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Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA 95616
3 To whom correspondence should be addressed. E-mail: qrrogers{at}ucdavis.edu.
KEY WORDS: • dietary choice • feline nutrition • methionine • threonine
Cats are more selective than dogs in the foods they eat. Selectivity can be exaggerated in situations often termed "finickiness" when cats are inadvertently trained to eat only one food by the owner routinely replacing a less-acceptable food with a desirable food. This finickiness does not appear to be learned taste aversion or caused by nutrient deficiency or excess and may be unique to cats. There is a paucity of information on the extent that dietary selection of cats is driven by odor, taste, texture (mouth feel), and metabolic feedback from the nutrients in the diet. Much is known about the effects of nutritional deficiencies and excesses on dietary choice in rats (1), whereas little is known for cats. Cats, unlike other species, will not select for sweetness (2) or for sodium even if they are sodium deficient (3), but when they are acidotic, cats will select diets containing extra sodium (4). Apart from positive palatability factors associated with proteins, cats do not select for or against protein even if the random choice results in protein deficiency and decreased weight gain (5). Cats are less sensitive than rats to leucine-isoleucine and valine antagonisms. A mild antagonism is exhibited by cats when isoleucine is limiting in the basal diet (6), but they do not avoid the diet containing a great excess of leucine (100 g/kg of diet) (7). The question arises, If cats do not select for or against protein, do they select for or against specific essential amino acids? The purpose of this study was to determine the dietary choice of cats when given threonine (Thr)- or methionine (Met)-deficient diets vs. more balanced diets.
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METHODS |
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 TOP METHODS RESULTSDISCUSSIONLITERATURE CITED |
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For the Met-choice study, after adaptation to a complete purified diet, adult cats (5.1 ± 0.4 kg, n = 8) were fed either a Met-free diet or 2 g of Met/kg of diet for 5 d. They were then offered purified diets that contained 272 g of amino acid mixture and either 0, 2, or 4 g of Met or a purified diet that contained 460 g of soybean-casein protein isolates/kg of diet (410 g of crude protein/kg of diet)6 with or without added Met. All diets contained concentrations of essential amino acids that exceeded the minimal requirements except those containing 0 and possibly 2 g of Met/kg of diet. Food intake was measured daily for 7–11 d and at 0.5, 1, 2, 4, and 16 h after cats were offered diet during the first 3 d. ANOVA was used to determine whether significant differences existed, and where differences were indicated, a t test using Bonferroni correction was used (Systat 10.2, SPSS, 2002).
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RESULTS |
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TOPMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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When kittens were offered a choice between diets containing 4 or 6 g of Thr/kg of diet, the kittens appeared to eat at random and ingested a mean of 30.3 vs. 29.6 g of each diet/d, respectively (Fig. 1).The minimal Thr requirement for growing kittens is 6 g/kg of diet (10). There was no indication throughout the 9-d period that kittens selected the Thr-adequate diet. When the kittens were offered a choice between a protein-free diet and either a Thr-adequate (6 g of Thr/kg of diet), a low-Thr or imbalanced diet (4 g of Thr/kg of diet), or a Thr-devoid diet, the kittens often (for 1–5 d of each period) ate significantly (P 
0.05) more of the amino acid–containing diets independent of whether the diet was adequate. Of the total daily food intake (mean of first 7 d), the kittens selected 73, 61, and 69% of the amino acid–containing diets versus the protein-free diet for the control, imbalanced, and Thr-devoid diets, respectively (daily choice not shown). The body weights of the kittens decreased gradually during the Thr-devoid– and imbalanced–choice studies, because their choices resulted in ingestion of inadequate quantities of Thr or total amino acids (crude protein). A small weight gain occurred when the choice included the control diet (a 35% complete amino acid diet).
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The results of the choice of adult cats adapted to the Met-devoid diet (n = 5) and the diet containing 2 g of Met/kg of diet (n = 5) were initially analyzed separately, but because the results were similar for both groups, they were pooled. Cats chose the diet that contained 2 g of Met/kg of diet over the diet containing no Met on d 1 (P = 0.04) and continued this selection throughout the 11-d period (P = 0.04–0.001) (Fig. 2). The choice for the diet that contained 2 g of Met diet over the Met-free diet occurred within 30 min (P = 0.06), and the difference was significant by 1 h (P = 0.04). When the cats were offered a choice between diets containing 2 or 4 g of Met/kg of diet, the cats initially chose the higher Met concentration (Fig. 3), which was significant only on d 3 (P = 0.003). The choice for the higher Met diet (4 g/kg of diet) gradually decreased with time until on d 6 and d 7, somewhat more (not significantly different) of the diet containing the lower Met concentration (2 g/kg of diet) was selected. To determine whether the selection for Met was for taste (palatability) or was a behavioral response resulting from a Met deficiency, cats were offered a choice between a fully adequate protein-containing (41% crude protein) diet with 0 or 2 g of Met/kg of diet (Fig. 4). Initially, the adult cats ate nearly equal amounts of each diet; they then gradually decreased the quantity of the Met-devoid diet until d 4, when they ate significantly more (P = 0.003) of the Met-containing diet, and the difference remained significant (P < 0.02) except for d 9 (P = 0.3).
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DISCUSSION |
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TOPMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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When adult cats were offered a choice between diets containing either 0 or 2 g of Met/kg of diet, the cats selected the Met-containing diet. When offered this choice, the cats initially chose the diet that contained the higher Met concentration, but starting on d 3 there was a gradual reversal until on d 6 the cats selected the same amount of each diet. From these results it is not clear whether the cats were selecting the Met taste, were responding to neural input to avoid the diet most deficient in Met (a learned taste aversion), or were selecting a diet based on a learned taste preference. The minimal Met requirement for adult cats to maintain nitrogen balance is 
2 g/kg of diet (19), but it is not known whether this quantity optimizes all metabolic needs for Met. However, with 6 g of Cys/kg of diet, the diet containing 4 g of Met/kg of diet provides plenty of total sulfur amino acids to sustain maximal nitrogen retention in growing kittens (20,21). To obtain information on these possibilities, we offered the adult cats a "fully adequate" dietary concentration of protein (41% crude protein that provided 2.5 g of Cys and 9 g of Met/kg diet) with and without the addition of 2 g of free Met/kg of diet. On d 1, the cats showed no selection for either diet, but gradually over then next 4 d the cats increased their intake of the Met-containing diet. This did not clarify the issue, because if the cats chose the Met because it improves the palatability of the diet, the cats should have selected the Met-containing diet on d 1; also, it does not seem reasonable to conclude that adult cats need more total sulfur amino acids than 11.5 g/kg of diet nor that 13.5 g/kg of diet, primarily from protein, is excessive. More research needs to be done to ascertain (using classic paradigms) (22) whether given the above choices, the cats may be choosing because of a learned taste aversion or a learned taste preference.
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FOOTNOTES |
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2 This work was partially supported by the Center for Companion Animal Health, School of Veterinary Medicine, University of California, Davis, CA 95616.
4 Present address: Florida International University-University Park, Miami, FL 33199.
5 For the Thr studies, the diets contained (in g/kg of diet) 325 amino acid mixture (14 Thr plus alanine, 20 L-arg·HCl, 8 L-Cys-Cys, 12 L-His·HCl, 18 L-Ile, 24 L-Leu, 28 L-Lys·HCl, 11 L-Met, 15 L-Phe, 10 L-Tyr, 4 L-Trp, 18 L-Val, 20 L-Ala, 10 L-Asp, 20 L-Asn, 30 L-Gln, 20 L-Glu, 13 L-Pro, 15 L-Ser, 15 Gly, and 25 Na acetate), 250 chicken fat, 197 starch, 150 dextrose, 40 salt mixture, 10 vitamin mixture, and 3 choline chloride. The low (imbalanced) and control (minimal requirement) diets contained 4 and 6 g of L-Thr/kg of diet, respectively.
6 For the Met studies, the diets contained (in g/kg of diet) 272 amino acid mixture (varied Met, 20 L-Arg·HCl, 6 L-Cys-Cys, 6 L-His, 10 L-Ile, 24 L-Leu, 20 L-Lys·HCl, 8 L-Phe, 9 L-Tyr, 14 L-Thr, 3 L-Trp, 12 L-Val, 24.5 L-Ala, 32 Gly, 14 L-Asp, 24 L-Asn·H2O, 24.5 L-Gln, 10.5 L-Glu, and 10 L-Pro), 200 animal tallow, 50 hydrogenated beef tallow, 100 starch, varied dextrose, 50 salt mixture, 5 vitamin mixture, 3 choline chloride, and 15 sodium acetate·H2O. The intact-protein diet contained 230 g of casein and 230 g of soybean protein concentrate/kg of diet (410 g of crude protein/kg of diet).
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LITERATURE CITED |
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TOPMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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1. Rozin, P. & Kalat, J. W. (1971) Specific hungers and poison avoidance as adaptive specializations of learning. Psychol. Rev. 78: 459–486.[Medline]
2. Beauchamp, G. K., Maller, O. & Rogers, J. G., Jr. (1977) Flavor preferences in cats (Felis catus and Panthera sp.). J. Comp. Physiol. Psychol. 91: 1118–1127.
3. Yu, S., Rogers, Q. R. & Morris, J. G. (1997) Absence of a salt (NaCl) preference or appetite in sodium-replete or depleted kittens. Appetite 29: 1–10.[Medline]
4. Cook, N. E., Rogers, Q. R. & Morris, J. G. (1996) Acid-base balance affects dietary choice in cats. Appetite 26: 175–192.[Medline]
5. Cook, N. E., Kane, E., Rogers, Q. R. & Morris, J. G. (1985) Self-selection of dietary casein and soy-protein by the cat. Physiol. Behav. 34: 583–594.[Medline]
6. Hargrove, D. M., Rogers, Q. R., Calvert, C. C. & Morris, J. G. (1988) Effects of dietary excesses of the branched-chain amino acids on growth, food intake and plasma amino acid concentrations of kittens. J. Nutr. 118: 311–320.
7. Hargrove, D. M., Morris, J. G. & Rogers, Q. R. (1994) Kittens choose a high leucine diet even when isoleucine and valine are the limiting amino acids. J. Nutr. 124: 689–693.
8. National Research Council (1996) Guide for the Care and Use of Laboratory Animals. Publication no. 85-23 (rev.). National Academy Press, Washington, D.C.
9. Titchenal, C. A., Rogers, Q. R., Indrieri, R. J. & Morris, J. G. (1980) Threonine imbalance, deficiency and neurologic dysfunction in the kitten. J. Nutr. 110: 2444–2459.
10. Hammer, V. A., Rogers, Q. R. & Morris, J. G. (1996) Dietary crude protein increases slightly the requirement for threonine in kittens. J. Nutr. 126: 1496–1504.
11. Hrupka, B. J., Lin, Y. M., Gietzen, D. W. & Rogers, Q. R. (1997) Small changes in essential amino acid concentrations alter diet selection in amino acid-deficient rats. J. Nutr. 127: 777–784.
12. Leung, P. M., Rogers, Q. R. & Harper, A. E. (1968) Effect of amino acid imbalance on dietary choice in the rat. J. Nutr. 95: 483–492.
13. Leung, P. M., Rogers, Q. R. & Harper, A. E. (1968) Effect of amino acid imbalance in rats fed ad libitum, interval-fed or force-fed. J. Nutr. 95: 474–482.
14. Koehnle, T. J., Russell, M. C. & Gietzen, D. W. (2003) Rats rapidly reject diets deficient in essential amino acids. J. Nutr. 133: 2331–2335.
15. Leung, P. M. & Rogers, Q. R. (1969) Food intake: regulation by plasma amino acid pattern. Life Sci. 8: 1–9.[Medline]
16. Hardy, A. J., Morris, J. G. & Rogers, Q. R. (1977) Valine requirement of the growing kitten. J. Nutr. 107: 1308–1312.
17. Rogers, Q. R., Strieker, M. J. & Morris, J. G. (1990) Effect of quantity and pattern of dietary amino acids on amino acid requirements of the dog and cat. In: International Symposium on Nutrition, Malnutrition and Diet of Dog and Cats (A. T. B. Edney, ed.), proceedings of symposium held in Hannover, Germany, Sept. 3–4, 1987, pp. 52–56. British Veterinary Association. English edition.
18. Morris, J. G. & Rogers, Q. R. (1978) Arginine: an essential amino acid for the cat. J. Nutr. 108: 1944–1953.
19. Burger, I. H. & Smith, P. M. (1987) Amino acid requirements of adult cats. In: Nutrition, Malnutrition and Dietetics in the Dog and Cat (Edney, A. T. B., ed.), proceedings of symposium held in Hannover, Germany, Sept. 3–4, 1987, pp. 49–51. British Veterinary Association. English edition.
20. Smalley, K. A., Rogers, Q. R. & Morris, J. G. (1983) Methionine requirement of kittens given amino acid diets containing adequate cystine. Br. J. Nutr. 49: 411–417.[Medline]
21. Strieker, M. J. (1991) The Effect of Dietary Crude Protein on Essential Amino Acid Requirements of Kittens. Ph.D. thesis, University of California, Davis.
22. Naito-Hoopes, M., McArthur, L. H., Gietzen, D. W. & Rogers, Q. R. (1993) Learned preference and aversion for complete and isoleucine-devoid diets in rats. Physiol. Behav. 53: 485–494.[Medline]
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0.36; n = 9 cats).
