Palatability and meal patterns

Physiology & Behavior, Vol. 30, pp. 915-918. Pergamon Press Ltd., 1983. Printed in the U.S.A.

Palatability and Meal Patterns SUZANNE

1

R. S U N D A Y

Manhattanville College, Purchase, N Y STEPHANIE

A. SANDERS

Kinsey Institute for Research in Sex, Gender and Reproduction, Indiana University, Bloomington, I N AND GEORGE

COLLIER

Rutgers, The State University, New Brunswick, N J R e c e i v e d 13 J a n u a r y 1983 SUNDAY, S. R., S. A. SANDERS AND G. COLLIER. Palatability and meal patterns. PHYSIOL BEHAV 30(6) 915-918, 1983.--Investigations of palatability have often been confounded by nutritional, pharmacological or methodological problems. The present experiment examined the free-feeding meal patterns of two differentially preferred yet nutritionally equivalent diets. When the two diets were offered sequentially (Experiment 1), no differences were found in meal frequency, meal size, or meal duration. However, when the diets were offered simultaneously in Experiment 2, the preferred diet was consumed more frequently, in larger meals, and at a faster feeding rate than the less preferred diet. Further, the meal patterns in Experiment 2 of the preferred diet were identical with those found for both diets in Experiment 1. Therefore, the meal patterns of two diets with differing palatability depend upon the method of presentation with differences appearing when the diets are offered simultaneously but not when they are offered sequentially. Palatability

Food preferences

Meal patterns

Feeding behavior

T R A D I T I O N A L L Y , intake and feeding patterns are thought to be regulated by homeostatic mechanisms. This view has led to the investigation of internal factors in determining the initiation and termination of feeding. Collier [1] has suggested that animals eat in anticipation of their physiological needs. This approach emphasizes the study of external, environmental factors such as food availability, abundance, caloric density, and palatability on meal parameters (meal size, meal frequency, and meal size-intermeal interval correlations). Palatability has been manipulated by adulterating a diet with either a noxious agent (such as quinine or sucrose octa acetate) to reduce its palatability or by adding fat, sugars, or saccharin to increase palatability. However, these variations in palatability are often confounded by pharmacological factors and with variation in nutritional variables such as caloric density, protein content, fat content and the type and amount of vitamins and minerals. In examining the relationship between palatability and meal patterns, Levitsky [6] found that adulterating a diet with quinine, thereby reducing its palatability, led to decreases in meal size and increases in meal frequency. Preliminary meal pattern data on rats receiving sucrose octa acetate (SOA) pellets (Sclafani, personal communication) were also characterized by small and frequent meals. However, this effect did not persist. Within 24 to 48 hours, nor-

mal meal patterns were recovered. The differences in the meal patterns with the quinine and the SOA adulterated diets most likely reflect the toxic effects of quinine. In past attempts to enhance palatability, fat has been added to Purina Chow. This procedure led to increased meal size and total caloric intake but did not affect meal frequency even when the diets were isocaloric [11]. When palatability experiments are conducted, one of two procedures is employed [2,3]. First, animals can be given preference tests. These generally involve short-term tests (often a few hours or less) with two or more differentiallypreferred diets offered simultaneously. Measurements of total intake of each of the diets and/or meal patterns are reported. Tests of this nature often mask toxic effects of diets (e.g., quinine) and may have little relevance to the long-term preferences the animal might show. The second approach reflects a long-term analysis of total intake and/or meal patterns and has usually involved differentiallypreferred diets being offered sequentially (with animals receiving one diet for a period of a week or more) or with groups of animals each receiving one of the diets. However, many studies using this paradigm have employed a simultaneous presentation (e.g., [3]). In this procedure, the diet consumed in larger amounts is labelled as the more preferred diet. However, the meal patterns of any diet which is consumed in larger amounts than another diet must, of neces-

~This research was supported in part by Grant No. HD 10588 from the National Institute of Health to George Collier. Experiment 1 was submitted in partial fulfillment for the degree of Masters of Science at Rutgers, The State University, New Brunswick, NJ by Stephanie A. Sanders.

Copyright © 1983 Pergamon Press Ltd.--O031-9384/83/060915-04503.00

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SUNDAY, SANDERS AND COLLIER

sity, involve an increase in meal size and/or meal frequency. This confounds differences in preference with differences in overall caloric and nutritional intake with respect to meal patterns. Therefore, it appears that many experiments examining palatability have been plagued by methodological problems. The present experiments examined the relationship between meal patterns and preference differences between two diets. The diets used in the present experiment were based on the results of previous work [11,12]. These diets were differentially preferred during simultaneous presentation, but did not differ with respect to total daily intake when offered separately. The diets were isocaloric, isonitrogenous, and had the same vitamin, mineral, and fat content. Previous tests of these diets [12] showed them to be comparable in terms of growth and total amount consumed when rats were presented with either one of the diets alone; yet, one was preferred seven to one over the other in a 24 hour intake, fleechoice paradigm. In Experiment 1, rats were given fleeaccess to one of the two diets, then they were shifted to the other diet. In Experiment 2, rats were provided access to the diets simultaneously. Total intake and meal patterns were monitored for both experiments. EXPERIMENT 1 METHOD

Subjects Twelve male, albino Sprague-Dawley rats (Charles River, Wilmington, MA) were used. The animals were 21-24 days of age at the start of the experiment. They were maintained on a 12-12 hr light/dark cycle.

Apparatus The experimental apparatus consisted of a standard Hoeltge cage to which a Plexiglas tunnel was attached. At the end of the feeding tunnel was a 4 cm diameter food cup access hole. Diets were placed in 24 food cups on the periphery of a 61 cm disc [7]. The disc was arranged under the feeding tunnel in such a fashion as to allow free access to only one food cup at a time. The tunnels were equipped with photocells to detect the presence of the rat. At the end of a meal (10 minutes after the last photocell break) the disc advanced, placing the next food cup into position. This procedure allowed a direct determination of both meal duration and the meal size [4].

Diets Diet 1 was a cereal-based diet, the composition of which appears in Table 1. Diet 2 was a one to two mixture of Diet 1 and a nutritionally-equivalent casein-based diet (Bio-Mix No. 101, Bio-Serve, Frenchtown, NJ). Diet 2 was known to be preferred seven to one in total dally intake over Diet 1 by weanling rats of this strain in a free-choice preference test [121.

TABLE 1 COMPOSITIONOF DIET 1 Ingredient Soybean Meal (48% protein) Corn Meal (8% protein) Fish Meal (60% protein) Alfalfa Meal (16% protein) Celluflour* Corn Starch HMW Salt Mix Modified* Vitamin Mix No. 740* Choline Dihydrogen Citrate* Corn Oil (Mazola)

39.50 27.00 4.50 3.50 5.02 8.00 5.00 0.89 1.06 5.53 100.00

Protein content Caloric content

24.38% 3.80 Kcal/g

*Supplied by Bio-Serve, Frenchtown, NJ.

breaks of at least 60 seconds duration during which at least 0.2 g of food was consumed. Meal termination was defined by a 10 minute period during which no photocell breaks occurred. As stated before, at this point the disc advanced to the next cup. All animals were allowed ad lib access to the appropriate diet for no less than seven days in an attempt to secure at least five days of data with minimal spillage. The maximum time an animal spent in a condition was ten days. Body weight, water intake, and amount of food consumed from each cup for each diet were recorded daily. Meal size and meal frequency were also measured. Pearson product moment correlations were used to determine intermeal interval, meal size, and meal duration relationships. Correlations were calculated for individual meals and intermeal intervals for each rat on each diet. Separate calculations were done for light and dark periods. RESULTS As can be seen in Table 2, mean meal frequencies, mean meal size, mean meal duration, and mean daily intake while showing an increase between Stages 1 and 2, did not differ systematically with respect to diet. Meal duration and meal size were positively correlated. These correlations for Diet 1 were .56 during the light, and .54 during the dark and for Diet 2 were .67 during the light and .65 during the dark. No consistent pattern of pre- or postmeal-intermeal interval correlations was found using either meal size or meal duration. (Premeal interval-meal size correlations ranged from - . 2 6 to +. 12, postmeal interval-meal size correlations ranged from - . 1 8 to +.14 for both diets.) EXPERIMENT 2

Procedure In Stage 1, six animals were given Diet 1 and six animals were given Diet 2. When sufficient data were collected, the animal's diet was shifted (Stage 2). Meal patterns were recorded using an Esterline-Angus event recorder. A meal was defined as a period of photocell

%

METHOD

Subjects Ten 21-24 day old male, albino Sprague-Dawley rats (Charles River, Wilmington, MA) were used. They were maintained on a 12-12 hr light/dark cycle.

PALATABILITY AND MEAL PATTERNS

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TABLE 2 MEAL PATTERNSFOR RATS OFFEREDTHE TWO DIETS SEQUENTIALLY Variable Mean meal frequency Mean meal size (g) Mean age for data analyzed (days) Mean daily intake (g)

Stage 1: Diet 1

Stage 2: Diet 2

Stage 1: Diet 2

Stage 2: Diet 1

Overall Diet 1

Overall Diet 2

9.48

10.78

10.07

11.56

10.48

10.34

1.22

1.51

1.31

1.61

1.39

1.40

26.3

37.8

26.4

37.2

31.8

30.6

11.57

16.28

13.19

13.61

14.57

14.48

TABLE 3 MEAL PATTERNS AND FOOD INTAKE FOR RATS OFFERED THE TWO DIETS SIMULTANEOUSLY Variable

Diet 1

Diet 2

t-values

Mean meal frequency Mean meal size (g) Mean feeding rate (g/min) Daily intake (g) Mean percent of daily intake

3.60

9.96

9.21t

0.60

1.36

3.16"

0.05

0.15

3.35*

2.25 14.90%

12.88 85.10%

11.23t

tween the diets were computed for meal size, meal frequency, feeding rate, and total intake. RESULTS

*p<0.05, two-tailed. tp<0.01, two-tailed.

Apparatus The apparatus described in Experiment 1 was used. However, each experimental animal had access to two tunnels and discs from their individual cage. Each disc operated independently.

Diets Diet 1 and Diet 2 from Experiment 1 were used.

Procedure Five experimental animals were selected from a group of ten on the basis of lack of food spillage. Animals were allowed ad lib access to the diets for no less than seven days in an attempt to secure at least five days of data with minimal spillage. Body weight, water intake, and amount of food consumed from each cup for each diet were recorded daily. Meal size and meal frequency were also measured. Cups in one disc contained Diet 1 and cups in the other disc contained Diet 2. Thus, the two diets were available simultaneously. Photocell breaks were recorded using running time meters. The same meal definition was used as in Experiment 1. Feeding rate (g/min) was computed. Correlated t-tests be-

As can be seen in Table 3, meal parameters differed when the two diets were offered simultaneously. The approximate ratio of cereal (Diet 1) to Biomix/cereal (Diet 2) was again seven to one. Meal frequency, meal size, feeding rate, and daily intake all showed significant differences, with all being greater for Diet 2 than for Diet 1. In comparison to Diet 1, Diet 2 was eaten approximately three times more often in meals of approximately twice the size and at about three times the rate. The meal frequency, meal size and total daffy intake found for Diet 2 differed little from that found in Experiment 1. The number and size of meals and total daffy intake for Diet 1 however, were far less than the values for either diet in Experiment 1. G E N E R A L DISCUSSION The present investigation was designed to examine the influence of palatability on feeding patterns while avoiding confounding of nutritional and pharmacological factors. Differentially preferred diets which were nutritionallyequivalent in protein, calorie, fat, vitamin, and mineral content and which promoted similar growth and total intake were used. No meal pattern differences were noted when these diets were offered sequentially. However, when the diets were offered simultaneously the preferred diet was eaten more frequently, in larger amounts, and at a faster rate than the less preferred diet. This suggests that when nutritional variables are held constant, palatability affects meal pattern parameters only in a free-choice paradigm when diets are presented simultaneously, not sequentially. The failure to find meal pattern differences during the sequential presentation (Experiment 1) indicates that while preference differences may be necessary to produce differential meal patterns between diets, they are not, in and of themselves, sufficient to yield such results. Therefore, the influence of palatability is not absolute, but depends on the simultaneous presentation of diets. The meal patterns observed for the preferred diet during the simultaneous presentation were similar to those ob-

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SUNDAY, SANDERS AND COLLIER

served for both diets when presented sequentially; whereas, the meal patterns were reduced for the less preferred diet. It appeared that offering the diets simultaneously reduced the appeal of the less preferred diet rather than enhancing the appeal of the preferred diet. This negative contrast persisted without change throughout the experiment. No consistent correlations were found between mealintermeal intervals. This result supports the findings of others (e.g., [8, 9, 10]). The lack of such correlations suggests that a simple, short-term depletion-repletion mech-

anism is not necessarily a major factor in determining meal patterns in freely-feeding rats, although this issue is still controversial [5]. While the present study is not exhaustive, it highlights the importance of minimizing nutritional and pharmacological confounds in the investigation of palatability and its influence on feeding. Further, the method of presentation of the differentially preferred diets (sequential vs. simultaneous) also affects the observed meal patterns.

REFERENCES

1. Collier, G. H. Determinants of choice. In: Nebraska Symposium on Motivation. edited by D. J. Bernstein. Lincoln: University of Nebraska Press, 1980, pp. 6%127. 2. Collier, G. and R. Bolles. Some determinants of intake of sucrose solutions. J Comp Physio! Psychol 65: 37%383, 1968. 3. Collier, G. and E. Hirsch. Nutrient factors in the control of sucrose intake. In: Taste and Development: The Genesis of Sweet Preference, edited by J. M. Weiffenbach. Washington, DC: U.S. Government Printing Office, 1977, pp. 330-344. 4. DeCastro, J. M. Meal pattern correlations: Facts and artifacts. Physiol Behav 15: 13-16, 1975. 5. LeMagnen, J. and M. Devos. Daily body energy balance in rats. Physiol Behav 29: 807-811, 1982. 6. Levitsky, D. Feeding patterns in rats in response to fasts and changes in environmentalconditions. Physiol Behav 5: 291-300, 1970.

7. Levitsky, D. Feeding conditions and intermeal relationships. Physiol Behav 12: 77%787, 1974. 8. Levitsky, D. and G. Collier. Effects of diet and deprivation on meal eating behavior in rats. Physio! Behav 5: 291-300, 1968. 9. Panksepp, J. Reanalysis of feeding patterns in the rat. J Comp Physiol Psychol 82: 78-94, 1973. 10. Premack, D. and W. Kintsch. A description of free responding in the rat. Learning Motiv 1: 321-326, 1970. 11. Sanders, S. Effects of diet variation on meal patterns. Paper presented at Eastern Psychological Association Conference, Boston, 1977. 12. Sanders, S. A meal pattern analysis of two differentiallypreferred, yet nutritionally-equivalent diets. Paper presented at Eastern Psychological Association Conference, Philadelphia, 1979.