- Email: [email protected]
Dietary Self-Selection and Meal Patterns of Obese and Lean Zucker Rats
Appetite: Journalfor Intake Research, 1983,4,281-293
Dietary Self-selection and Meal Patterns of Obese and Lean Zucker Rats MELVIN P. ENNS and JOEL A. GRINKER St. Lawrence University, Canton and The Rockefeller University, New York
Following two weeks of baseline measures on laboratory chow, young obese and lean male Zucker rats were given access to separate macro nutrients for 18 weeks. First, the rats were given access to soybean meal, dextrinized starch, and lard for eight weeks.Daily caloric intake of obese rats was greater on laboratory chow than that oflean rats and remained so following the switch to the self-selectiondiet. While obese and lean rats consumed similar proportions of each macronutrient over the eight-week period, there were substantial changes in the pattern of intake across weeks. Obese rats decreased daily caloric intake primarily by decreasing lard consumption. In contrast, lean rats maintained daily caloric intake while increasing the proportion of calories from fat. The addition of a 25% sucrose solution (weeks 11-18) produced an elevated daily caloric intake for both obese and lean rats, achieved through an increased carbohydrate intake with a simultaneous decrease in fat intake. A diurnal pattern of intake was maintained for each macronutrient. Compared to controls, rats on the self-selection diet did not show differential growth. Measurement of daily meal patterns for soybean meal, sucrose, and lard du;ing the last four days of the experiment showed that obese rats ate significantly larger meals comprised of all three food components than did lean rats. For both obese and lean rats, one-component meals were primarily sucrose with greater intake during the light period. Lean rats showed a greater tendency to indulge in these between-meal "snacks" than did obese rats. These data suggest that palatability and nutrient source as wellas length of exposure are critical determiners of nutrient selection and total daily caloric intake.
The obesity of the Zucker rat is associated with metabolic and behavioral alterations including overeating and underactivity (Cleary, Vasselli & Greenwood, 1980; Enns, Wecker & Grinker, 1982; Grinker, Drewnowski, Enns & Kissileff, 1980; Radcliffe & Webster, 1978; Stern & Johnson, 1977). Some investigators have suggested that the hyperphagia seen in both genetically obese male and female Zucker rats is related to a protein deficiency or need. The primary evidence for this hypothesis has come from studies in which the genetically obese rat tolerates a greater percentage of its diet as protein (casein) than the lean rat (Radcliffe & Webster, 1976; 1978). However, other investigators have reported that obese and lean female Zucker rats allowed to feed ad libitum from either of two food cups differing in protein concentration (10 or 60% casein plus carbohydrate and fat) eat nearly identical- amounts of protein while consuming excess calories from the carbohydrate and fat sources (Anderson, Leprohon, Chambers & Coscina, 1979). In contrast, it has been reported that the adult obese male Zucker rat, allowed to self-select dietary intake from three constituents, This research was supported in part by NSF Grant BNS 7609957 and NIH Grant AM 27980. We would like to thank Allison Cohen, Kris Neth and Jana Spalding for their excellent technical assistance. Portions of this research were reported at EPA, 1980. Current address of Joel A. Grinker: Human Nutrition Program, School of Public Health, The University of Michigan, Ann Arbor, MI 48109-2029, U.S.A. 0195-6663/83/040281 + 13 $03·00/0
© 1983 Academic Press Inc. (London) Limited
282
M. P. ENNS AND J. A. GRINKER
consumes a greater proportion of its diet as fat (corn oil) and a lower proportion of its diet as protein (casein) than the lean rat (Castonguay, Hartman, Fitzpatrick & Stern, 1982). The duration of the observation periods, the age of the rats, and especially the specific dietary constituents may account for the differing results reported in these studies of the obese Zucker rat. For example, it has been noted that the self-selection pattern of the developing Sprague-Dawley rat differs from the pattern of the adult Sprague-Dawley rat. In the developing lean rat, percent daily caloric intake offat (corn oil) and carbohydrate vary in an inverse manner across days, while percent daily caloric intake of protein remains constant (Sunday, unpublished). In the adult lean rat given protein, fat, and carbohydrate separately as semi-synthetic gels, daily caloric intake of fat shows day-to-day irregularities while daily caloric intake of protein and carbohydrate remain constant (Piquard, Schaefer & Haberey, 1978). However, both young and mature rats can regulate total caloric intake independently from protein intake (Johnson, Li, Coscina & Anderson, 1978; Musten, Pearce & Anderson, 1974; Rozin, 1968). The present experiment examined the consequences of a self-selection paradigm in obese and lean Zucker rats. Daily food intake, developing shifts in dietary self-selection, contributions of dietary components differing in palatability, and composition as well as the partitioning of total intake into separate meals were analyzed. Examinations of meal patterns have proven useful in the analysis of the circadian distributions offeeding (Becker & Kissileff, 1974; Kissileff, 1970), the consequences of pharmacological agents (Blundell & Latham, 1978;Grinker et al., 1980) and the metabolic correlates offeeding patterns (DeCastro & Balagura, 1975; LeMagnen & Devos, 1970), as well as the purely descriptive analysis of the feeding patterns of animal models of obesity (Becker & Grinker, 1977; Becker & Kissileff, 1974; Sclafani & Berner, 1976; Strohmayer & Smith, 1979).We now show that meal pattern analysis can also provide useful information on the patterning and consumption of individual macronutrients. METHOD
Subjects
The experimental subjects were seven obese (fa/fa) and seven lean (Fa/-) experimentally naive 45-day-old male Zucker rats obtained from the Harriet Bird Memorial Laboratory, Stow, MS. The control subjects were an equivalent group of seven lean, experimentally naive male Zucker rats. The body weights of the obese and lean rats were 191·9 g (8'9) (mean ± SEM) and 172·1 g (2'8) respectively. The animals were housed individually in a temperature (22 ± 1°C) and humidity (55 ± 2%) controlled room maintained on a 12: 12 light/dark schedule (5 a.m.-5 p.m.). Procedure
Upon arrival in our laboratory at St. Lawrence University the rats were given ad libitum access to a standard laboratory chow diet for approximately two weeks (Charles River Rat Chow, Agway Inc., Syracuse, NY: 22% protein, 5% fat, 51'5% carbohydrate by weight) (Phase I: two weeks). Next the rats were switched to a diet which allowed self-selection of dietary constituents (Phase II: eight weeks). The diet included soybean meal (Agway Inc., Syracuse, NY: 48% protein, 30% starch, 0'5% fat), lard (commercial: unsalted), and dextrinized starch (National Starch and Chemical Co.,
DIET SELF-SELECTION IN GENETIC OBESITY
283
Bridgewater, CT.). Each component was in a separate Wahmann non-spill food cup (Wahmann Mfg. Co., Timonium, MD.) and supplemented with a 2'2% vitamin diet fortification mixture (Kratz & Levitsky, 1979),4'0% USP XIV salt mixture and 0·15% DL methionine (Collier, Leshner & Squibb, 1969)(Nutritional Biochemical, Cleveland, OR.). Vitamins, minerals and DL methionine were placed in each nutrient to equate for taste. Thus, the intake of each dietary component resulted in the consumption of all dietary essentials with the exception of those under investigation. Lastly, the diet was altered by the addition of a palatable carbohydrate (25% sucrose solution, wt/vol) (Phase III: four weeks). During each phase, food intake, water intake, and body weight were recorded on alternate days. Twelve-hour intakes of each dietary component, recorded at the beginning of the light (5 a.m.) and dark (5 p.m.) periods, were obtained on Days 33-36 in Phase II and Days 23-26 in Phase III. Intake of each dietary component was converted from grams to kcal when evaluating caloric intake (soybean meal: kcal = g x 2·89;dextrinized starch; kcal = g x 3·35;lard: kcal = g x 8·43;sucrose: kcal = g x 4·00). At the termination of Phase III, the rats were transferred to our laboratory at the Rockefeller University. After an additional five to six weeks while the rats were maintained on the diet given in Phase III, feeding patterns were examined in detail over a four-day period. The rats were housed individually in a temperature (20 ± 1°C) controlled room maintained on a 12: 12 light/dark schedule (5 a.m.-5 p.m.). Using an on-line PDP-8 computer, every lick of solid or liquid food was recorded 24 h daily via low-amplitude "drinkometers" or solid food "eatometers" (see Drewnowski & Grinker, 1978; Strohmayer, Silverman & Grinker, 1980 for a complete description). For data collection purposes, a bout offeeding was defined as a minimum of ten responses in 1 min, separated by 2 min without feeding. Three dietary components (lard, sucrose and soybean meal) were available to the rats during this four-day period. The rats were serviced during a I-h period beginning 3 h before the onset of the dark cycle. The elective pattern of the various macronutrients over 24 h was examined by dividing the total daily food intake into discrete meals using a IS-min intermeal interval (IMI). A IS-min IMI has been used previously in our laboratory (Drewnowski & Grinker, 1978;Grinker et al., 1980)for describing the feeding patterns of obese Zucker rats and the data are qualitatively similar to data obtained with a 2-min IMI. The use of a IS-min IMI is also consistent with results reported by Kissileff (1970). The IS-min IMI was used to partition the daily food intake into three-, two- and one-component meals. A three-component meal was defined as the consumption of each of the three dietary constituents with no separations of over 15 min between successive bouts of feeding. Of the remaining bouts of feeding, a two-component meal was defined as the consumption of each of two dietary constituents without a separation of over IS-min between successive bouts. Finally, a one-component meal was defined as eating without a separation of over 15 min between successive bouts (15min IMI). The major statistical analyses were analysis of variance tests with repeated measures over days. RESULTS
Food Intake over 24 h
Obese rats consumed significantly more laboratory chow per day than lean rats (Phase I; Days 11-14; gena": F(I, 12)= 20·8, p <0·01). Neither obese nor lean rats altered
284
M. P. ENNS AND J. A. GRINKER
daily caloric intake immediately following the switch to a self-selection diet (Phase II: Days 1--4). There were, however, significant shifts in the calories consumed as protein, carbohydrate and fat when the rats began self-selecting dietary components. Both obese and lean rats decreased the daily caloric intake of protein [F(1,12)=25'51, p
access to Charles river rat chow or a self-selection diet (soybean meal, dextrinized starch, lard) Protein
Total -
Obese PhI: Chow (Days 11~14) PhIl: Self-selection (Days 1--4) Lean PhI: Chow (Days 11-14) PhIl: Self-selection (Days 1--4)
_
.
_
.
~
-
~
Carbohydrate
Fat
- - - -
111·3 (13-6) 116·8 (20'1)
28·9 (4'9) 22·1 (5'0)
67-6 (11-4) 26·0 (5'5)
14·8 (2'5) 68·6 (17-4)
86'6 (6'0) 80·6 (13-6)
22'5 (1-6) 20·5 (3-6)
52-6 (3'7) 23·8 (7'7)
11'5 (0'8) 36·3 (19'8)
a The means represent the average of the last four days of chow and the first four days of selfselection.
Over the total eight-week period of Phase II, the obese rats ate significantly greater absolute amounts of soybean meal [geno: F(1,12)=21'6, p geno x days: F(23,276) = 2,75, p
DI ET SELF-S ELECTION IN GENETIC OBESITY
285
T ABLE 2 Mean (SEM) percentage of daily calories (kcal) consumed as protein, carbohydrate, and fa t of obese and lean Zuck er rats allowed to selfselect dietary components in Pha se Il (soybean meal, dextrinized starch, lard) and Phase III (soyb ean meal, dextrinized starch, lard, sucrose)
% Protein
% Fat
% C ho
23-2 (1-2) 21-5 (0-9)
48-4 56-2 (2-1)
28-4 (3-0) 22-3 (1-4)
18-4 (0-6) 14-5 (0-5)
15-8 (2-5) 19-5 (2-1)
65-8 (H) 66-0 (2-3)
A_ Phase II (eight weeks) Obese Le an
B_ Phase III (four weeks) Obese Lean
(3-8)
Obese (follo l A _PHASE D
.,..,
o .~ u
.~
"8
o Lord
Leon (Fo/-)
00-S t ordl
o ~~~ G,J 2
10
'9
26 34 42
Days
2
10
0
Soybeon
aCHO Prote in
SucrosewI/va il (25%
19
Days
FIGURE 1. Mean (SEM) 24-h cal o ric intakes (kcal) of each diet component for obese (fa/fa) a nd lean (Fa/-) male Zucker rats in Phase II (soybean meal, dextrinized starch, lard) and Phase III (soy bean meal, dextrinized sta rch, lard, 25% sucrose wt /vol l,
were eat ing significa ntly more protein [t(12)=2-81, p <0-05] and carbohydrate [t(12 ) = 3-22, p < 0-01] and slightly less fat [t(12) = -I -53, N] than lean rats (see Figure 1, A]. During the first four days of Phase III , when the sucrose solution was given as an additional choice in the self-selection diet, dail y sucrose consumption was 35-2 (3-6) kcal for ob ese rats and 29-6 (3-3) kcal for lean rats [t(12) = 1-17, NS]. In comparison with levels ob served immediately preceding the introduction of the sucrose solution (Phase II: Da ys 53-56), there was a significant decrease in lard intake [lard: F(l,12) = 128-49, p < 0-0I] for both obese and lean rats , and a significant decrea se in starch intake
286
M. P. ENNS AND J. A. GRINKER
(starch-geno x diet: F(l,12)= 7'59, p <0,01) for obese rats. These reductions were not sufficient, however, to prevent both obese and lean rats from increasing daily caloric intake [total cals: F(1,12) = 25A1,p <0'01]. The addition ofthe sucrose solution did not significantly alter the intake of soybean meal for either obese or lean rats [see Figure 1, B]. Thus, when compared with the intake of Phase II (Days 53-56), the additional calories from carbohydrate (sucrose) (Phase III: Days 1--4) replaced calories from fat (lard) and/or carbohydrate (dextrinized starch) but not from protein (soybean meal) for obese and lean rats. 70 ~ 60
.5
~
(b)
50
u 40 'g "8 30
c:
(a)
20 10
:: ::
~~ ~~
~ :::
DL
DL
DL Lean Ooese
~
f:~
o Lard ~ Sucrase (25%)
$ o~ D-Storch Soybean Meal
DL
0 : Dark; L : Light
Lea n Obese
FIGURE 2. Mean 12-h caloric intakes (kcal) of each diet component for obese (fa/fa) and lean (Far-) male Zucker rats sampled from four-day blocks in (a) Phase II (soybean meal, dextrinized starch, lard) and (b) Phase III (soybean meal, dextrinized starch, lard, sucrose 25% wt/vol),
Over the four-week period of Phase III, obese rats ate significantly greater amounts of soybean meal than did lean rats [geno: F(1,12)= 25,11, p
In Phase II, all rats consumed more calories in the dark than in the light (dark: obese = 64%; lean = 73%). This diurnal pattern was evident with all dietary components [see Figure 2(A)] [diurnal: F(1,12)-total cals=60'1, p
The daily intake of water, when only laboratory chow was available (Phase I: Days 11-14), was greater for obese [44'3 (3'4) ml] than for lean rats [33'2 (1-1) ml] [t(12) =3'14, p<0'05] reflecting the greater food intake by obese rats. Daily water intake
DIET SELF-SELECTION IN GENETIC OBESITY
287
during Phase II of self-selection was 37·8 (1'6) ml for obese and 23·9 (1-4) ml for lean rats [t (12)=6'45), p
During Phase I, II and III, growth curves for obese rats were essentially identical to normative data for obese Zucker rats (Zucker & Zucker, 1961). In spite of the reductions in caloric intake across the eight weeks of Phase II, the rate of growth for obese rats was equal to or greater than normative controls. Growth curves for lean rats given the self-selection diet and lean rats given laboratory chow were identical. Both growth curves were higher but essentially parallel to normative data for lean Zucker rats (Zucker & Zucker, 1961) (see Figure 3). Phase II
Phase I
PhaseID
640 s:
560
.~
'"
480
:g
400
....-.~
c-,
. r;~~;';~~:':::::~----
-
r'~*"
Obese self-selection (n'?) Obese controls" (n'56) - - .... Leon self-selection (n=7) 0--_ Leon controls" (n=27l ...... Leon chow (n'?)
~:;~'
-
~,.". ~
160
o
1 2
3
4
5
6
7
8
9
10 II
12 13 14
Weet
Meal Pattern Analyses
The pattern of dietary self-selection for six obese and six lean rats was examined at the Rockefeller University over a four-day period following an additional five to six week exposure to the self-selection diet of Phase III. At this time the rats were six months of age, had been on a self-selectiondiet for 17-18 weeks and had access to a 25% sucrose solution for 9-10 weeks. Soybean meal, lard, and a 25% sucrose solution were available during testing. Obese rats weighed 699·6 g (20'3). Lean rats allowed to selfselect dietary components weighed 488·5 g (11'3), while lean rats maintained on laboratory chow weighed 472·3 g (10'5) (t = 1,78, NS). The obese rats consumed more total daily calories than lean rats [geno: F(1,9) =63'9, p
288
M. P. ENNS AND J. A. GRINKER -0
E
::>
III
c 0 o
'"
4·0 3·0
~\tj
2·0
c
1·0
0
CIl
:2
Set time
(a)
CIl
t
1·6
t
I· 2
f
~o ~--~>
f
V-,k-I £"
2 4 Dark
0·8 0·4
"0... -0
6 8 Light
Set time
(b)
~
~,~~ 2 4 Dark
e-c._
8 6 Light
-0
CIl
E ::>
10·0
E
8·0
~
6·0
c
4·0
III
8
~ 0
Set time
(c)
C
t
~L-.jJ I ,( ! Y 'f
CD
::;:
0y¥
4 2 Dark
6 Light
3- h time blocks FIGURE 4. Temporal profiles of intake (g or ml ± SEM) for each diet component [(a) soybean meal; (b) lard; (c) sucrose] for obese (fa/fa) and lean (Fa/«) male Zucker rats. Eight consecutive 3-h time blocks are shown.
various micronutrients are shown in Figure 4. These profiles confirm the circadian distribution of feeding [3-h blocks: F(7,70)-soybean=17'62, p
When the total daily caloric intake was divided into three-component (soybean meal, lard and sucrose), two-component (soybean meal and lard, soybean meal and sucrose, or lard and sucrose), and one-component meals (soybean meal, lard, or sucrose), interesting differences emerged between the obese and lean rats. The proportion of the total calories consumed in three-component meals was greater for obese than lean rats [F(I,9) = 17'9, p
3-Component 2-Component l-Component meals meals meals Obese Light Dark Lean Light Dark
37·1 (5'9) 65·0 (5'0)
33-8 (B) 26·9 (6'3)
29·1 (7'0) 8·1 (3'0)
16·5 (4'2) 45·1 (5'0)
37·1 (5'2) 35·5 (4'0)
46·4 (2'3) 19-4 (4'1)
289
DIET SELF-SELECTION IN GENETIC OBESITY
The proportion of the total ca lories cons umed in two-component meals was similar for obese and lean rats and equally distributed in the dark and in the light (see Table 3). A majority of the two -component meals for obese rats [65 '8% (5'7)] and fo r lean rats [ 77'5% (3'0)] was made up of soybean meal and sucrose. The remaining twocomponent mea ls were equally divided into lard and sucrose meals, or soy bean mea l and lar d mea ls. Consequently, in subsequent analyses of mea l number an d meal size in two-component meals, all such combinations were included. The proportion ofthe total calories consumed in one -component meals was greater for lean than obese rats [F(l,9) = 12'1, p
Obese Light Dark Lean Light Dark
3-Component meals
2-Component meals
l -Component meals
Total meals
1·6 (0'3) 4·0 (0'5)
2-4 (0'2) 3-6 (0'8)
J8 (0'6) 2·7 (1'( )
7-8 (0'8) to· 3 (1'0)
0·8 (0'2) 3·2 (0'3)
2·7 (004) 4·6 (0'6
5·6 (1'0) 4·7 (l'0)
9·1 (1-1) 12·5 (1'2)
Meal Size
Obese rats ate la rger three-component meals than did lean rats [gen o: F( 1,9) =24,08, p
290
M. P. ENNS AND J. A. GRINKER TABLE
S
Mean (SEM) daily meal size of3-, 2- and I-component meals ofobese and lean Zucker rats allowed to self-select dietary components (soybean meal, lard, sucrose) 3-Component 2-Component I-Component meals meals meals -._-_._._-_._-~-~-----
Obese Light Dark Lean Light Dark
3-2
9·1 (0·4) 11·9 (1·2)
5·3 (0·6) 5·3 (0-4)
(1·0) 2·3 (0-4)
5·8 (0·5) 8·6 (0·6)
4·1 (0'4) 4·8 (0·5)
2·7 (0'4) 2·8 (0·6)
Since meal size differed between obese and lean rats for the three-component meals, it is of some interest to examine the relative composition of these meals. For obese rats, the proportion of the three-component meal consumed as soybean meal, lard, and sucrose was 30·3, 42·2 and 27·S% respectively. For lean rats, the proportion consumed as soybean meal, lard, and sucrose was 22·1, 4S·2 and 32·7%. Thus, the only significant difference between obese and lean rats was in the proportion consumed as soybean meal [F(I,9) = 8,20, p < O·OS]. Finally, for obese rats sucrose was the initial component in SO% of the three-component meals while for lean rats sucrose appeared first in 37·S% of the three-component meals. DISCUSSION
When compared with lean rats, obese rats were hyperphagic when given laboratory chow (Phase I), remained hyperphagic when switched to a self-selection diet (Phase II) and gained weight at a faster rate. All rats increased caloric intake when given a sucrose solution as an additional dietary choice (Phase III). However, after access to the sucrose solution for four weeks, the body weights of obese rats did not differ from normative data of obese Zucker rats fed laboratory chow. Additionally, in agreement with previous reports (Kratz & Levitsky, 1979), the body weights oflean rats, given the selfselection diet and access to a sucrose solution for 8-10 weeks, were not significantly different from the body weights of lean rats given laboratory chow and water. Each of the macronutrients (soybean meal, starch, lard, sucrose) as well as a majority of the three-component meals were consumed on a diurnal schedule by obese and lean rats. This diurnal pattern has also been reported for Zucker rats given laboratory chow (Grinker et al., 1980). Meal pattern analyses allowed both a temporal accounting and a quantification of the distribution of macronutrients to daily caloric intake. Consistently with previous findings (Becker & Grinker, 1977), meal pattern analyses indicated that the primary difference between the food intake of obese and lean rats was an enhanced meal size. This enhanced meal size was apparent only in the three-component meals and obese rats ate a greater proportion of their total calories as three-component meals. Furthermore, for obese rats a greater proportion of the threecomponent meals was derived from soybean meal. Thus, the size and timing of the
DIET SELF-SELECTION IN GENETIC OBESITY
291
three-component meal mirrored the amount and distribution of daily caloric intake. The data from one-component meals suggest that the sucrose solution was utilized as a "snack" throughout the 24-h period by both obese and lean rats, with greater intake in the light. Surprisingly, the lean rats showed a greater tendency to indulge in these between-meal "snacks" than did obese rats. These findings are consistent with reported intakes of sucrose or other carbohydrates in obese populations (Grinker, 1981). Young obese and lean rats placed on a self-selection diet showed short-term as well as long-term changes in dietary intake. These changes were associated with both the macronutrients available and the length of exposure to each. Initially, obese rats ate significantly greater amounts offat than did lean rats (Phase II: Days 1-4). These results are consistent with recent reports suggesting that genetically obese rodents display an enhanced appetite for fat (Castonguay et al.; 1982; Romsos & Ferguson, 1982). As length of exposure increased (Phase II: eight weeks), however, this difference disappeared and the proportions of total daily calories consumed as protein, fat and carbohydrate were similar for obese and lean rats. When a palatable source of carbohydrate (a 25% sucrose solution) was offered as an additional dietary component, obese and lean rats rapidly decreased the consumption of fat as sucrose consumption increased. Obese rats appeared to adjust for their sucrose intake by consuming more soybean meal than lean rats. Consequently, a greater proportion of their total calories was consumed as protein. While these results are not inconsistent with the hypothesis that the hyperphagia of the obese rats is related to a protein deficiency (Radcliffe & Webster, 1976; 1978), the changes in the pattern of intake with age and alterations in the diet make it difficult to give unqualified support to this hypothesis. Additionally, since soybean meal consists of carbohydrate (30%) and a minimal amount of fat (0'5%) as well as protein (48%), changes in intake of soybean meal may also involve the regulation of these components. These results offer a clear demonstration that the form as well as the assumed palatability of the nutrients employed will influence the selection pattern. It is also clear from recent reports that environmental or nutrient restraints, such as food deprivation, can alter the pattern of dietary self-selection (McArthur & Blundell, 1982; Piquard et al., 1978; Piquard, Schaefer, Haberey, Chanez & Peret, 1979). Thus, when postulating a mechanism relating energy requirements and taste responsiveness to food intake, caution must be exercised. Information regarding the specific nutrients employed, age of animals, deprivation conditions and, more critically, the length of exposure on the self-selection diet must be included (Lat, 1967). This experiment represents one of a growing number of experiments which have monitored dietary self-selection over a sustained period of time so that changes in patterns of selection become evident. The results of this experiment as well as the data from several recent experiments by Kanarek and her associates (Kanarek & Beck, 1980; Kanarek, Marks-Kaufman & Lipeles, 1980; Kanarek, Feldman & Hanes, 1981) should serve as a warning against facile conclusions about excessive intake of fat or carbohydrate in producing or sustaining animal obesity.
REFERENCES
Anderson, G. H., Leprohon, C, Chambers, 1. W., & Coscina, D. V. Intact regulation of protein intake during the development of hypothalmic or genetic obesity in rats. Physiology and Behavior, 1979,23,751-755.
292
M. P. ENNS AND J. A. GRINKER
Becker, E. E., & Grinker, 1. A. Meal patterns in the genetically obese Zucker rat. Physiology and Behavior, 1977,18,685-692. Becker, E. E., & Kissileff,H. R. Inhibitory controls offeeding by the ventromedial hypothalamus. American Journal of Physiology, 1974,226,383-396. Blundell, J. E., & Latham, C. J. Pharmacological manipulation of feeding behavior: Possible influences of serotonin and dopamine on food intake. In Central mechanisms of anorectic drugs, S. Garrattini & R. Samanin (Eds.). New York: Raven Press, 1978. Castonguay, T. W., Hartman, W. 1., Fitzpatrick, E. A., & Stern, 1. S. Dietary self-selection and the Zucker rat. Journal of Nutrition, 1982,112,796-800. Cleary, M. P., Vasselli, J. R., & Greenwood, M. R. C. Development of obesity in the Zucker obese (fa/fa) rat in the absence of hyperphagia. American Journal of Physiology, 1980, 238, E284-292. Collier, G., Leshner, A. I., & Squibb, R. L. Dietary self-selection in active and non-active rats. Physiology and Behavior, 1969,4, 79-82. DeCastro, J. M., & Balagura, S. Meal patterning in the streptozotocin-diabetic rat. Physiology and Behavior, 1975, 15, 259-261 Drewnowski, A., & Grinker, 1. A. Food and water intake and meal and activity patterns of obese and lean Zucker rats following chronic and acute treatment with A-tetrahydrocannabinol. Pharmacology, Biochemistry and Behavior, 1978, 9, 619-630. Enns, M. P., Wecker, J., & Grinker, J. A. Interrelationships among activity, food intake and weight gain in genetically obese and lean Zucker rats. Physiology and Behavior, 1982, 28, 1059-1064. Grinker, J. A. Sucrose, meal patterns and obesity. In J. M. Tanzer (Ed.). Animal models in carioloqy. IRL: Washington, D.C., 1981. Grinker, J. A., Drewnowski, A., Enns, M., & Kissileff, H. Effects of d-amphetamine and fenfluramine on feeding patterns and activity of obese and lean Zucker rats. Pharmacology, Biochemistry and Behavior, 1980,12,265-275. Johnson, D. J., Li, E. T. S., Coscina, D. V., & Anderson, G. H. Different diurnal rhythms of protein and non-protein energy intake by rats. Physiological and Behaviour, 1978,21,777780. Kanarek, R. B., & Beck, J. M. Role of gonadal hormones in diet selection and food utilization in female rats. Physiology and Behavior, 1980,24, 381-386. Kanarek, R. B., Feldman, P. G., & Hanes, C. Pattern of dietary self-selection in VMH-Iesioned rats. Physiology and Behavior, 1981, 27, 337-343. Kanarek, R. B., Marks-Kaufman, R., & Lipeles, B. 1. Increased carbohydrate intake as a function of insulin administration in rats. Physiology and Behavior, 1980,25, 779-782. Kissileff, H. R. Free feeding in normal and "recovered lateral" rats monitored by a pelletdetecting eatometer. Physiology and Behavior, 1970,5,163-173. Kratz, C. M., & Levitsky, D. A. Dietary obesity: differential effects with self-selection and composite diet feeding techniques. Physiology and Behavior, 1979,22,245-249. Lat.J, Self-selection of dietary components. In C. F. Code (Ed.). Handbook ofphysiology. Section 6: The alimentary canal (Vol. 1). Washington, D.C.: American Physiological Society, 1967. LeMagnen, J., & Devos, M. Metabolic correlates of the meal onset in the free food intake of rats. Physiology and Behavior, 1970,5, 805-814. McArthur, R. A., & Blundell, J. E. Effects of age and feeding regimen on protein and carbohydrate self-selection. Appetite, 1982,3, 153-162. Musten, B., Peace, D., & Anderson, G. H. Food intake regulation in the weanling rat: selfselection of protein and energy. Journal of Nutrition, 1974, 104, 563-572. Piquard, F., Schaefer, A., & Haberey, P. Influence of fasting and protein deprivation on food selfselection in the rat. Physiology and Behavior, 1978,20, 771-778. Piquard, F., Schaefer, A., Haberey, P., Chanez, M., & Peret, 1. The effects of dietary self-selection upon the overshoot phenomenon in starved-refed rats. Journal of Nutrition, 1979, 109, 1035-1044. Radcliffe, J. D., & Webster, A. 1. F. Regulation offood intake during growth in fatty and lean female Zucker rats given diets of different protein content. British Journal ofNutrition, 1976, 36, 457--469. Radcliffe, 1. D., & Webster, A. J. F. Sex, body composition and regulation offood intake during growth in the Zucker rat. British Journal of Nutrition, 1978,39,483--492.
DIET SELF-SELECTION IN GENETIC OBESITY
293
Romsos, D. R., & Ferguson, D. Self-selected intake of carbohydrate, fat and protein by obese (ob/ob) and lean mice. Physiology and Behavior, 1982, 28, 301~305. Rozin, P. Are carbohydrate and protein intakes separately regulated? Journal of Comparative and Physiological Psychology, 1968,65,23-29. Sclafani, A., & Berner, C. N. Influence of diet and palatability on the meal taking behavior of hypothalamic hyperphagic and normal rats. Physiology and Behavior, 1976,16,355-361 Stern, J. S., & Johnson, P. R. Spontaneous activity and adipose cellularity in the genetically obese Zucker rat (fa/fa). Metabolism, 1977,26,371-380. Strohmayer, A. 1., & Smith, G. P. Abnormal feeding and posprandial behavioral responses to food deprivation in genetically obese mice (C57B1/6J-ob/ob). Physiology and Behavior, 1979,22, 1157-1162. Strohmayer, A. J., Silverman, G., & Grinker, 1. A device for the continuous recording of solid food ingestion. Physiology and Behavior, 1980,24,789-791. Zucker, L. M., & Zucker, T. F. Fatty, a new mutation in the rat. The Journal ofHeredity, 1961,52, 275-278.
Received 12 April, 1983; revision 20 September, 1983
Dietary Self-selection and Meal Patterns of Obese and Lean Zucker Rats MELVIN P. ENNS and JOEL A. GRINKER St. Lawrence University, Canton and The Rockefeller University, New York
Following two weeks of baseline measures on laboratory chow, young obese and lean male Zucker rats were given access to separate macro nutrients for 18 weeks. First, the rats were given access to soybean meal, dextrinized starch, and lard for eight weeks.Daily caloric intake of obese rats was greater on laboratory chow than that oflean rats and remained so following the switch to the self-selectiondiet. While obese and lean rats consumed similar proportions of each macronutrient over the eight-week period, there were substantial changes in the pattern of intake across weeks. Obese rats decreased daily caloric intake primarily by decreasing lard consumption. In contrast, lean rats maintained daily caloric intake while increasing the proportion of calories from fat. The addition of a 25% sucrose solution (weeks 11-18) produced an elevated daily caloric intake for both obese and lean rats, achieved through an increased carbohydrate intake with a simultaneous decrease in fat intake. A diurnal pattern of intake was maintained for each macronutrient. Compared to controls, rats on the self-selection diet did not show differential growth. Measurement of daily meal patterns for soybean meal, sucrose, and lard du;ing the last four days of the experiment showed that obese rats ate significantly larger meals comprised of all three food components than did lean rats. For both obese and lean rats, one-component meals were primarily sucrose with greater intake during the light period. Lean rats showed a greater tendency to indulge in these between-meal "snacks" than did obese rats. These data suggest that palatability and nutrient source as wellas length of exposure are critical determiners of nutrient selection and total daily caloric intake.
The obesity of the Zucker rat is associated with metabolic and behavioral alterations including overeating and underactivity (Cleary, Vasselli & Greenwood, 1980; Enns, Wecker & Grinker, 1982; Grinker, Drewnowski, Enns & Kissileff, 1980; Radcliffe & Webster, 1978; Stern & Johnson, 1977). Some investigators have suggested that the hyperphagia seen in both genetically obese male and female Zucker rats is related to a protein deficiency or need. The primary evidence for this hypothesis has come from studies in which the genetically obese rat tolerates a greater percentage of its diet as protein (casein) than the lean rat (Radcliffe & Webster, 1976; 1978). However, other investigators have reported that obese and lean female Zucker rats allowed to feed ad libitum from either of two food cups differing in protein concentration (10 or 60% casein plus carbohydrate and fat) eat nearly identical- amounts of protein while consuming excess calories from the carbohydrate and fat sources (Anderson, Leprohon, Chambers & Coscina, 1979). In contrast, it has been reported that the adult obese male Zucker rat, allowed to self-select dietary intake from three constituents, This research was supported in part by NSF Grant BNS 7609957 and NIH Grant AM 27980. We would like to thank Allison Cohen, Kris Neth and Jana Spalding for their excellent technical assistance. Portions of this research were reported at EPA, 1980. Current address of Joel A. Grinker: Human Nutrition Program, School of Public Health, The University of Michigan, Ann Arbor, MI 48109-2029, U.S.A. 0195-6663/83/040281 + 13 $03·00/0
© 1983 Academic Press Inc. (London) Limited
282
M. P. ENNS AND J. A. GRINKER
consumes a greater proportion of its diet as fat (corn oil) and a lower proportion of its diet as protein (casein) than the lean rat (Castonguay, Hartman, Fitzpatrick & Stern, 1982). The duration of the observation periods, the age of the rats, and especially the specific dietary constituents may account for the differing results reported in these studies of the obese Zucker rat. For example, it has been noted that the self-selection pattern of the developing Sprague-Dawley rat differs from the pattern of the adult Sprague-Dawley rat. In the developing lean rat, percent daily caloric intake offat (corn oil) and carbohydrate vary in an inverse manner across days, while percent daily caloric intake of protein remains constant (Sunday, unpublished). In the adult lean rat given protein, fat, and carbohydrate separately as semi-synthetic gels, daily caloric intake of fat shows day-to-day irregularities while daily caloric intake of protein and carbohydrate remain constant (Piquard, Schaefer & Haberey, 1978). However, both young and mature rats can regulate total caloric intake independently from protein intake (Johnson, Li, Coscina & Anderson, 1978; Musten, Pearce & Anderson, 1974; Rozin, 1968). The present experiment examined the consequences of a self-selection paradigm in obese and lean Zucker rats. Daily food intake, developing shifts in dietary self-selection, contributions of dietary components differing in palatability, and composition as well as the partitioning of total intake into separate meals were analyzed. Examinations of meal patterns have proven useful in the analysis of the circadian distributions offeeding (Becker & Kissileff, 1974; Kissileff, 1970), the consequences of pharmacological agents (Blundell & Latham, 1978;Grinker et al., 1980) and the metabolic correlates offeeding patterns (DeCastro & Balagura, 1975; LeMagnen & Devos, 1970), as well as the purely descriptive analysis of the feeding patterns of animal models of obesity (Becker & Grinker, 1977; Becker & Kissileff, 1974; Sclafani & Berner, 1976; Strohmayer & Smith, 1979).We now show that meal pattern analysis can also provide useful information on the patterning and consumption of individual macronutrients. METHOD
Subjects
The experimental subjects were seven obese (fa/fa) and seven lean (Fa/-) experimentally naive 45-day-old male Zucker rats obtained from the Harriet Bird Memorial Laboratory, Stow, MS. The control subjects were an equivalent group of seven lean, experimentally naive male Zucker rats. The body weights of the obese and lean rats were 191·9 g (8'9) (mean ± SEM) and 172·1 g (2'8) respectively. The animals were housed individually in a temperature (22 ± 1°C) and humidity (55 ± 2%) controlled room maintained on a 12: 12 light/dark schedule (5 a.m.-5 p.m.). Procedure
Upon arrival in our laboratory at St. Lawrence University the rats were given ad libitum access to a standard laboratory chow diet for approximately two weeks (Charles River Rat Chow, Agway Inc., Syracuse, NY: 22% protein, 5% fat, 51'5% carbohydrate by weight) (Phase I: two weeks). Next the rats were switched to a diet which allowed self-selection of dietary constituents (Phase II: eight weeks). The diet included soybean meal (Agway Inc., Syracuse, NY: 48% protein, 30% starch, 0'5% fat), lard (commercial: unsalted), and dextrinized starch (National Starch and Chemical Co.,
DIET SELF-SELECTION IN GENETIC OBESITY
283
Bridgewater, CT.). Each component was in a separate Wahmann non-spill food cup (Wahmann Mfg. Co., Timonium, MD.) and supplemented with a 2'2% vitamin diet fortification mixture (Kratz & Levitsky, 1979),4'0% USP XIV salt mixture and 0·15% DL methionine (Collier, Leshner & Squibb, 1969)(Nutritional Biochemical, Cleveland, OR.). Vitamins, minerals and DL methionine were placed in each nutrient to equate for taste. Thus, the intake of each dietary component resulted in the consumption of all dietary essentials with the exception of those under investigation. Lastly, the diet was altered by the addition of a palatable carbohydrate (25% sucrose solution, wt/vol) (Phase III: four weeks). During each phase, food intake, water intake, and body weight were recorded on alternate days. Twelve-hour intakes of each dietary component, recorded at the beginning of the light (5 a.m.) and dark (5 p.m.) periods, were obtained on Days 33-36 in Phase II and Days 23-26 in Phase III. Intake of each dietary component was converted from grams to kcal when evaluating caloric intake (soybean meal: kcal = g x 2·89;dextrinized starch; kcal = g x 3·35;lard: kcal = g x 8·43;sucrose: kcal = g x 4·00). At the termination of Phase III, the rats were transferred to our laboratory at the Rockefeller University. After an additional five to six weeks while the rats were maintained on the diet given in Phase III, feeding patterns were examined in detail over a four-day period. The rats were housed individually in a temperature (20 ± 1°C) controlled room maintained on a 12: 12 light/dark schedule (5 a.m.-5 p.m.). Using an on-line PDP-8 computer, every lick of solid or liquid food was recorded 24 h daily via low-amplitude "drinkometers" or solid food "eatometers" (see Drewnowski & Grinker, 1978; Strohmayer, Silverman & Grinker, 1980 for a complete description). For data collection purposes, a bout offeeding was defined as a minimum of ten responses in 1 min, separated by 2 min without feeding. Three dietary components (lard, sucrose and soybean meal) were available to the rats during this four-day period. The rats were serviced during a I-h period beginning 3 h before the onset of the dark cycle. The elective pattern of the various macronutrients over 24 h was examined by dividing the total daily food intake into discrete meals using a IS-min intermeal interval (IMI). A IS-min IMI has been used previously in our laboratory (Drewnowski & Grinker, 1978;Grinker et al., 1980)for describing the feeding patterns of obese Zucker rats and the data are qualitatively similar to data obtained with a 2-min IMI. The use of a IS-min IMI is also consistent with results reported by Kissileff (1970). The IS-min IMI was used to partition the daily food intake into three-, two- and one-component meals. A three-component meal was defined as the consumption of each of the three dietary constituents with no separations of over 15 min between successive bouts of feeding. Of the remaining bouts of feeding, a two-component meal was defined as the consumption of each of two dietary constituents without a separation of over IS-min between successive bouts. Finally, a one-component meal was defined as eating without a separation of over 15 min between successive bouts (15min IMI). The major statistical analyses were analysis of variance tests with repeated measures over days. RESULTS
Food Intake over 24 h
Obese rats consumed significantly more laboratory chow per day than lean rats (Phase I; Days 11-14; gena": F(I, 12)= 20·8, p <0·01). Neither obese nor lean rats altered
284
M. P. ENNS AND J. A. GRINKER
daily caloric intake immediately following the switch to a self-selection diet (Phase II: Days 1--4). There were, however, significant shifts in the calories consumed as protein, carbohydrate and fat when the rats began self-selecting dietary components. Both obese and lean rats decreased the daily caloric intake of protein [F(1,12)=25'51, p
access to Charles river rat chow or a self-selection diet (soybean meal, dextrinized starch, lard) Protein
Total -
Obese PhI: Chow (Days 11~14) PhIl: Self-selection (Days 1--4) Lean PhI: Chow (Days 11-14) PhIl: Self-selection (Days 1--4)
_
.
_
.
~
-
~
Carbohydrate
Fat
- - - -
111·3 (13-6) 116·8 (20'1)
28·9 (4'9) 22·1 (5'0)
67-6 (11-4) 26·0 (5'5)
14·8 (2'5) 68·6 (17-4)
86'6 (6'0) 80·6 (13-6)
22'5 (1-6) 20·5 (3-6)
52-6 (3'7) 23·8 (7'7)
11'5 (0'8) 36·3 (19'8)
a The means represent the average of the last four days of chow and the first four days of selfselection.
Over the total eight-week period of Phase II, the obese rats ate significantly greater absolute amounts of soybean meal [geno: F(1,12)=21'6, p
DI ET SELF-S ELECTION IN GENETIC OBESITY
285
T ABLE 2 Mean (SEM) percentage of daily calories (kcal) consumed as protein, carbohydrate, and fa t of obese and lean Zuck er rats allowed to selfselect dietary components in Pha se Il (soybean meal, dextrinized starch, lard) and Phase III (soyb ean meal, dextrinized starch, lard, sucrose)
% Protein
% Fat
% C ho
23-2 (1-2) 21-5 (0-9)
48-4 56-2 (2-1)
28-4 (3-0) 22-3 (1-4)
18-4 (0-6) 14-5 (0-5)
15-8 (2-5) 19-5 (2-1)
65-8 (H) 66-0 (2-3)
A_ Phase II (eight weeks) Obese Le an
B_ Phase III (four weeks) Obese Lean
(3-8)
Obese (follo l A _PHASE D
.,..,
o .~ u
.~
"8
o Lord
Leon (Fo/-)
00-S t ordl
o ~~~ G,J 2
10
'9
26 34 42
Days
2
10
0
Soybeon
aCHO Prote in
SucrosewI/va il (25%
19
Days
FIGURE 1. Mean (SEM) 24-h cal o ric intakes (kcal) of each diet component for obese (fa/fa) a nd lean (Fa/-) male Zucker rats in Phase II (soybean meal, dextrinized starch, lard) and Phase III (soy bean meal, dextrinized sta rch, lard, 25% sucrose wt /vol l,
were eat ing significa ntly more protein [t(12)=2-81, p <0-05] and carbohydrate [t(12 ) = 3-22, p < 0-01] and slightly less fat [t(12) = -I -53, N] than lean rats (see Figure 1, A]. During the first four days of Phase III , when the sucrose solution was given as an additional choice in the self-selection diet, dail y sucrose consumption was 35-2 (3-6) kcal for ob ese rats and 29-6 (3-3) kcal for lean rats [t(12) = 1-17, NS]. In comparison with levels ob served immediately preceding the introduction of the sucrose solution (Phase II: Da ys 53-56), there was a significant decrease in lard intake [lard: F(l,12) = 128-49, p < 0-0I] for both obese and lean rats , and a significant decrea se in starch intake
286
M. P. ENNS AND J. A. GRINKER
(starch-geno x diet: F(l,12)= 7'59, p <0,01) for obese rats. These reductions were not sufficient, however, to prevent both obese and lean rats from increasing daily caloric intake [total cals: F(1,12) = 25A1,p <0'01]. The addition ofthe sucrose solution did not significantly alter the intake of soybean meal for either obese or lean rats [see Figure 1, B]. Thus, when compared with the intake of Phase II (Days 53-56), the additional calories from carbohydrate (sucrose) (Phase III: Days 1--4) replaced calories from fat (lard) and/or carbohydrate (dextrinized starch) but not from protein (soybean meal) for obese and lean rats. 70 ~ 60
.5
~
(b)
50
u 40 'g "8 30
c:
(a)
20 10
:: ::
~~ ~~
~ :::
DL
DL
DL Lean Ooese
~
f:~
o Lard ~ Sucrase (25%)
$ o~ D-Storch Soybean Meal
DL
0 : Dark; L : Light
Lea n Obese
FIGURE 2. Mean 12-h caloric intakes (kcal) of each diet component for obese (fa/fa) and lean (Far-) male Zucker rats sampled from four-day blocks in (a) Phase II (soybean meal, dextrinized starch, lard) and (b) Phase III (soybean meal, dextrinized starch, lard, sucrose 25% wt/vol),
Over the four-week period of Phase III, obese rats ate significantly greater amounts of soybean meal than did lean rats [geno: F(1,12)= 25,11, p
In Phase II, all rats consumed more calories in the dark than in the light (dark: obese = 64%; lean = 73%). This diurnal pattern was evident with all dietary components [see Figure 2(A)] [diurnal: F(1,12)-total cals=60'1, p
The daily intake of water, when only laboratory chow was available (Phase I: Days 11-14), was greater for obese [44'3 (3'4) ml] than for lean rats [33'2 (1-1) ml] [t(12) =3'14, p<0'05] reflecting the greater food intake by obese rats. Daily water intake
DIET SELF-SELECTION IN GENETIC OBESITY
287
during Phase II of self-selection was 37·8 (1'6) ml for obese and 23·9 (1-4) ml for lean rats [t (12)=6'45), p
During Phase I, II and III, growth curves for obese rats were essentially identical to normative data for obese Zucker rats (Zucker & Zucker, 1961). In spite of the reductions in caloric intake across the eight weeks of Phase II, the rate of growth for obese rats was equal to or greater than normative controls. Growth curves for lean rats given the self-selection diet and lean rats given laboratory chow were identical. Both growth curves were higher but essentially parallel to normative data for lean Zucker rats (Zucker & Zucker, 1961) (see Figure 3). Phase II
Phase I
PhaseID
640 s:
560
.~
'"
480
:g
400
....-.~
c-,
. r;~~;';~~:':::::~----
-
r'~*"
Obese self-selection (n'?) Obese controls" (n'56) - - .... Leon self-selection (n=7) 0--_ Leon controls" (n=27l ...... Leon chow (n'?)
~:;~'
-
~,.". ~
160
o
1 2
3
4
5
6
7
8
9
10 II
12 13 14
Weet
Meal Pattern Analyses
The pattern of dietary self-selection for six obese and six lean rats was examined at the Rockefeller University over a four-day period following an additional five to six week exposure to the self-selection diet of Phase III. At this time the rats were six months of age, had been on a self-selectiondiet for 17-18 weeks and had access to a 25% sucrose solution for 9-10 weeks. Soybean meal, lard, and a 25% sucrose solution were available during testing. Obese rats weighed 699·6 g (20'3). Lean rats allowed to selfselect dietary components weighed 488·5 g (11'3), while lean rats maintained on laboratory chow weighed 472·3 g (10'5) (t = 1,78, NS). The obese rats consumed more total daily calories than lean rats [geno: F(1,9) =63'9, p
288
M. P. ENNS AND J. A. GRINKER -0
E
::>
III
c 0 o
'"
4·0 3·0
~\tj
2·0
c
1·0
0
CIl
:2
Set time
(a)
CIl
t
1·6
t
I· 2
f
~o ~--~>
f
V-,k-I £"
2 4 Dark
0·8 0·4
"0... -0
6 8 Light
Set time
(b)
~
~,~~ 2 4 Dark
e-c._
8 6 Light
-0
CIl
E ::>
10·0
E
8·0
~
6·0
c
4·0
III
8
~ 0
Set time
(c)
C
t
~L-.jJ I ,( ! Y 'f
CD
::;:
0y¥
4 2 Dark
6 Light
3- h time blocks FIGURE 4. Temporal profiles of intake (g or ml ± SEM) for each diet component [(a) soybean meal; (b) lard; (c) sucrose] for obese (fa/fa) and lean (Fa/«) male Zucker rats. Eight consecutive 3-h time blocks are shown.
various micronutrients are shown in Figure 4. These profiles confirm the circadian distribution of feeding [3-h blocks: F(7,70)-soybean=17'62, p
When the total daily caloric intake was divided into three-component (soybean meal, lard and sucrose), two-component (soybean meal and lard, soybean meal and sucrose, or lard and sucrose), and one-component meals (soybean meal, lard, or sucrose), interesting differences emerged between the obese and lean rats. The proportion of the total calories consumed in three-component meals was greater for obese than lean rats [F(I,9) = 17'9, p
3-Component 2-Component l-Component meals meals meals Obese Light Dark Lean Light Dark
37·1 (5'9) 65·0 (5'0)
33-8 (B) 26·9 (6'3)
29·1 (7'0) 8·1 (3'0)
16·5 (4'2) 45·1 (5'0)
37·1 (5'2) 35·5 (4'0)
46·4 (2'3) 19-4 (4'1)
289
DIET SELF-SELECTION IN GENETIC OBESITY
The proportion of the total ca lories cons umed in two-component meals was similar for obese and lean rats and equally distributed in the dark and in the light (see Table 3). A majority of the two -component meals for obese rats [65 '8% (5'7)] and fo r lean rats [ 77'5% (3'0)] was made up of soybean meal and sucrose. The remaining twocomponent mea ls were equally divided into lard and sucrose meals, or soy bean mea l and lar d mea ls. Consequently, in subsequent analyses of mea l number an d meal size in two-component meals, all such combinations were included. The proportion ofthe total calories consumed in one -component meals was greater for lean than obese rats [F(l,9) = 12'1, p
Obese Light Dark Lean Light Dark
3-Component meals
2-Component meals
l -Component meals
Total meals
1·6 (0'3) 4·0 (0'5)
2-4 (0'2) 3-6 (0'8)
J8 (0'6) 2·7 (1'( )
7-8 (0'8) to· 3 (1'0)
0·8 (0'2) 3·2 (0'3)
2·7 (004) 4·6 (0'6
5·6 (1'0) 4·7 (l'0)
9·1 (1-1) 12·5 (1'2)
Meal Size
Obese rats ate la rger three-component meals than did lean rats [gen o: F( 1,9) =24,08, p
290
M. P. ENNS AND J. A. GRINKER TABLE
S
Mean (SEM) daily meal size of3-, 2- and I-component meals ofobese and lean Zucker rats allowed to self-select dietary components (soybean meal, lard, sucrose) 3-Component 2-Component I-Component meals meals meals -._-_._._-_._-~-~-----
Obese Light Dark Lean Light Dark
3-2
9·1 (0·4) 11·9 (1·2)
5·3 (0·6) 5·3 (0-4)
(1·0) 2·3 (0-4)
5·8 (0·5) 8·6 (0·6)
4·1 (0'4) 4·8 (0·5)
2·7 (0'4) 2·8 (0·6)
Since meal size differed between obese and lean rats for the three-component meals, it is of some interest to examine the relative composition of these meals. For obese rats, the proportion of the three-component meal consumed as soybean meal, lard, and sucrose was 30·3, 42·2 and 27·S% respectively. For lean rats, the proportion consumed as soybean meal, lard, and sucrose was 22·1, 4S·2 and 32·7%. Thus, the only significant difference between obese and lean rats was in the proportion consumed as soybean meal [F(I,9) = 8,20, p < O·OS]. Finally, for obese rats sucrose was the initial component in SO% of the three-component meals while for lean rats sucrose appeared first in 37·S% of the three-component meals. DISCUSSION
When compared with lean rats, obese rats were hyperphagic when given laboratory chow (Phase I), remained hyperphagic when switched to a self-selection diet (Phase II) and gained weight at a faster rate. All rats increased caloric intake when given a sucrose solution as an additional dietary choice (Phase III). However, after access to the sucrose solution for four weeks, the body weights of obese rats did not differ from normative data of obese Zucker rats fed laboratory chow. Additionally, in agreement with previous reports (Kratz & Levitsky, 1979), the body weights oflean rats, given the selfselection diet and access to a sucrose solution for 8-10 weeks, were not significantly different from the body weights of lean rats given laboratory chow and water. Each of the macronutrients (soybean meal, starch, lard, sucrose) as well as a majority of the three-component meals were consumed on a diurnal schedule by obese and lean rats. This diurnal pattern has also been reported for Zucker rats given laboratory chow (Grinker et al., 1980). Meal pattern analyses allowed both a temporal accounting and a quantification of the distribution of macronutrients to daily caloric intake. Consistently with previous findings (Becker & Grinker, 1977), meal pattern analyses indicated that the primary difference between the food intake of obese and lean rats was an enhanced meal size. This enhanced meal size was apparent only in the three-component meals and obese rats ate a greater proportion of their total calories as three-component meals. Furthermore, for obese rats a greater proportion of the threecomponent meals was derived from soybean meal. Thus, the size and timing of the
DIET SELF-SELECTION IN GENETIC OBESITY
291
three-component meal mirrored the amount and distribution of daily caloric intake. The data from one-component meals suggest that the sucrose solution was utilized as a "snack" throughout the 24-h period by both obese and lean rats, with greater intake in the light. Surprisingly, the lean rats showed a greater tendency to indulge in these between-meal "snacks" than did obese rats. These findings are consistent with reported intakes of sucrose or other carbohydrates in obese populations (Grinker, 1981). Young obese and lean rats placed on a self-selection diet showed short-term as well as long-term changes in dietary intake. These changes were associated with both the macronutrients available and the length of exposure to each. Initially, obese rats ate significantly greater amounts offat than did lean rats (Phase II: Days 1-4). These results are consistent with recent reports suggesting that genetically obese rodents display an enhanced appetite for fat (Castonguay et al.; 1982; Romsos & Ferguson, 1982). As length of exposure increased (Phase II: eight weeks), however, this difference disappeared and the proportions of total daily calories consumed as protein, fat and carbohydrate were similar for obese and lean rats. When a palatable source of carbohydrate (a 25% sucrose solution) was offered as an additional dietary component, obese and lean rats rapidly decreased the consumption of fat as sucrose consumption increased. Obese rats appeared to adjust for their sucrose intake by consuming more soybean meal than lean rats. Consequently, a greater proportion of their total calories was consumed as protein. While these results are not inconsistent with the hypothesis that the hyperphagia of the obese rats is related to a protein deficiency (Radcliffe & Webster, 1976; 1978), the changes in the pattern of intake with age and alterations in the diet make it difficult to give unqualified support to this hypothesis. Additionally, since soybean meal consists of carbohydrate (30%) and a minimal amount of fat (0'5%) as well as protein (48%), changes in intake of soybean meal may also involve the regulation of these components. These results offer a clear demonstration that the form as well as the assumed palatability of the nutrients employed will influence the selection pattern. It is also clear from recent reports that environmental or nutrient restraints, such as food deprivation, can alter the pattern of dietary self-selection (McArthur & Blundell, 1982; Piquard et al., 1978; Piquard, Schaefer, Haberey, Chanez & Peret, 1979). Thus, when postulating a mechanism relating energy requirements and taste responsiveness to food intake, caution must be exercised. Information regarding the specific nutrients employed, age of animals, deprivation conditions and, more critically, the length of exposure on the self-selection diet must be included (Lat, 1967). This experiment represents one of a growing number of experiments which have monitored dietary self-selection over a sustained period of time so that changes in patterns of selection become evident. The results of this experiment as well as the data from several recent experiments by Kanarek and her associates (Kanarek & Beck, 1980; Kanarek, Marks-Kaufman & Lipeles, 1980; Kanarek, Feldman & Hanes, 1981) should serve as a warning against facile conclusions about excessive intake of fat or carbohydrate in producing or sustaining animal obesity.
REFERENCES
Anderson, G. H., Leprohon, C, Chambers, 1. W., & Coscina, D. V. Intact regulation of protein intake during the development of hypothalmic or genetic obesity in rats. Physiology and Behavior, 1979,23,751-755.
292
M. P. ENNS AND J. A. GRINKER
Becker, E. E., & Grinker, 1. A. Meal patterns in the genetically obese Zucker rat. Physiology and Behavior, 1977,18,685-692. Becker, E. E., & Kissileff,H. R. Inhibitory controls offeeding by the ventromedial hypothalamus. American Journal of Physiology, 1974,226,383-396. Blundell, J. E., & Latham, C. J. Pharmacological manipulation of feeding behavior: Possible influences of serotonin and dopamine on food intake. In Central mechanisms of anorectic drugs, S. Garrattini & R. Samanin (Eds.). New York: Raven Press, 1978. Castonguay, T. W., Hartman, W. 1., Fitzpatrick, E. A., & Stern, 1. S. Dietary self-selection and the Zucker rat. Journal of Nutrition, 1982,112,796-800. Cleary, M. P., Vasselli, J. R., & Greenwood, M. R. C. Development of obesity in the Zucker obese (fa/fa) rat in the absence of hyperphagia. American Journal of Physiology, 1980, 238, E284-292. Collier, G., Leshner, A. I., & Squibb, R. L. Dietary self-selection in active and non-active rats. Physiology and Behavior, 1969,4, 79-82. DeCastro, J. M., & Balagura, S. Meal patterning in the streptozotocin-diabetic rat. Physiology and Behavior, 1975, 15, 259-261 Drewnowski, A., & Grinker, 1. A. Food and water intake and meal and activity patterns of obese and lean Zucker rats following chronic and acute treatment with A-tetrahydrocannabinol. Pharmacology, Biochemistry and Behavior, 1978, 9, 619-630. Enns, M. P., Wecker, J., & Grinker, J. A. Interrelationships among activity, food intake and weight gain in genetically obese and lean Zucker rats. Physiology and Behavior, 1982, 28, 1059-1064. Grinker, J. A. Sucrose, meal patterns and obesity. In J. M. Tanzer (Ed.). Animal models in carioloqy. IRL: Washington, D.C., 1981. Grinker, J. A., Drewnowski, A., Enns, M., & Kissileff, H. Effects of d-amphetamine and fenfluramine on feeding patterns and activity of obese and lean Zucker rats. Pharmacology, Biochemistry and Behavior, 1980,12,265-275. Johnson, D. J., Li, E. T. S., Coscina, D. V., & Anderson, G. H. Different diurnal rhythms of protein and non-protein energy intake by rats. Physiological and Behaviour, 1978,21,777780. Kanarek, R. B., & Beck, J. M. Role of gonadal hormones in diet selection and food utilization in female rats. Physiology and Behavior, 1980,24, 381-386. Kanarek, R. B., Feldman, P. G., & Hanes, C. Pattern of dietary self-selection in VMH-Iesioned rats. Physiology and Behavior, 1981, 27, 337-343. Kanarek, R. B., Marks-Kaufman, R., & Lipeles, B. 1. Increased carbohydrate intake as a function of insulin administration in rats. Physiology and Behavior, 1980,25, 779-782. Kissileff, H. R. Free feeding in normal and "recovered lateral" rats monitored by a pelletdetecting eatometer. Physiology and Behavior, 1970,5,163-173. Kratz, C. M., & Levitsky, D. A. Dietary obesity: differential effects with self-selection and composite diet feeding techniques. Physiology and Behavior, 1979,22,245-249. Lat.J, Self-selection of dietary components. In C. F. Code (Ed.). Handbook ofphysiology. Section 6: The alimentary canal (Vol. 1). Washington, D.C.: American Physiological Society, 1967. LeMagnen, J., & Devos, M. Metabolic correlates of the meal onset in the free food intake of rats. Physiology and Behavior, 1970,5, 805-814. McArthur, R. A., & Blundell, J. E. Effects of age and feeding regimen on protein and carbohydrate self-selection. Appetite, 1982,3, 153-162. Musten, B., Peace, D., & Anderson, G. H. Food intake regulation in the weanling rat: selfselection of protein and energy. Journal of Nutrition, 1974, 104, 563-572. Piquard, F., Schaefer, A., & Haberey, P. Influence of fasting and protein deprivation on food selfselection in the rat. Physiology and Behavior, 1978,20, 771-778. Piquard, F., Schaefer, A., Haberey, P., Chanez, M., & Peret, 1. The effects of dietary self-selection upon the overshoot phenomenon in starved-refed rats. Journal of Nutrition, 1979, 109, 1035-1044. Radcliffe, J. D., & Webster, A. 1. F. Regulation offood intake during growth in fatty and lean female Zucker rats given diets of different protein content. British Journal ofNutrition, 1976, 36, 457--469. Radcliffe, 1. D., & Webster, A. J. F. Sex, body composition and regulation offood intake during growth in the Zucker rat. British Journal of Nutrition, 1978,39,483--492.
DIET SELF-SELECTION IN GENETIC OBESITY
293
Romsos, D. R., & Ferguson, D. Self-selected intake of carbohydrate, fat and protein by obese (ob/ob) and lean mice. Physiology and Behavior, 1982, 28, 301~305. Rozin, P. Are carbohydrate and protein intakes separately regulated? Journal of Comparative and Physiological Psychology, 1968,65,23-29. Sclafani, A., & Berner, C. N. Influence of diet and palatability on the meal taking behavior of hypothalamic hyperphagic and normal rats. Physiology and Behavior, 1976,16,355-361 Stern, J. S., & Johnson, P. R. Spontaneous activity and adipose cellularity in the genetically obese Zucker rat (fa/fa). Metabolism, 1977,26,371-380. Strohmayer, A. 1., & Smith, G. P. Abnormal feeding and posprandial behavioral responses to food deprivation in genetically obese mice (C57B1/6J-ob/ob). Physiology and Behavior, 1979,22, 1157-1162. Strohmayer, A. J., Silverman, G., & Grinker, 1. A device for the continuous recording of solid food ingestion. Physiology and Behavior, 1980,24,789-791. Zucker, L. M., & Zucker, T. F. Fatty, a new mutation in the rat. The Journal ofHeredity, 1961,52, 275-278.
Received 12 April, 1983; revision 20 September, 1983