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Journal of the American College of Nutrition, Vol. 17, No. 1, 79-85 (1998)
Published by the American College of Nutrition

The Timing of Protein Feeding and Dietary Protein Levels Affect Taste Preference, Serum Zinc Concentration and Glossal Epithelial Morphology in Growing Rats

Ritsuko Tabuchi, M Home Econ, RD and Ikuo Ohara, D Agr, DMS, FACN

Laboratory of Nutrition, Faculty of Home Economics, Kobe Women’s University, Kobe, JAPAN

Address reprint requests to: Ikuo Ohara, D Agr, DMS, Faculty of Home Economics, Kobe Women’s University, Kobe 654, JAPAN


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Objective: The present study investigated the effect of timing of protein feeding on taste preference, serum zinc concentration and morphological changes of tongue epithelia.

Methods: Weanling rats were fed one of three diets: protein-free and 2.5 or 8% purified egg protein (PEP) diets for 14 days (first feeding period). Rats in each group were further subdivided into three dietary treatments, totaling nine groups and fed one of the same three diets for 14 days (second feeding period). Preference tests between 3 or 6 mmol/L NaCl vs. deionized water were conducted in both feeding periods. At the end of each period, blood and tongue were collected for the measurement of serum zinc concentrations and morphological observations.

Results: The rats fed the protein-free diet in the first feeding period did not discriminate 3 mmol/L NaCl from deionized water in both feeding periods, even though adequate protein was administered in the second feeding period. The rats fed the 2.5 or 8% PEP diet in the first feeding period discriminated 3 mmol/L NaCl from deionized water regardless of the protein levels in the diet during the second feeding period. Serum zinc concentration was proportionate to protein content in the diet at the end of each feeding period (p<0.0001), and low serum zinc concentration was restored by protein feeding. Abnormal changes of tongue epithelia were observed only in the rats fed the protein-free diet throughout the two feeding periods.

Conclusions: The results suggest that protein nutrition immediately after weaning is important to maintain normal taste function.

Key words: timing protein feeding, taste preference, serum zinc concentration, taste pore, rats

Abbreviations: PEP=purified egg protein


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Tasting behavior is modified generally by nutritional conditions [1–6] e.g., trace mineral, vitamin and protein deficiency, and properties of food [7] e.g., visual cues, odors and texture. One of the most important factors influencing taste preference or taste sensitivity is protein nutrition [8–11]. Although taste relates to zinc nutriture [12–16], a previous study from our laboratory demonstrated that low taste sensitivity, low serum zinc concentration and morphological changes of tongue epithelia induced by dietary protein insufficiency were not restored by dietary zinc supplementation [10].

The relationship between tasting behavior and dietary protein type [17] or early experiences of visual cues [7] and food consumption [18] on growing animals has been reported, but there is a paucity of data concerning the effect of early protein feeding on taste preference.

It is also well known that gustatory functions relate not only to zinc but also retinol [19]. Plasma zinc is correlated with either plasma retinol or plasma retinol binding protein in humans [20,21], so it is possible that retinol may be related to the low taste sensitivity induced by protein insufficiency. In this experiment, however, we focused on the interrelationship between zinc and protein in gustatory function.

The objective of the present study was to examine the effect of the timing of protein feeding on taste preference, serum zinc concentration and morphological changes of tongue epithelia in growing rats.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals
Female weanling (3 weeks of age) Sprague Dawley rats (n=93) were obtained from Japan SLC (Hamamatsu, Japan). Animals were housed in individual stainless steel wire mesh cages (21x24x20 cm) in a well-ventilated room at 22±1°C with the relative humidity of 40 to 60% and a 12-hour light/dark cycle. Rats were allowed free access to food and deionized water throughout the feeding periods. Individual body weight and dietary intake were assessed daily. The experimental protocol was approved by the Animal Care and Use Committee of Kobe Women’s University.

Diets
The composition of the diets is shown in Table 1. Eight percent purified egg protein (PEP) level served as a positive control which showed maximum growth of rats from our experiment [22], 2.5% PEP level was viewed as a low protein diet which showed maintaining initial body weight [8] and the protein-free diet was a negative control.


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  		Table 1. Composition of Experimental Diet

 
The analyzed zinc concentrations of the protein-free and 2.5 or 8% PEP diets were 20.4, 22.8 and 29.2 µg/g, respectively. The protein diets contained higher levels of zinc than the protein-free diet, since PEP contained 131 µg/g of zinc. All the test diets even in protein-free diet satisfied with the requirement level of NRC [23]. Dietary zinc concentrations were assessed by atomic absorption spectrophotometry (Hitachi Type Z-8200, Tokyo, Japan) after wet ashing with HNO3:H2SO4 (3:1, v:v). The zinc level was not adjusted to equalize among three diets, as we reported previously that the zinc supplementation under similar conditions did not affect: intakes of NaCl solution and deionized water tested by two-choice method, serum zinc concentration, morphological change of tongue epithelia, and percentage of fungiform papillae with normal taste pore [10].

Experimental Procedure
Rats were assigned one of three dietary treatments and only two rats from the groups were reallocated to equalize initial mean body weight in each group. No significant differences in any parameters were observed among groups by this reallocation. Rats in each group (n=31/group) received one of three levels of protein diets for 14 days. On day 14, seven rats per each dietary treatment (first feeding period) were anesthetized with ether and blood was collected from posterior vena cava. The tip of each rat’s tongue was excised immediately. Serum was obtained by centrifuging the blood at 200xg for 10 minutes at 4°C and was stored at -20°C until analyzed.

The remaining 24 rats in each dietary group were subdivided into three dietary treatments, totaling nine groups and further received one of the same three diets for 14 days (second feeding period). On day 28, the rats were treated as described on day 14.

The 14-day feeding periods were chosen to clarify which time is more important for protein feeding in the first 28 days after weaning, since our previous study demonstrated that in 28 days, protein insufficiency affected taste sensitivity or preference in weanling rats [8,10].

Animals were trained to approach and sample a highly preferred solution of 0.82 mmol/L saccharin solution vs. deionized water on day 7 in a two-choice situation as mentioned previously [8]. We confirmed that the rats were able to discriminate preferred taste by this procedure. Two-choice preference tests for 24 hours were conducted between a different single concentration of NaCl solution vs. deionized water in the first (days 10 and 13) and the second (days 23 and 26) feeding periods. The following concentrations of NaCl were used: 3 (days 10 and 23) and 6 (days 13 and 26) mmol/L. The position of the drinking bottles was alternated every test day. On the other days, they were given only deionized water for drinking.

Serum zinc concentration was determined by atomic absorption spectrophotometry as mentioned above.

For observation by electron microscopic scanning (Hitachi Type S-430, Tokyo, Japan), the tips of the tongue were treated as described before [8]. The tongues’ surfaces were observed and the number of rats with abnormal tongue changes were counted. Criteria of abnormalities were treated as reported previously [10]. Twenty fungiform papillae from the tips of each rat’s tongue were picked and the number of these with normal taste pores were counted as reported previously [6].

Statistical Analysis
Values shown are means±SEM. Statistical significances of the differences between the groups concerning all measured variables were calculated by one-way analysis of variance, further comparison being made using Fisher’s protected leastsignificant difference test [24]. Differences between preference for NaCl solution and preference for deionized water were analyzed by paired t-test. Statistical analyses were performed with StatView 4.02 Software (Abacus Concepts, Berkeley, CA). A difference was considered significant with p<0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Body Weight
At the end of the first feeding period (day 14), the rats fed the protein-free diet showed retarded growth (Table 2). The rats fed the 2.5% PEP diet maintained their initial weights. The rats fed the 8% PEP diet grew well. There were significant differences among three dietary treatments (p<0.0001). Same patterns as in the first feeding period were observed in the second feeding period (p<0.0001).


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  		Table 2. Body Weight and Protein and Zinc Intakes of the Rats Fed Either of Protein-Free or 2.5 or 8% PEP Diet1

 
Protein and zinc intakes (Table 2) were proportionate to protein content in the diet in both feeding periods (p<0.0001).

Preference Test
Total volume intakes were proportionate to protein content in the diet (Table 3; p<0.001).


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  		Table 3. Total Volume Intake and Proportional Intake of NaCl Solution of the Rats Fed Either of Protein-Free or 2.5 or 8% PEP Diet

 
In the first feeding period, rats fed the 2.5 or 8% PEP diet discriminated between 3 mmol/L NaCl solution and deionized water, while the rats fed the protein-free diet discriminated only 6 mmol/L NaCl solution from deionized water.

In the second feeding period, all rats fed the protein-free diet in the first feeding period discriminated only 6 mmol/L NaCl from deionized water except those fed the 2.5% PEP diet in the second feeding period. The rats fed the 2.5 or 8% PEP diet in the first feeding period discriminated 3 mmol/L NaCl from deionized water.

Serum Zinc Concentration
Serum zinc concentrations of the rats fed experimental diets were proportionate to protein content in the diet in the both feeding periods (Table 4; p<0.0001). In the second feeding period, protein nutrition status just prior to the collection of sera influenced greater to the serum zinc concentration than that of the first feeding period.


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  		Table 4. Serum Zinc Concentration of the Rats Fed Either of Protein-Free or 2.5 or 8% PEP Diet1

 
Morphological Observation of Tongue Epithelia
In the first feeding period, no morphological abnormality of tongue epithelia were observed in any dietary treatments (Table 5). There were no significant differences among three dietary treatments on the percentages of fungiform papillae with normal taste pores (Table 6).


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  		Table 5.

 

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  		Table 6.

 
In the second feeding period, morphological changes of tongue epithelia were observed in only the rats fed the protein-free diet for 28 days (both in the first and second feeding periods). The lower percentage of fungiform papillae with normal taste pores of the rats fed the protein-free diet differed significantly than those of the other groups (p<0.0001).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The present study demonstrated that protein nutrition immediately after weaning is important to maintain normal taste function, though the importance of dietary protein levels, not of zinc supplementation, on taste preference, serum zinc concentration and morphological changes of tongue epithelia was already reported in the previous study [10].

It was also reconfirmed that taste preference was impaired by protein deficiency as previously reported [8,10]. Taste preference in the first feeding period was impaired by protein deficiency within 2 weeks of weaning. Furthermore, the impaired taste preference was not restored by protein feeding for 2 weeks during the second feeding period, since the rats fed the protein-free diet in the first feeding period and then received the 8% PEP diet in the second feeding period did not discriminate 3 mmol/L NaCl solution from deionized water in the second feeding period. The rats fed the 8% PEP diet in the first feeding period and then received the protein-free diet in the second feeding period discriminated 3 mmol/L NaCl from deionized water despite the protein-free diet in the second feeding period. These results suggest that good protein nutrition immediately after weaning plays an important role in the normal taste preference than in later periods.

In the rats fed the protein-free diet in the first feeding period, only the rats fed 2.5% PEP diet in the second feeding period discriminated 3 mmol/L NaCl solution from deionized water (Table 3). Considering the values of percent preference for 3 mmol/L NaCl among groups were similar, it is probable this NaCl concentration, 3 mmol/L, is the borderline of discrimination in this experiment. Rats fed 2.5% PEP diet in the first feeding period and then fed protein-free diet in the second feeding period did not discriminate 6 mmol/L NaCl. Large individual differences may be responsible for this result.

Serum zinc concentrations were proportionate to dietary protein level, especially just prior to the collection of the blood. Taste preference, however, did not necessarily parallel current serum zinc concentration (Tables 3 and 4). Zinc treatment has been generally adopted for patients with taste disorders [2]. These inconsistencies indicate that measurements of zinc distribution in the body other than serum are needed.

Abnormal morphological change of tongue epithelia was not observed in any dietary treatment groups during the first feeding period. The proportion of abnormal morphological change of tongue epithelia corresponded to the percentage of fungiform papillae with normal taste pores. Thus, glossal epithelial morphology was not influenced by protein deficiency within 2 weeks of weaning, whereas sodium chloride perception and serum zinc concentration of the rats were reduced by dietary protein insufficiency at the same time.

Meanwhile, tongue epithelia were affected morphologically by protein deficiency during the second feeding period. Taste preference and serum zinc concentration, however, were not necessarily related to the results of morphological observations.

The low percentage of fungiform papillae with normal taste pores is the low number of active taste buds. Miller and Reedy [25] reported that the higher taste bud density of humans gave significantly higher average intensity rating for substances, e.g., NaCl, than the lower density group. Extrapolating to rats, the results of taste preference ought to correspond with the results of the percentage of fungiform papillae with normal taste pores. Despite their decreased taste discrimination, the glossal epithelial morphology of not only the rats fed the protein-free diet in the first feeding period, but also the rats fed the 8% PEP diet in the second feeding period who received the protein-free diet in the first feeding period, was unaffected and superficially normal. One possible reason of this phenomenon is that a time lag exists between structure and function in the degeneration of fungiform papillae.

In conclusion, protein nutrition immediately after weaning is critical for normal taste function, even serum zinc concentration and morphological change of tongue epithelia can be recovered in the course of subsequent lifetime.


   ACKNOWLEDGMENTS
 
We wish to thank Akiko Goroumaru and Kumiko Onai for their technical help.

Received March 1, 1997. Accepted July 1, 1997.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Hambidge KM, Casey CE, Krebs NF: Zinc. In Mertz W (ed): "Trace Elements in Human and Animal Nutrition." Orlando, FL: Academic Press, pp 1–137, 1986.
  2. Tomita H: Zinc in taste and smell disorders. In Tomita H (ed): "Trace Elements in Clinical Medicine." Tokyo: Springer-Verlag, pp 15–37, 1990.
  3. Tordoff MG: Salt intake of rats fed diets deficient in calcium, iron, magnesium, phosphorus, potassium, or all minerals. Appetite 18: 29–41, 1992.[Medline]
  4. Kimura S, Kim CH, Ohtomo IM, Yokomukai Y, Komai M, Morimatsu F: Nutritional studies of the roles of dietary protein levels and umami in preference response to sodium chloride in experimental animals. Physiol Behav 49: 997–1002, 1991.[Medline]
  5. Mori M, Kawada T, Ono T, Torii K: Taste preference and protein nutrition and L-amino acid homeostasis in male Sprague-Dawley rats. Physiol Behav 49: 987–995, 1991.[Medline]
  6. Ohara I, Tabuchi R, Kimura M, Itokawa Y: Decline of taste sensitivity in protein deficient adult rats. Physiol Behav 57: 921–926, 1995.[Medline]
  7. Pick HL, Kare MR: The effect of extraneous cues on the measurement of taste preference in the chicken. J Comp Physiol Psychol 55: 342–345, 1962.[Medline]
  8. Ohara I, Tabuchi R, Kimura M, Itokawa Y: Preference for sodium chloride is reduced in protein-deprived juvenile rats. J Nutr 124: 901–905, 1994.
  9. Okiyama A, Torii K, Tordoff MG: Increased NaCl preference of rats fed low-protein diet. Am J Physiol 270 39: R1189–R1196, 1996.
  10. Tabuchi R, Ohara I: Influence of zinc supplementation to diets at graded levels of protein on taste sensitivity, morphological changes of tongue epithelia and serum zinc concentration in growing rats. J Am Coll Nutr 15: 303–308, 1996.[Abstract]
  11. Vazquez M, Pearson PB, Beauchamp GK: Flavor preferences in malnourished Mexican infants. Physiol Behav 28: 513–519, 1982.[Medline]
  12. Brosvic GM, Slotnick BM, Henkin RI: Decreased NaCl sensitivity in zinc-deprived rats. Physiol Behav 52: 527–533, 1992.[Medline]
  13. Catalanotto FA, Nanda R: The effects of feeding a zinc-deficient diet on taste acuity and tongue epithelium in rats. J Oral Path 6: 211–220, 1977.
  14. Hambidge KM, Hambidge C, Jacobs M, Baum JK: Low levels of zinc in hair, anorexia, poor growth and hypogeusia in children. Pediatr Res 6: 868–874, 1972.
  15. Hasegawa H, Kishi T, Suzuki M, Tomita H: Abnormal taste reactivities in zinc-deficient rats. Taste and Smell 14: 69–71, 1980.
  16. Tomita H, Tomiyama H, Kishi T: Pathology of taste buds. Adv Neurol Sci 25: 344–353, 1981.
  17. Naim M, Ohara I, Kare MR, Levinson M: Interaction of MSG taste with nutrition: Perspectives in consummatory behavior and digestion. Physiol Behav 49: 1019–1024, 1991.[Medline]
  18. Oscai LB, McGarr JA: Evidence that the amount of food consumed early life fixes appetite in the rat. Am J Physiol 235 4: R141–R144, 1978.
  19. Muto Y: Food ingestion and intracorporeal adaptation. In "Digestion · Absorption." Daiichisyuppan: Tokyo, pp 257–322, 1988 (In Japanese).
  20. Ishida H, Takahashi H, Suzuki H, Hongo T, Suzuki T, Shidoji Y, Yoon KH: Interrelationship of some selected nutritional parameters relevant to taste for salt in a group of college-aged women. J Nutr Sci Vitaminol 31: 585–598, 1985.
  21. Suzuki T, Ishida H, Suzuki H, Hongo T, Kobayashi K, Yoshinaga J, Ohba T, Shidoji Y: Minerals and vitamin-A status in relation to gustatory functions for salt in healthy female college students. J Nutr Sci Vitaminol 34: 209–222, 1988.
  22. Tabuchi R, Ohara I: The relationships between dietary protein levels and minerals affecting taste sensitivity for sodium chloride in rats. (Abstract). FASEB J 9 (4): A737, 1995.
  23. National Research Council: "Nutrient Requirements of Laboratory Animals." No. 10, 3rd ed. Washington, DC: National Academic Press, 1978.
  24. Armitage P, Berry G: "Statistical Methods in Medical Research." Oxford, UK: Blackwell Scientific Publications, 1994.
  25. Miller IJ JR, Reedy FE JR: Variations in human taste bud density and taste intensity perception. Physiol Behav 47: 1213–1219, 1990.[Medline]



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