Dietary self-selection vs. complete diet: Body weight gain and meal pattern in rats

Physiology&Behavior,Vol.51, pp. 995-999, 1992

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Dietary Self-Selection vs. Complete Diet: Body Weight Gain and Meal Pattern in Rats C H R I S T I A N E L A R U E - A C H A G I O T I S , j C. M A R T I N , P. V E R G E R A N D J. L O U I S - S Y L V E S T R E

Laboratoire de Neurobiologie de la Nutrition, Bdt B. Universit~ Pierre et Marie Curie, 4, place Jussieu, 75252 Paris Cedex 05, France Received 27 March 1991 LARUE-ACHAGIOTIS, C., C. MARTIN, P. VERGER AND J. LOUIS-SYLVESTRE.Dietary self-selection vs. completediet: Body weightgain and mealpattern in rats. PHYSIOL BEHAV 51(5) 995-999, 1992.--Food intake and body weightgain of male adult Wistar rats were examined in two groups of animals. One group (n = 14) was allowedto select its diet from separate sources of protein (casein, 3.1 kcal/g), fat (lard and sunflower oil, 7.9 kcal/g) and carbohydrate (CHO, starch and sucrose, 3.3 kcal/g). Another group (n = 10) received a nutritionally complete diet (3.3 kcal/g). After 2 weeks of adaptation to the diets, body weights and meal patterns were recorded for at least 4 days. The total caloric intake was nearly identical for the two groups of rats. Rats given dietary choice gained lessweight over 4 days than rats fed chow and showed reduced feed efficiency.During the 24-h period, self-selectingrats consumed 20.8% of calories as proteins, 21% as fats and 58.2% as CHO. Self-selectingrats ate significantlyless calories during the day than did rats given chow. The chow diet consisting of 17.3% calories as protein, 7.6% as fat and 75.1% as CHO. When comparing the self-selectinggroup nutrient intakes to those of chow-fed group it was observed that 24-h protein calorie intakes were identical in both groups. Fat intake was significantlyhigher and CHO reduced as compared to chow-fed rats. During the day, CHO intake was higher in self-selectingrats, and fat intake was not significantlyreduced. During the night, protein and fat intakes were significantlyhigher in self-selectingrats, while CHO intake was significantlydecreased, particularly in the last periods of the night. The distribution of food intake over the nycthemeral cycle could be responsiblefor the difference in body weight gain. It could be hypothesized that the specific needs of the rat varied across the nycthermeral cycle. Rats fed a standard diet ingest the complete diet in order to fulfillparticular nutrient requirements at different times of the day. Dietary self-selection

Feedingpatterns

Chowdiet

Body weight gain

rats fed a nutritionally complete diet were heavier than two experimental groups allowed to select from separate sources of protein, fat, and carbohydrate. This weight difference was attributed to the casein being the protein source. An identical observation was made by Elkin et al. (l) on chicks. Self-selecting chicks were able to separately regulate their protein and energy intake, but exhibited lower weight gains and/or poor feed conversion values as compared to control animals fed lab chow. Several years later Tempel et al. (17) observed that rats, under normal laboratory conditions, exhibited specific temporal patterns of food intake and macronutrient selection that varied across the nycthemeral cycle. It could be hypothesized that if the specific needs of the rat varied across the nycthemeral cycle, rats fed a standard chow ingested the complete diet in order to fulfill particular nutrient requirements at different times of the day. In the present work, two groups of rats were used. They were fed either one nutritionally complete food (chow) or three separate sources of protein, fat, and carbohydrate. The same ingredients were used for the complete diet. The total caloric intake, body weight gain, and meal patterns were compared in selfselecting and chow-fed rats over the same period.

IN early studies examining nutritional needs of laboratory animals, Richter (l 3) observed that rats allowed to select their own diets from pure nutrients chose those that led to normal growth and development. On the other hand, Kon (5) reported that given a choice of sucrose, casein, fat, and a salt mixture offered separately, rats did not thrive as well as when the substances were offered in a single composite mixture. Failure of animals to gain body weight (b.wt.) on self-selection regimes appears to result from insufficient protein intake (4). The proportion of daily energy ingested as protein varied as a function of the type of protein used in the diet. Only 30% of rats would not eat protein when either casein, lactalbumin, or fibrin were used; when all sources were offered simultaneously, only 10% of the animals failed to maintain an adequate intake of protein (4). It was also observed that rats given casein consumed more protein and gained more weight than animals given lactalbumin (3,4). In a first pilot experiment, we observed that rats fed lab chow gained more weight than self-selecting rats which were of the same weight at the beginning of the study. The difference in weight was not related to food intake. The chow protein sources of the lab chow were fish (3%), soya (l 3. 1%0),and cereal (l 1.2%). Comparable results have already been reported (18): Control

Requests for reprints should be addressed to C. Larue-Achagiotis.

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996

I~ARUE-ACHAGIOI'IS El .~1 METHOD

Animals Male adult Wistar rats were used. Animals were housed individually and placed in quiet temperature-controlled rooms (23 +- I°C) with a 12-12 hour light-dark cycle (lights on: 1800). Rats in the chow-fed group (n = 10) were housed in Plexiglas cages. They had access to water and food (powdered diet) at all times. The feeding patterns were continuously recorded on chart paper by means of pens connected to electric strain-gauge microbalances. Rats in the self-selection group (n = 20) were provided with separate sources of the three macronutrients: Protein, fat, and carbohydrate (CHO). Intakes of the three foods were recorded separately (one chanel for each item). Le Magnen and Devos (10) showed that rats are meal eaters: on the basis of a statistical analysis of feeding patterns, a meal is generally defined as an eating episode following and preceding at least 40 rain of no eating. This criterion was used in the present study.

Diets The chow diet (metabolizable energy, 3.31 kcal/g) was composed of 17% protein (casein), 3% fat (lard), 70% CHO (corn starch), 4% cellulose powder, 5% salt mixture, and 1% vitamins; the composition was identical to the chow (UAR, AO4) typically used in the laboratory. The carbohydrate component of the self-selection diet (metabolizable energy, 3.34 kcal/g) was composed of 85% corn starch, 8% commercial grade sucrose, 2% powder cellulose, 4% salt mixture, and 1% vitamins. The protein component (3.12 kcal/g) consisted of 93% casein, 2% cellulose powder, 4% salt mixture, and 1% vitamins. The fat component (7.88 kcal/g) contained 91% lard, 2% sunflower oil, 2% cellulose powder, 4% salt mixture, and 1% vitamins. The protein and CHO diets were in powdered form, the fat diet was semisolid.

Procedure Chow-fed rats were allowed 1 week to adapt to the new complete diet. Thereafter, their feeding patterns were continuously recorded for I week. The last 4 days were considered for statistical analysis. Self-selecting rats were allowed to adapt to the diets for a 2week period. The three foods were placed in separate food cups anchored in front of stainless steel cages. The position of the foods were changed daily in a random fashion. Water was available ad lib. Following the 2-week adaptation period, rats were placed in square Plexiglas cages equipped with three weighing food cups in order to record the intake of the three foods (protein, fat, CHO) simultaneously. Food intake was recorded for 1 week, and the last 4 days were used for statistical analysis. It should be emphasized that the present method did not require any manipulation of the food cups except refilling them once a day (0900-1000). It has been observed that the introduction of food at any time triggers some feeding activity in rats; food cups were refilled only at the beginning of the light period in order to prevent interference at the start of the dark period when feeding activity is usually intense. Nocturnal intakes were analyzed in terms of 3-h cumulative intakes in both groups. Daytime intakes were analyzed as one period (0-12 h). Another hour-by-hour analysis was conducted over the 24-fi cycle. Body weight was recorded every day at the same hour (09001000) for the two groups of rats.

&ati,stical Analvsi,s Differences between groups were assessed using studenfs ltests. In the self-selection group, macronutrient selection during different periods was compared using a one-way analysis of variance for repeated measures followed by Dunnett tests. RESUI_TS

Some animals (n = 5) in group 2 failed to maintain body weight due to inadequate nutritional intake during the 2-week habituation (reference period). Because of the large difference in weigh as compared to other animals, those rats were excluded. One rat was excluded afterwards due to spillage of the CHO diet. Spillage in the other animals was minimal due to the food cups used and described earlier (7). So, 14 rats were maintained.

Body Weight and 24-Hour Intake Rats of both groups had the same b.wt. at the beginning of the experimental period (299.1 _+_4.3 g in self-selection group versus 296.1 +_ 3 g in chow-fed group). Body weight gains over the 4 days were significantly higher for the chow-fed group (22.6 +_ 1.2 g) than for the self-selection group (17.4 + 1.7 g; t = 2.61. p < 0.05). Caloric intakes over these 4 days were nearly identical for the self-selection group (303.8 + 9.5 kcal) and the chow-fed group (305.5 -+ 5.9 kcal). Thus, feed efficiency was significantly lower for the self-selection group (56.7 _+ 5.2 mg b.wt. kcal) than for the chow-fed group (73.8 _ 3 mg b.wt./kcal; t = 2.56, p < 0.05). In the self-selection group no correlation was observed between b.wt. gain over the 4 days and caloric intake over any period of the nycthemeral cycle (total 24-h intake, day and night intakes, and intake in the 4 phases of the night). In the chow-fed group, the 4-day b.wt. gain was significantly correlated with total caloric intake (r = 0.69, p < 0.05) and more selectively with caloric intake over the last period of the night (r = 0.68, p < 0.05). Intakes during other night- or daytime periods were not significantly correlated with b.wt. gain.

Dark-Light Feeding Pattern Rats of the self-selection group consumed only 10.9% of their total caloric intake during the light part of their nycthemeral cycle, while rats of the chow-fed group consumed about 18% of their total daily intake during the same period. This difference was statistically significant (t = 2.18; p < 0.05). In contrast, selfselecting rats ate more during the night than did chow-fed rats, but this difference did not reach statistical significance. Finally, self-selecting rats ate significantly less than chow-fed rats in the last part of the night, F(3, 27) = 4.5, p < 0.05.

Sel.l:Seleeting Group--Macronutrient Selection Over the 24 h, self-selecting rats consumed approximately equal amounts of calories as protein (20.9 + 1.7%; range: 10.6% to 31%) and fat (21.8 _+ 3.1%; range: 0.9% to 40.5%), while CHO constituted the major part of the diet (57.3 + 3.5% ranging from 41 to 87.5%) (Table 1; Fig. 1). A significant inverse relationship appeared between the amounts of protein (kcal) and CHO consumed (r = -0.57, p < 0.05) and between lipid and CHO intakes (r = -0.84, p < 0.05). During the light period, CHO calorie intake was higher than protein and fat intakes, but variability from rat to rat was large. Protein intake was significantly lower than fat intake. During the night, CHO intake was higher than protein and fat intakes. At the beginning of the night (0-3 h) rats consumed

SELF-SELECTION VS, S T A N D A R D DIETS

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that four rats behaved differently from the group. A statistical analysis in the other l0 rats showed a significant increase of protein intake from the beginning to the end of the night, F(3, 27) = 7.21,p < 0 . 0 1 : 0 - 3 h: 2.3 kcal, 3-6 h: 3.3 kcal, 6-9 h: 3.7 kcal, 9-12 h: 4.8 kcal. Fat intake did not vary significantly over time during the night periods, F(3, 39) = 0.06, NS. However, the A N O V A revealed a significantly variability between rats, F(13, 39) -- 3.93, p < 0.05. A more detailed, hour-by-hour analysis of feeding patterns (Fig. 2) showed a higher total caloric intake during the second hour of the night that was significantly different from other values, F ( l l , 143) = 5.24, p < 0.05. This increased intake was mainly due to a significantly augmented C H O intake, F(11,143) = 4.4 l, p < 0.05; increased fat intake was not significant. During the last hour of the night rats ate less and consumed the same a m o u n t of calories as protein (29.8%), fat (36.7%) and C H O (33.5%). During the daytime, the small increase in total caloric intake at 9 - l 0 h was due to the refilling of food cups (rats sampled the fresh foods).

TABLE 1 MEAN CALORIC INTAKE (kcal) IN SELF-SELECTING AND CHOW-FED RATS Total 24 h Self Chow Day Self Chow Night Self Chow Night: 0-3 h Self Chow Night: 3-6 h Self Chow Night: 6-9 h Self Chow Night: 9-12 h Self Chow

NS 76.0 +_ 2,4 76.4 + 1.5 * 8,2 -+ 1.8 13.7+ 1.6 NS 67.8 _+ 3.1 62,7___2.1 NS 20,6 _ 1.5 18.3-+0.8 NS 17,1 _+ 1.4 17.5 _+ 1.8 NS 15.1 + 1.0 14.3 + 0.9 NS 15.0 + 1.8 12.6_+ 1.1

Protein

Fat

CHO

NS 15.9 + 1.4 13,3 ___0.3 1" 1.0 _+ 0.3 2.4+0.3 i 14.9 _+ 1.2 10.9-+0.4 NS 3,4 + 0.7 3.2+0.1 NS 3.3 + 0.4 3.0 + 0.3 t 3.8 + 0.4 2.5 + 0,2 $ 4.3 + 0.4 2.2+0.2

* 16.6 _ 3.5 5.9 + 0.4 NS 2.4 _+ 0.7 1.1 +0,1 t 14.1 _+ 2.4 4.8+0.2 NS 3.6 -+ 0.9 1.4_+0.1 * 3.4 _+0.7 1.3 _+ 0,1 t 3.5 -+ 0.6 1.1 _+ 0.1 * 3.7 -+ 0.8 1.0 -+ 0.1

~t 43.5 _+ 2.7 57.3 _+ 1.1 :~ 4.8 _+ 0.2 10,3_+ 1.2 1" 38.7 _+ 2.1 47.1 _+ 1.6 NS 13.5 -+ 0.7 13.7_+0.6 NS 10.4 _+ 0.9 13.1 _+ 1.4 "~ 7.8 -+ 0.8 10.8 _+ 0.7 NS 6.9 ___ 1.2 9.4_+0.8

Feeding Patterns Rats began to eat within 27 + 7 rain after lights-off in the self-selection group, and 52.2 _+ 5.3 rain in chow-fed group (t = 2.83, p < 0.05. During the night, meal number was significantly smaller in the self-selecting group (5.4 _+ 0.2 vs. 6.45 -+ 0.3 in the chowfed group; t = 2.96, p < 0.01), while meal size was significantly higher (12.9 _+ 1 kcal vs. 9.8 + 0.4 kcal, t = 2.53, p < 0.05). In self-selecting animals, 301 nocturnal meals were observed, of which most were composite meals; 19% were C H O only, 0.7% fat only, and 2.7% protein only. The first meal of the night (16.3 + 2.3 kcal) lasted 74.7 _+ 9 min and consisted of significantly more C H O (62.1%) than protein (17.9%) or fat (20%). The second meal was 12.7 _+ 1.7 kcal lasted 58.5 + 6.5 rain and consisted of C H O (57.6%), protein (19.2%) and fat (23.2%). It is noteworthy that the first meal of the night was different among individual rats: Four rats had a small C H O meal (7.7 + 1 kcal), three rats consumed a huge meal (more than 20 kcal) alternating between

* p < 0.05. i'p < 0.01. ~;p < 0.001.

nearly four times as much C H O as protein or fat. This 3-h time period accounted for 31% of the total 24-h C H O intake. Carbohydrate intake declined slowly from the beginning to the end of the night, F(3, 39) = I l.Sl, p < 0.01. Protein intakes were not different over the 4 periods of the night. However, there was a nonsignificant increase at the end of the night. It was observed

NIGHT

Kcal

DAY

L

14-

1 PROTEIN I~1 FAT

12-

CARBOHYDRATE 10 8<

6i ~ ~ ii

O-3h

3-6h

6-9h

9-12h

0-12h

FIG. 1. Mean caloric intake from protein, fat and carbohydrate during the 4 periods of the night and during the day, on the self-selection regime (n = 14 rats).

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LARUE-ACHAGIOFIS EI AI,

Kcal

but the other eight rats had a significant postprandial correlation (range r = 0.749 to r = 0.877).

10-

Macronutrient Intakes in Self Selecting and Chow-Fed Groups

CARBOHYDRATE

The chow diet consisted of 17.3% calories as protein, 7.6% as fat and 75.1% as CHO. When nutrient intakes (expressed in kcal) in the self-selection group were compared to those of the chow-fed group (Table 1), it was shown that the 24-h protein calorie intakes were identical. The 24-h fat calorie intake was significantly higher and CHO calorie intake significantly reduced in the self-selecting as compared to chow-fed group. During the daytime, in the self-selecting group, protein consumption was significantly smaller and CHO intake higher than in the chowfed group; fat intake was not significantly reduced. Over the nighttime, in the self-selecting group, protein and fat intakes were significantly higher while CHO was significantly decreased as compared to chow-fed animals. Moreover, self-selecting rats ate significantly more protein in the last two periods of the night; fat intake was significantly higher in the last three periods of the night; CHO intakes were the same in the first period and were decreased in the other three periods of the night.

ImB FAT PROTEIN

6-

DISCUSSION

0

3

6

NIGHT

9

12

6

DAY

12

hours

FIG. 2. Mean hourly caloric intake from protein, fat and carbohydrate on the self-selection regime (n = 14 rats). *Significantly different from the other night values.

CHO, fat, and protein several times. The second meal was small if it followed a large one, or large if it followed a small one. Most rats began their first night meal with CHO (86.4%), very few with fat (6.8%) or protein (6.8%). Moreover, most nocturnal meals began with CHO (71.4%), a few with protein (19.2%) or fat (9.4%). The size of the last meal of the night was 12.1 ___ 1.2 kcal. It consisted of protein (31.1 + 5.1%), fat (20.7 + 3.1%), and CHO (48.2 + 4.3%). The last meal contained more protein (in %) and less CHO (in %) than the first meal of the night, while fat content remained unchanged. There was no significant correlation between the size of this last meal and the 4-day b.wt. gain of each rat; however, a negative trend (r = -0.43, NS) was noted. A significant negative correlation between the protein content of the last meal and the b.wt. gain was observed (r = -0.54, p < 0.05). During the daytime, meal number was significantly smaller in the self-selection versus chow-fed groups (1.8 + 0.3 vs. 2.8 + 0.3; t = 2.15, p < 0.05). Meal size was slightly reduced (3.8 +_ 0.5 kcal vs. 4.8 + 0.28 kcal; t = 1.61, NS) but the difference was not significant due to the great variability of intake in self-selecting rats. Two rats of the self-selection group never ate during the day. Only four out of 12 rats had a meal including the three diets, and this happened during the last part of the day. During the night, the correlation between the size of a meal and the postmeal interval [postprandial correlation; (10, ! 1)] was highly significant in all rats of self-selecting group (range: r = 0.479 to r = 0.912). No preprandial correlation appeared. In chow-fed rats, the correlation was not significant in two animals,

Adult male Wistar rats maintained on either a chow diet or a self-selection diet demonstrated similar total caloric intakes but different body weight gain over the observation period. A significant increase in feed efficiency was noted in chow-fed rats. Moreover, differences in night and daytime calorie intakes were significant between the two groups of rats. Over the 24 h, self-selecting rats choose a large part of calories as CHO (58%) and identical percentages of protein and fat. Our results agree with the pioneer study of Richter (13). However, in the Tempel et at. study (17), rats distributed their daily caloric intakes more equally than did our rats, each nutrient representing a third of daily calories. In the present study, self-selecting rats ate small amounts of calories (10.9%) during the day. The eating patterns in the light phase were highly idiosyncratic. Rats mostly ate CHO (58%) and only 12.2% protein and 29.2% fat. This contrasts with what was reported by Tempel et al. (17): their rats were eating more protein and fat than CHO during the light period. The differences observed could be due to the fact that rats were of a different strain (Sprague-Dawley) although of identical body weight (16). During the night, self-selecting rats exhibited a high preference for CHO and ate the similar amounts of fat and protein. The observation that self-selecting rats ate very little during the light phase suggests that their nutritional needs were efficiently satisfied by the self-selection diet: consequently only very modest needs for specific nutrients led the animals to complete their daily requirements during the daytime. Rats allowed to self-select dietary components regulate their intake of protein and energy independently (2). It has been previously observed (2,12) that protein intake was not precisely controlled but could be allowed to fluctuate between lower and upper limits without any adverse consequences for the rat. In the present study our animals ate at least 10.6% and at the most 30% of their calories from protein, and this agrees with previous observations. Precise regulation of CHO and fat was not observed; they are fuel nutrients. Animals can use CHO, fat and protein interchangeably. However, they preferentially used CHO at the beginning of the night. It has been shown that norepinephrine (NE) level is high at the beginning of the dark phase and that insulin (11) and corticosterone (6) peak at the onset of the dark phase. These three hormones facilitate the intake of

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energy-rich nutrients (8). In contrast, serotonin appears to suppress CHO intake while sparing protein intake. The progressive decrease in CHO intake during the night likely reflects the increasing influence ofserotonin. Serotonin and NE interact closely in the control of energy and nutrient balance. Through this interaction, they affect not only intake of a specific macronutrient but also modulate the ratio as well as the time course of nutrient intake. The feeding patterns reported here are at variance with those of Tempel et al. (17). In our hands, fat was never consumed alone, and no increase in fat intake appeared at the end of the night. The differences in strains between the Tempel et al. (17) study and ours seems also to account for fat intake difference (16). The fat sources in the two studies were identical. This is the first time that a significant postprandial correlation has been demonstrated during the night in self-selecting rats, using the meal criterion proposed by Le Magnen and Devos (10) for rats eating a monotonous chow diet. This correlation indicates that rats are satiated for a period of time which is a function of the amount eaten in the preceding meal. Since no preprandial correlation was noted, meal size appeared to be a function of the sensory qualities of the food (9,14). The larger meals in selfselecting animal leads to longer satiation and, therefore, fewer meals. These various observations are totally consistant with the patterns reported in rats eating a monotonous diet ( 10, i 1). The 40-min criterion for meal definition thus appears a useful tool for identifying physiological mechanisms under various meal conditions. What could explain the difference in b.wt. gain between the 2 groups of rats? They ingested the same amount of calories. Diets were composed of the same items, the protein used was the same. Energy expenditures are unlikely to be the source of the low feed efficiency in self-selecting rats. This observation cannot be attributed to thermogenesis: self-selecting rats consumed more fats and less CHO, and diet-induced thermogenesis is lower with fats. The difference in b.wt. gain cannot be attributed to motor activity: if the difference in weight gain was due to activity, then according to the equation of Taylor et al. (15),

such activity should be equivalent to running continuously at 38 m/h (at least) for the whole 4-day period. Such a difference in activity seems unlikely in rats housed in identical cages. Excretion (feces and urine) cannot be responsible for b.wt. difference, it could be due to a poorer enzymatic and digestive preparation in self-selecting rats, but this hypothesis seems improbable since chow and choice diets were prepared with the same components. The variation in b.wt. gain can perhaps be attributed to less lipolysis occurring during the daytime in the chow-fed group. The day/night ingestion ratio was higher in the chow-fed group. It could be hypothesized that chow-fed rats are obliged to eat the whole diet in order to fulfill their protein needs at the end of the night; so doing they increased their CHO intake in excess of their needs. As a result, in order to meet their 24-h protein requirement rats eat during the day and, thus, ingested the whole diet. In the self-selecting group an inverse relation between the protein content of the last meal of the night and the weight gain was observed. In the chow-fed group there was a positive correlation between weight gain and food intake (and, therefore, protein) of the last period of the night. Moreover, self-selecting rats ate more protein at the end of the dark phase and significantly more than the chow-fed group. These differences in protein intakes at the end of the night could be responsible for the lower feed efficiency of self-selecting rats and the higher daytime intake in chow-fed rats. The present results emphasize the limitations of dietary studies in which animals are allowed access to a single nutritionally complete food. Although rats were of the same strain, same weight, and purchased from the same supplier, there was a great variability between animals when placed on a self-selecting diets. Individual differences are an important topic for further study. ACKNOWLEDGEMENTS The authors are grateful to Mrs. M. T. Lanteaume and Mr. J. Blet for preparing the various diets. We thank France Bellisle for her kind review of the manuscript and contribution to the Englishcorrection and Adam Drewnowski for the final revision.

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