have developed a salt appetite during evolution. Also as sodium occurs mainly in extracellular fluid, and sodium concentration in extracellular fluid in animals is similar, strict carnivores would have had little opportunity to experience foods with great differences in sodium content. In the present study, we investigated the salt (NaCl) preference of sodium-replete kittens and salt appetite in sodium-depleted kittens by giving them a choice of purified diets containing various sodium concentrations. M  M These studies adhered to the guide for the care and use of laboratory animals developed by the Institute of Laboratory Animal Resources of the National Research Council and were approved by the Animal Use and Care Administrative Advisory Committee of University of California at Davis. Animals Twenty-four specific-pathogen-free domestic short-hair kittens (14 male and 10 female) about 7 weeks of age were used. The kittens were obtained from the Nutrition Pet Care Center, University of California at Davis, U.S.A. Kittens were chosen for this study to minimize the influence of dietary experience. In addition, it is easier to accustom kittens than adult cats to purified diets. Diets A casein-lactalbumin based purified basal diet was formulated as follows (g/kg): casein, 222·5; lactalbumin, 222·5; animal fat, 300; sucrose, 100; starch, 90·5; taurine, 1·5; choline chloride, 3; vitamin mixture, 10 (Williams, Morris & Rogers, 1987) and a mineral mixture, 50 containing (g/kg diet): CaHPO4, 19·5; MgSO4, 2·25; KCl, 10; K2HPO4, 4·5; CaCO3, 5·50; MnSO4.H2O, 0·019; FeSO4.7H2O, 0·47; NaF, 0·007; KI, 0·0015; ZnSO4.7H2O, 0·223; CuSO4, 0·04; SnSO4, 0·005; NaSeO3, 0·0015; (NH4)6Mo7O4.4H2O, 0·002; CrCl3.6H2O, 0·013; NiCl2.6H2O, 0·015 and NH4VO3.4H2O, 0·001. Experimental diets with different sodium concentrations were made by adding NaCl to the basal diet at the expense of starch. Dietary sodium concentrations were verified by an atomic absorption spectrophotometer (PerkinElmer 3030B, Clay Adams, NJ, U.S.A.).

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Kitten Housing and Management The kittens weaned at 7 weeks of age (week 1 of the experiment) were divided into two groups. One group of 12 kittens (6 males and 6 females) was given a sodium-adequate diet (2 g Na/kg diet). Another group of 12 kittens (8 males and 4 females) was given a sodium-deficient diet (0·1 g Na/kg diet). Tap water was provided for all kittens which were housed as 2 or 3 kittens per cage with a litter box containing wood shavings in temperature controlled rooms (22±2°C) with a light:dark cycle of 14:10 for 3 weeks. Thereafter, the kittens were housed individually and given the same diet in two similar stainless steel food bowls separated by a bowl of deionized water (Barnstead/Thermolyne NANOpure II, Dubuque, IA, U.S.A.) for another 3 weeks to accustom them to the feeding conditions.

Choice Tests When the kittens were 13 weeks of age (week 7 of the experiment), both groups of kittens were further divided into two subgroups by stratified randomization on the basis of sex and body weight to test for sodium preference (sodium-replete kittens) and sodium appetite (sodium-depleted kittens). The sodium-replete kittens were divided into a sodium-replete control (RCo, 3 males and 3 females) and a sodium-replete choice group (RCh, 3 males and 2 females). One female kitten failed to accustom to the sodium-adequate diet and was removed from the experiment. Similarly, the sodium-depleted kittens were divided into a sodium-depleted control (DCo, 4 males and 2 females) and a sodium-depleted choice (DCh, 4 males and 2 females) group. At the end of week 11, 2 male, and 2 female kittens were removed from DCo and DCh groups, respectively because these kittens had an extensive loss of body weight as a result of severe sodium deficiency, and 1 female kitten in RCo group was removed to balance the numbers of kittens between RCo and RCh groups. Kittens were allocated in cages so that positions of the cages were balanced for sex and treatments. A two-bowl choice test was used to determine sodium preference and sodium appetite of kittens that were offered a sodium-adequate diet vs. a sodium-deficient diet. Two food bowls (A and B) always contained the same diet for the RCo and DCo groups and had diets with different sodium concentrations for the RCh and DCh groups except during week 13 when both bowls had a sodium-adequate diet (2 g Na/kg diet). Dietary sodium concentrations tested were 2, 10, 5, 3·5, 3, 2, 2 or 3 g Na/kg diet for the RCo group during weeks 7, 8, 9, 10, 11, 12, 13 and 14, respectively. For the RCh group, one food bowl had the same diet as for the RCo group and the other contained a sodium-deficient diet (0·1 g Na/kg diet). Both food bowls for the DCo group contained the sodium-deficient diet during weeks 7–10, and had the same diet as the RCo group thereafter because the body conditions of the kittens in the DCo group had deteriorated as a result of the severe sodium deficiency. Dietary sodium concentrations in each food bowl for the DCh group were the same as those for the RCh group. Contrast to the sodium-deficient diet, a sodium adequate diet (2 g Na/kg diet) that was slightly above the sodium requirement of kittens (1·6 g Na/kg diet, Yu & Morris, 1997) was tested during week 7. Then, sodium preference and appetite were challenged with a diet of high sodium concentration (10 g Na/kg diet: about 6 times

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sodium requirement and about 2 times sodium concentration in commercial dry cat foods) during week 8. As the kittens in both RCh and DCh groups consumed less diet containing 10 Na/kg diet, a series of gradually decreased dietary sodium concentrations was tested during weeks 9–12 to determine at what sodium concentration kittens were unable to make a choice. Kittens in both RCh and DCh groups were given the same diet (2 g Na/kg diet) in both food bowls during week 13, and a diet containing 3 g Na/kg diet vs. the sodium-deficient diet was tested again during week 14 to determine whether there was an extinction of the learned aversion to the high sodium diet. A sufficient quantity of food was provided in each food bowl to meet the total daily need of each kitten. The positions of two food bowls in each cage was changed daily to avoid a misinterpretation of a side preference. Food intakes from both food bowls were measured every 1 or 3 days and body weights were measured weekly. Plasma Aldosterone Assay About 3 ml of blood sample was taken from the jugular vein of unanesthetized kittens at weekly intervals from week 6 through 14 (except week 9) between 10:00 and 15:00 h. Plasma was separated by centrifuging the blood samples at 1100 g for 20 min, and was stored at −20°C until analysis which occurred within 2 months of sampling. Plasma aldosterone concentration was measured using a commercial RIA kit (Coat-A-Count, Diagnostic Products Corporation, Los Angeles, CA, U.S.A.) developed for measuring aldosterone in human serum and plasma. The application of the kit for cat plasma was validated in our laboratory by a dilutional parallelism test and by measuring added d-aldosterone (Sigma Chemical Co., St. Louis, MO, U.S.A.) in a pooled cat plasma. The recovery was 103 and 95% at concentrations of 0·6 and 2·2 nmol of added aldosterone per liter cat plasma, respectively. Statistical Analysis Independent Student’s t-tests were used to compare the differences of food intakes between bowl A and B in each group, and of total food intake (bowl A plus bowl B), body weight, and plasma aldosterone concentrations between choice groups and their counterpart control groups if variances between tested groups were similar (F test), otherwise the Mann-Whitney U tests were used. Significant level of difference was preset as p<0·05.

R Body Weight Sodium-depleted kittens had a lower rate of body weight gain than sodiumreplete kittens (Fig. 1). The RCo and RCh groups had similar body weight throughout the experiment. The body weight of the DCh group was somewhat higher but was not significantly higher (p>0·05) than that of the DCo group after week 6 of the experiment. The difference in body weight between the DCh and DCo groups became progressively greater until the end of week 10. The body weight of kittens in the DCo group increased rapidly when the kittens were given the sodium-adequate diets

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F 1. Body weight of kittens given diets containing various amounts of sodium. The sodium-replete or depleted status of kittens were induced by giving diets containing 2 or 0·1 g Na/kg from weeks 1 to 6. The sodium-replete (R, N=6; N=5 after week 11) and depleted (D, N=6; N=4 after week 11) control groups were given diets with various sodium concentrations as indicated by numbers in the figure (g Na/kg diet). The sodium-replete (R, N= 5) and depleted (D, N=6; N=4 after week 11) choice groups were offered the same diet given to the sodium-replete control group, plus the sodium-deficient diet in a separate bowl, except for week 13 when both food bowls contained the same sodium-adequate diet. There was no statistical difference in body weight between control and choice groups of the sodium-replete or depleted kittens. Β, R control (RCo); Χ, R choice (RCh); Φ, D control (DCo); Ε, D choice (DCh).

after week 10, and caught up with that of the kittens in the DCh group within 2 weeks. Plasma Aldosterone The kittens in the RCo and RCh groups had similar plasma aldosterone concentrations at the end of week 6 (Fig. 2). Plasma aldosterone concentrations of kittens in the DCo and DCh groups were also similar, but significantly elevated when compared to that of the RCo and RCh groups before week 10. From weeks 7 to 12, plasma aldosterone concentration was consistently higher in the RCh group than in the RCo group, but the DCo group had a significantly higher plasma aldosterone concentration (p<0·05) than the DCh group at the end of weeks 7, 8 and 10, respectively. Plasma aldosterone in the DCo group progressively fell after week 10, and reached the level of the RCo group within 3 weeks. Food Intake On the first day of week 7, kittens in all groups (RCo, RCh, DCo and DCh) ate similar amounts of food from bowl A and B (Fig. 3). Total food intakes were slightly higher in RCo and RCh than those in DCo and DCh but these differences were not significant (p>0·05). The food intake from bowl A or B was similar throughout the

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F 2. Plasma aldosterone concentrations of kittens given diets containing various amounts of sodium. The sodium-replete or depleted status of kittens were induced as presented in Fig. 1. The sodium-replete (R, N=6; N=5 after week 11) and depleted (D, N=6; N=4 after week 11) control groups were given diets with various sodium concentrations as indicated by numbers in the figure (g Na/kg diet). The sodium-replete (R, N=5) and depleted (D, N= 6; N=4 after week 11) choice groups were offered the same diet given to the sodium-replete control group, plus the sodium-deficient diet in a separate bowl, except for week 13 when both food bowls contained the sodium-deficient diet in a separate bowl, except for week 13 when both food bowls contained the sodium-adequate diet. ∗Significantly different from its counterpart control group (p<0·05). Β, R control (RCo); Χ, R choice (RCh); Φ, D control (DCo); Ε, D choice (DCh).

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F 3. Food intake of kittens given a choice for sodium adequate or deficient diets on the first day of week 7. The sodium-replete (R) or depleted (D) kittens were induced as presented in Fig. 1. Kittens in the control (N=6 for both R and D) or choice (N=5 and 6 for R Choice and D Choice, respectively) groups were offered two food bowls for the choice test, i.e., bowl A and B. Dietary sodium concentration (g Na/kg diet) in each food bowl is indicated by a number on the top of each bar. Φ, Bowl A; ∆, Bowl B; Ε, Bowl A+B.

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F 4. Food intake of kittens given a choice for sodium adequate or deficient diets. The sodium-replete (R) or depleted (D) kittens were induced as presented in Fig. 1. Kittens in the control (N=6 for R and D; N=5 and 4 respectively after week 11) or choice (N=5 and for R and D, respectively; N=4 for D Choice after week 11) groups were offered two food bowls for choice test, i.e., bowl A (open bars) and B (hatched bars). Dietary sodium concentration (g Na/kg diet) in each food bowl is indicated by a number on the top of each bar. ∗Significantly different from bowl A of the selection group (p<0·05). (A) R control (RCo); (B) R choice (RCh); (C) D control (DCo); (D) D choice (DCh).

experiment in the control groups of both the sodium-replete and depleted kittens (Fig. 4, R. Control and D. Control) except during week 12 when food intake from bowl B was higher than that from bowl A in the DCo groups (Fig. 4, D. Control, p<0·05). There was no difference in food intake during week 7 when the kittens in the RCh and DCh groups were offered diets of 2 vs. 0·1 g Na/kg diet for choice (Fig. 4). Both sodium-replete and sodium-depleted kittens preferred the sodium-deficient diet (0·1 g Na/kg diet) to the diet containing 10, 5, 3·5, 3 or 2 g Na/kg during weeks 8, 9, 10, 11 and 12, respectively (Fig. 4, R. Choice and D. Choice). However, there was no difference in food intake (week 14) between bowl A (0·1 g Na/kg diet) and B (3 g Na/kg) in the DCh group but a greater intake from bowl B (3 g Na/kg) occurred for the RCh group (p<0·05) after the kittens of both groups had been given a diet containing 2 g Na/kg (in both bowl A and B) for 1 week (week 13). Total food intake (bowl A and B, Fig. 5) was similar between the RCo and RCh groups but kittens in the DCh group consumed more food than those in the DCo group from weeks 8 to 11 (p<0·05).

D Neither sodium-replete nor depleted kittens showed salt preference or appetite (Figs 3 and 4). Both sodium-replete and depleted kittens showed either indifference for diets containing 2 or 0·1 g Na/kg diet, or an aversion for a diet of 10 g Na/kg

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Food intake (g/week)

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F 5. Total food intake of kittens given diets containing various amounts of sodium. The sodium-replete (R) or depleted (D) kittens were induced as presented in Fig. 1. Kittens in the sodium-replete control (N=6; N=5 after week 11) and choice (N=5) groups, and the sodium-depleted control (N=6; N=4 after week 11) and choice (N=6; N=4 after week 11) groups were given diets with various sodium concentrations as presented in Fig. 4. ∗Significantly different from its counterpart control group (p<0·05). Φ, R control (RCo); ∆, R choice (RCh); Ρ, D control (DCo); Ε, D choice (DCh).

diet in favor of 0·1 g Na/kg diet. These results are in agreement with those reported by Carpenter (1956) who demonstrated in a two-bottle test that sodium-replete cats did not possess an innate salt preference. A salt appetite may be induced by feeding a low sodium diet in Fischer 344 rats that have no innate salt appetite (Rowland & Fregly, 1988; Midkiff, Fitts, Simpson & Bernstein, 1987 & 1985). However, sodium-depleted kittens (as indicated by significantly elevated plasma aldosterone concentration) did not show any salt appetite (Figs 3 and 4). On the contrary, the sodium-replete kittens developed sodium deficiency (as indicated by elevated plasma aldosterone concentration) by consistently choosing a sodium-deficient diet over a sodium-adequate diet during weeks 8–11. This response of kittens was different from that of ruminants, rabbits and rats which, when sodium depleted, voluntarily chose solutions containing higher salt (Denton, Nelson & Tarjan, 1985; Denton, Orchard & Weller, 1969; McCutcheon & Levy, 1972), showing an advantage of a salt appetite as a survival mechanism. Although indifferent to diets containing 0·1 or 2 g Na/kg diet during week 7, kittens showed an apparent aversion to a diet containing 2 g Na/kg during week 12 (Fig. 4, R. Choice and D. Choice) after kittens had been given the choice of diets containing 10, 5, 3·5 or 3 g vs. 0·1 g Na/kg diet. This avoidance may be results of a learned aversion established to diets containing the higher levels of sodium. To further examine the effect of pretreatment of sodium, the kittens in the choice groups (RCh and DCh) were offered the same diet (2 g Na/kg diet) in both bowl A and B during week 13 and were given diets containing 3 or 0·1 g Na/kg diet for choice during week 14. Instead of showing an aversion for 3 g Na/kg diet as occurred during week 11, kittens in the RCh group preferred the diet containing 3 g Na/kg to that containing 0·1 g Na/kg diet during week 14 (Fig. 4 R. Choice, p<0·05) while

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the kittens in the DCh group showed no preference for diets containing either 3 or 0·1 g Na/kg diet (Fig. 4, D. Choice). It appears that the kittens’ preference or aversion for a dietary sodium concentration was influenced by the immediate previously experienced dietary sodium concentration. A sodium concentration as high as 10 g Na/kg diet (about six times the sodium requirement, Yu & Morris, 1977) which kittens partially rejected when given a choice did not affect the total food intake if it is the only food presented, as the total food intakes of the RCo group were similar between weeks 7 and 8 (Fig. 5). Choice or rejection per se of one diet over another did not affect total food intake as energy requirement was met. This was confirmed by the total food intakes being similar between the RCo and RCh groups throughout the experiment (Fig. 5). Total food intakes in DCh group were consistently and significantly (p<0·05) higher than those in DCo during weeks 8–11 apparently because of compensatory growth as a result of the improvement of sodium status of kittens in DCh group (Fig. 2). Sodium depleted kittens were found to become anorectic (Yu & Morris, 1997). In conclusions, kittens have neither an innate nor a sodium deficiency-induced salt appetite. Sodium status of kittens has no effect on their choice for diets containing various levels of sodium. A diet containing 10 g Na/kg is avoided by kittens when they are offered a choice between this diet and a sodium-deficient diet. However, food intake is similar among kittens given diets containing various adequate amounts of sodium when no choice is available.

R Abraham, S. F., Blaine, E. H., Denton, D. A., McKinley, M. J., Nelson, J. F., Shulkes, A., Weisinger, R. S. & Whipp, G. T. (1975). Phylogenetic emergence of salt taste and appetite. In D. A. Denton & J. P. Coghlan, (Eds), Olfaction and taste V. Pp. 253–260. New York: Academic Press, Inc. Brown, C. J., Thrasher, T. N., Keil, L. C. & Ramsay, D. J. (1980). Chronic cerebroventricular infusions of angiotensin II stimulate excessive drinking but not salt appetite in dogs. Society for Neuroscience Abstracts, 6, 529. Carpenter, J. A. (1956). Species differences in taste preferences. Journal of Comparative and Physiological Psychology, 49, 139–144. Denton, D. A. (1982). The hunger for salt. Berlin: Springer-Verlag. Denton, D. A. (1969). Salt appetite. Nutrition Abstracts & Reviews, 39, 1043–1049. Denton, D. A., Orchard, E. & Weller, S. (1969). The relation between voluntary sodium intake and body sodium balance in normal and adrenalectomized sheep. Communications in Behavioral Biology, 3, 213–221. Denton, D. A., Nelson, J. F. & Tarjan, E. (1985). The voluntary correction of sodium deficiency by the rabbit. Physiology and Behavior, 34, 181–187. Dethier, V. G. (1977). The taste of salt. American Scientist, 65, 744–751. Fitzsimons, J. T. & Moore-Gillon, M. J. (1980). Drinking and antidiuresis in response to reductions in venous return in the dog: neural and endocrine mechanisms. Journal of Physiology, London, 308, 403–416. Fregly, M. J. (1980). On the spontaneous intake of NaCl solution by dogs. In M. R. Kare, M. J. Fregly & R. A. Bernard (Eds), Biological and behavioral aspects of salt intake, Pp. 55–68. New York: Academic Press. McCutcheon, B. & Levy, C. (1972). Relationship between NaCl rewarded bar-pressing and duration of sodium deficiency. Physiology and Behavior, 8, 761–763. Midkiff, E. E., Fitts, D. A., Simpson, J. B. & Bernstein, I. L. (1985). Absence of sodium chloride preference in Fisher-344 rats. American Journal of Physiology, 249, R438–442.

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Midkiff, E. E., Fitts, D. A., Simpson, J. B. & Bernstein, I. L. (1987). Attenuated salt appetite in response to sodium deficiency in Fisher-344 rats. American Journal of Physiology, 252, R562–566. Rowland, N. E. (1990). Sodium appetite. In E. D. Capaldi & T. L. Powley (Eds), Taste, experience, and feeding, Pp. 94–104. Washington DC: American Psychological Association. Rowland, N. E. & Fregly, M. J. (1988). Sodium appetite: Species and strain differences and role of renin-angiotensin-aldosterone system. Appetite, 11, 143–178. Williams, J. M., Morris, J. G. & Rogers, Q. R. (1987). Phenylalanine requirement of kittens and the sparing effect of tyrosine. Journal of Nutrition, 117, 1102–1107. Yu, S. & Morris, J. G. (1977). The minimum sodium requirement of growing kittens defined on the basis of plasma aldosterone concentration. Journal of Nutrition, 127, 494–501. Received 24 July 1996, revision 20 December 1996