Physiological to starvation
and behavioral responses in the golden hamster
KATARINA TOMLJENOVIC BORER, NEIL ROWLAND, ARWIN MIROW, ROBERT C. BORER, JR., AND ROBERT P. KELCH Departments of Physical Education and Pediatrics, University of Michigan, Ann Arbor, Michigan 48109; and Department of Psychology, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 BORER, KATARINA TOMLJENOVI~, NEIL ROWLAND, ARWIN MIROW, ROBERT C. BORER, JR., AND ROBERT P. KELCH. Physiological and behavioral responses to starvation in the golden hamster. Am. J. Physiol. 236(2): ElOkE112, 1979 or Am. J. Physiol.: Endocrinol. Metab. Gastrointest. Physiol. S(2): El05E112, 1979.-Physiological and behavioral re-
sponsesof adult hamsters to starvation were studied by measuringfood intake, weight recovery, serumconcentrations of glucose, insulin, free fatty acids and P-hydroxybutyrate, and ketonuria in animals subjectedto different vveight losses, diets, and durations of fast. Hamsters were debilitated by fasts longer than 12 h or leading to greater than 20%weight loss. Hamsters’ feeding patterns were unmodified by fasts ranging between 5 and 12 h and showed no circadian periodicity. Hamsters predominantly recovered from weight losses without increasing their food consumption (unlessthey were offered a diet of pellets and seeds)and without changing their meal patterns, at a rate of weight gain proportional to the magnitude of preceding weight loss if provided with uninterrupted accessto food. By 8 h of fast, blood metabolites were indicative of mobilization of body fat. Hamsters are thus behaviorally unresponsiveto duration of fast, but compensate physiologically for weight losseswith proportional increases in the rate of weight gain. meal patterns; hyperphagia; weight recovery; glucose;insulin; free fatty acids; @hydroxybutyrate; weight loss;diet composition
golden hamster. Although this rodent is able to increase its food intake during dietary dilution (3l), chronic insulin treatment (28), and exercise with concomitant increased linear growth and rapid weight gain (4-6, 8), they are apparently unable to adjust to schedules of restricted food availability. When food is given for only a fraction of a day, there is little or no compensatory overeating (19,31,34) compared to the rat, which learns to take very large meals (20, 27). Silverman (30, 31) proposed a “rate limiting step” hypothesis according to which the hamster is unable to eat, digest, absorb, or anabolize food fast enough to support high food intakes and rapid weight gains. However another study that differs from that of Silverman and Zucker (31) in both diet and the method of restriction has reported overeating and full weight recovery after starvation in golden hamsters (7). In the present experiments we have further investigated the phenomenon of poststarvation feeding in the hamster, with special reference to this discrepancy. We have systematically examined the role of meal frequency during food restriction on the postfast meal patterns and weight gain. We have also investigated the role of degree of weight loss and of diet composition on postfast eating and weight gain. MATERIALS
PHYSIOLOGICAL
ADAPTATIONS
regulating the availability
of metabolic fuels (e.g., lo), along with behavioral strategies such as hoarding or hyperphagia, enable animals to survive in their environment of fluctuating food supply. Periodic famine has demanded the evolution of efficient mechanisms of energy conservation during, and rapid anabolism after, the fast. However, the relative contributions of physiological mechanisms and behavioral changes in refeeding after starvation are unclear. Thus although the much-studied rat is capable of increasing daily food intake after a fast (2, 20-22), the increase in food consumption is neither proportional to the duration of the fast nor is it essential to weight gain (23). Starvation-induced weight loss in the rat leads to recovery to normal weight during ad libitum refeeding, the duration of which is proportional to the weight deficit (11, 23). This response does not seem to be the case in the 0363-6100/79/0000-0000$01.25
AND METHODS
Animals. Golden hamsters (Mesocricetus auratus Waterhouse) were purchased from Engle Laboratory Animals, Farmersburg, IN or from Charles River Laboratories, Lakeview, NJ. Both males and females with initial weights of about 100 g were used. Data have been combined because no sex or age differences were evident in the results. Housing and general procedures. Hamsters were housed singly in suspended wire cages with Purina Formulab chow (pellets or powder) available as described below and with tap water available from bottles with stainless steel nozzles. In addition, for experiment 6, unshelled sunflower seeds were also available. To prevent the loss of pellet fragments and of sunflower seeds through the wire floor in the cages of animals whose food was rationed, food boxes (10 x 16 cm x 5 cm high) with close grain wire floor and acrylic sides were provided. Powdered diets were avilable in small porce-
Copyright 0 1979 the American Physiological Society
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BORER
lain dishes. All food measurements were made to to. 1 g after extracting any food from the pouches and correcting for spillage. Animals were assigned to experimental groups on a matched-weight basis. Recording of meal patterns. A wire cage (18 x 24 cm) with an acrylic top was fitted on one wall with a removable &cm deep recess. The bottom of the recess was. shaped as a cup and served as the container for powdered food. Food intake was determined by removing and weighing recess plus contents. A red light and photocell detector were placed in opposite walls of the cup so that each time the animal’s head entered the recess, the photobeam was interrupted and the signal registered on Esterline Angus event recorder with chart speed of 7.6 cm/h. A few hamsters used the recess as a nest and were discarded. Most hamsters produced clearly identifiable meals defined as a minimum of lmin duration flanked by at least 10 min of no activity. This criterion readily eliminated random exploratory entries from analysis, and individual patterns so analyzed were stable from day-to-day. We assume, but do not prove, that meal size is proportional to meal duration. Meal pattern data were collected in Pittsburgh, whereas most of the food restriction studies were carried out in Ann Arbor. In both laboratories the conditions were comparable, with controlled temperature (22°C) and a light dark cycle (12L:12D). Data analysis. Data were expressed as mean t SE. Differences in food intakes were averaged for the first 7 days of refeeding after weight loss and examined by analysis of variance (ANOVA) and Duncan’s multiple range test (9). All weight changes were expressed as percentage of change from the initial value. Rates of weight gain were determined from slopes of the least square linear regressions of body weight as a function of time during the first 7 days of refeeding or the last 7 days of the experiment. Differences in these rates of weight gain were examined with ANOVA and Scheffe simultaneous inference procedure (29). Deprivation and recovery data are presented separately so that the 1st day of recovery coincided in all the animals despite slight within-group variations in the length of time required to attain a target weight loss. Experi ment 1. Refeeding and recovery from a 20% weight loss incurred by food rationing access to food. Five groups of hamsters
or restricted
were used in this experiment. Six male animals were restricted to one 5-g ration daily (approximately one-half ad libitum intake) until their body weight had declined to 80% of ‘ihe starting level. At this time food was restored ad libitum. A control group (n = 6, male) had food ad libitum throughout. A third group (n = 6, female) was given restricted access to food according to a schedule of alternating 12-h periods of feed and fast; each phase comprised 6 h light and 6 h dark. The fourth group (n = 13, female) was restricted according to alternating 24h period of feed and fast. Both restriction schedules were in effect until 20% weight loss was achieved. The f&h group was another ad libitum fed control (n = 6, female). In each case, food (pellets) and body weight were measured daily during restriction and during 14-
ET
AL.
26 days of ad libitum refeeding. Experiment 2. Role of diet in refeeding afier starvation. Four weight-matched groups of six females were used in this experiment. Two groups were subjected to 20% weight loss by daily rations of 5-g pellets, whereas the other two groups were fed pellets ad libitum. In the refeeding period, one of the fasted groups was given pellets alone (a high carbohydrate diet, 3.5 kcal/g), whereas the other group was given a choice for the first 12 days of refeeding between pellets and sunflower seeds (a high-fat, low-carbohydrate source, 6.8 kcalig). The ad libitum fed control groups were similarly partitioned between dietary conditions. Experiment 3. Influence of intermeal intervals during food restriction on subsequent food intake and recovery of body weight. A 20% weight loss was induced in four
experimental groups of female hamsters by rationing their food intake to 5 g/day. This food ration was equally divided among the scheduled meals. The 5-h group (n = 6) received four l-h meals, each separated by 5 h of fast; the 7-h group (n = 6) received three l-h meals, each separated by 7 h of fast; the 11-h group (n = 7) received two l-h meals separated by 11-h fasts. These groups thus differed both in intermeal interval as well as in the total time available for feeding (4, 3, and 2 h, respectively). An additional group, the 10-h group (n = 6), was given two 2-h meals daily to control for the variable duration of food presentation. Thus 5-h and loh groups both had 4 h of food access but differed in intermeal interval. Food intake and body weight were noted daily during restriction and in the subsequent ad libitum refeeding period. Experiment 4. Effects of duration of fast on serum concentrations of glucose, insulin, free fatty acids and ketone bodies, and on ketonuria. Female hamsters (n = 22) that had fully recovered from previous deprivation experiments and males (n = 70) without prior history of
deprivation were used in this experiment. Blood metabolite levels were measured after 0, 4, 8, 12, 16, 20, and 24 h of fast (with water available). Blood was collected by heart puncture under ether anesthesia (one sample in females) or by decapitation (all males; second sample in females) , and the collected blood was chill .ed on ice. Serum was separated by centrifugation (5,000 rpm, 4°C) and stored at -20°C for subsequent assay of glucose, free fatty acids, P-hydroxybutyrate, and insulin. Urine was aspirated from the bladder after decapitation, and one drop applied to Acetest reagent tablets (Ames) for semiquantitative colorimetric determination of ketonuria. Serum glucose concentration was determined by the glucose oxidase method (Glucostat , Worthington). Free fatty acid concentrations were determined by a radiochemical method (14, 15). Serum P-hydroxybutyrate was determined with a fluorimetric enzymatic method (35). Insulin concentrations were determined with a double antibody radioimmunoassay using rat insulin as standard 1251-labeled porcine insulin, and porcine insulin antiserum developed in the guinea pig. The curve obtained with serial dilutions of hamster sera was parallel to the inhibition curve with rat insulin. Experiment
5.
Effects
of differing
magnitudes
of
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STARVATION
IN
THE
GOLDEN
weight loss on refeeding hamsters. Four groups
El07
HAMSTER
and weight
recovery
in fasted
of female hamsters were assigned to weight losses of 0% (n = 7), 8% (n = 8), 17% h = 5), or 22% (n = 8), the reductions in weight being achieved through daily restriction of food to a 5-g ration. During the refeeding phase, these hamsters received sunflower seeds in addition to chow pellets. The experiment was replicated with three groups of female hamsters maintained on pellets only; subjected to 0% (n = 6), 22% (n = 6), and 29% (n = 11) weight loss. as above. Daily weight measures were taken throughout, but food intake was measured only in the second replication.
Experiment 6. Meal patterns in ad libitum fed and postfasting hamsters. Meal patterns of powdered chow
were recorded continuously in three groups of male hamsters. Four were ad libitum fed controls. Four sustained a 20% weight loss during restriction to 5 g of food daily, given as a single ration in the morning. Eight hamsters were completely fasted for four days, during which time a 17% weight loss occurred. Meal patterns were studied during ad libitum refeeding and weight gain. RESULTS
AND
DISCUSSION
Experiment 1. Refeeding and recovery fFom a 20% weight loss incurred by food rationing or restricted access to food. The results of the food rationing experiment (groups 1 and 2) are shown in Fig. 1. The 20%
weight loss was achieved in A7 days. Sometimes the hamsters did not consume the whole ration of 5 g. When free feeding was restored, food intake rose to control levels, and rapid weight gain ensued so that body weight was in the control range after 12 days. Control animals maintained the slow rate of ponderal growth characteristic of adult hamsters (5). -o\” 5 II01
(3 w 100 3L I > 90 n g
80 1
The results of the restricted access experiment are shown in Fig. 2. Hamsters subjected to alternating 12-h periods of feeding and fast lost weight at -0.9 t 0.2 gl day and required 14-33 days to reach the target 20% weight loss. In contrast, the 24-h group lost weight at -2.3 t 0.1 g/day and had lost 20% of initial weight in 711 days. Consistent with their slower weight loss, the 12-h group consumed more food, 6.7 t 0.2 g/day, than did the 24-h group, 8.3 g/2 days or 4.1 t 0.2 g/day (t = 7.512, P c 0.001). When returned to ad libitum feeding, the 12-h group, but not the 24-h animals, significantly increased their rate of weight gain over that of the control hamsters, which were gaining weight at moderately rapid rates (2.2 t 0.2, 1.3 t 0.1, and 1.0 t 0.2 g/ day, respectively, F = 10.0, P c 0.001). During the last week of the experiment, the respective weight gains were 0.7 t 0.1, 0.3 t 0.1, and 0.0 t 0.3 g/day (F = 4.98, P < 0.02), th a t is, the 24-h group was now gaining weight at a subnormal rate. On the last experimental day, weight of both the 12- and 24-h groups was below control levels (F = 86.38, P c O.OOl), consistent with their slowed growth rate. This experiment shows that rationed animals regain the control weight without overeating (Fig. l), whereas time-restricted animals show recovery of predeprivation, but not of control weights (Fig. 2). We thus replicate the findings of Silverman and Zucker (31) who found absence of postfast hyperphagia and incomplete weight recovery after fasting. The failure of the 24-h group to recover norm .a1 rates of weight gain, however, suggested that some feature of the restricted access schedule is disruptive to the refeeding phase. In experiment 2 we examined whether the absence of postfast hyperphagia persisted under a different dietary regime, and in experiments 3 and 4 we looked at the variation in intermeal intervals as a possible source of feeding disturbances and of metabolic changes.
c
g IX I- 12c x
a
;
IIC
3 ~
IOC
s m
9c 8C
-
0,
; x
a If
1
I
05 FAST
T I6 RECOVERY ( days) FIG. 1. Weight change and food intake in female hamsters subjected to 20% weight loss by restriction of food intake to 5 g/day (0, n = 6) compared with ad libitum fed controls (0, n = 6).
12 IO 8
0”
6
0 IL
4 0 DE
IO 20 PRIVATION
30
0
IO 20 RECOVERY
30
(days)
2. Weight change and food intake in female hamsters subjected to alternate 12-h (0, n = 6) or 24-h periods (A, n = 13) of food access and fast compared with ad libitum fed controls (0, n = 6). FIG.
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BORER ET AL.
Experiment 2. Role of diet in refeeding after starvation. The 20% weight loss was completely recovered to control levels within 14 days of refeeding in both dietary
Experiment 3. Influence of intermeal intervals during food restriction on subsequent food intake and recovery of body weight. The spacing of the daily ra-
conditions (Fig. 3). There were differences in food intake over the first 7 days of refeeding which could be attributed to dietary (F = 162.03, P < 0.001) and weight loss @’ = 88.83, P c 0.001) variables. Mean daily food intake during the first 7 days of recovery was 46.5 t 0.8 kcal in underweight hamsters fed pellets and seeds, 40.1 t 0.8 in control ha .msters fed pellets and seeds, 37 .9_t 0.4 in underweight hamsters fed pellets only, and 31.7 t 0.5 in control hamsters eating pellets. There was no change in the percentage of sunflower seeds eaten by underweight hamsters (74.5 t 3.3) when compared to nondeprived hamsters (75.6 t 3.3). ’ There -is clear (25%) hyperphagia in both the starved and the normal controls fed the sunflower seed choice that confirms the previous report by Borer and Kooi (7). A high-fat diet thus promoted rapid weight gain in ad libitum fed hamsters and a very rapid weight recovery in starved animals. In this experiment, in contrast to experiment 1, we found a 26% postfast hyperphagia in animals fed -a pellet diet alone. The strength of this finding is, however, mitigated by the low intakes (31.7 kcal or 9.0 g) of the control animals relative to other ad libitum fed groups reported in this study.
tioned food affected the food consumption during the restriction period (Fig. 4). Only the 5-h group consumed most of their 5-g allotment (4.9 t 0.2 g). The food intake of the other groups was reduced in proportion to the intermeal interval. The respective intakes of 7-h, 11-h, and 10-h groups were 3.8 t 0.3, 2.3 t 0.3, and 2.2 t 0.2 g/day, with no trend to increase the proportion consumed with days on the schedule. The differences in food intake as a function of duration of the intermeal interval were significant (F = 26.5, P < 0.01) in all comparisons except between the lo- and 11-h groups. As expected from these differences in food intake, the rates of weight loss were significantly different (J’ = 7.5, P < 0.01): -2.2 t 0.3, -2.4 t 0.2, -4.5 t 0.6, and -3.7 t 0.3 g/day for 5-, 7-, ll-, and 10-h groups, respectively. The length of the intermeal interval had no effect on the food intake and weight gain when food was again available ad libitum. Daily food intake of all five groups was indistinguishable, and all of the restricted groups regained body weights that were above their predeprivation weight, but less than the weights of ad libitum fed controls (F = 2.536, P < 0.05). This experiment shows that the weight loss is a function of meal spacing, and that the food intake is a function of duration of food access. Hamsters ingested about 1.1 g during the l-2 h of access to food regardless of the duration of intervening fast. In addition, hamsters ate 1.1 g of food during one or two consecutive
)- 120 x ‘3 I IO
o\” - 120 Ix I IO (3
W 3 100 > (7
90
i 100
- 80 G ; 60 W 50
> 90 a
0 a
x 4 t-
z-
g -0 w x a Iz-
40 30
n 0 0 IL
1
I
0 IO DEPRIVATION (days)
I
I
1
0 IO 20 RECOVERY
FIG. 3. Weight change and food intake of female hamsters subjected to 20% weight loss by rationing of food intake to 5 g/day, as a function of dietary choice during first 10 days of refeeding. One group of fasted hamsters (0, n = 6) received pellets and sunflower seeds, and other (H, n = 6), pellets only. Matched nondeprived groups of hamsters were given either pellets and sunflower seeds (0, n = 6) or pellets only (0, n = 6). Arrows indicate times sunflower seeds were introduced and removed.
cl 0 0 LL
80 12 10 8 6. 4s 2. 0
I
I
0 IO DE PRIVATION
1
0
I
I
I
IO 20 30 RECOVERY
(days) FIG. 4. Weight change and food intake of hamsters subjected to 5 g/day food rationing with food available at intervals of 5 h (A, n = 6), 7 h Co, n = 6), 11 h (0, n = 7), or 10 h (m, n = 6) compared with ad libitum fed controls (0, n = 6). Duration of access to food was four lh periods for 5-h group, two 2-h periods for 10-h group.
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STARVATION
IN
THE
GOLDEN
El09
HAMSTER
hours of access to food (10-h group) but multiples of that amount during hourly exposures to food that were spaced at intervals of 5-11 h. This result indicates that, after a food intake of about 1.1 g, there is an obligatory fast of about 2 h before hamsters will initiate another meal regardless of the duration of intervening fast. This important observation, which might be consistent with the “rate limiting hypothesis” (30), will be considered later. Experiment 4. Effects of duration of fast on serum concentrations of glucose, insulin, free fatty acids and ketone bodies, and ketonuriu. The results are summa-
rized in Table 1. Blood glucose fell only slightly during the fast, reflecting excellent glucostasis. Insulin levels fell to a basal level within 8 h. Free fatty acids (FFA) and ketone bodies showed a large increase by the 8th h of fast and remained elevated thereafter. Six out of ten males showed moderate to severe ketonuria at 24 h. These data indicate that the hamster, like other mam1. Blood metabolite
TABLE
durations
levels at selected
of food deprivation Duration
Serum
Concn
Glucose,
mg/dl
(10 d)
Insulin,
mg/ml
(10 CT)
FFA, nM/ml
(6 Q)
Ketones,
(6 Q)
nM/ml
of Fast, h
ANOVA
of 0
4
118.1 +6.1 2.6 20.8 17.0 k4.4 37.2 26.8
118.3 210.6 1.7 20.7 12.7 22.7 56.8 27.7
8
16
24
F
P
0.05, Fig. 5, left). In the replication (Fig. 5, right), there were significant weight differences on the last day of the experiment (F = 6.04, P < 0.04) due to the failure of the 22% weight loss group to attain more than 87% of final control weights (P < 0.001) although they did recover their predeprivation weight. Ponderal growth rate of the control hamsters in the experimental replication (Fig. 5, right) was higher than in the control hamsters used in the original experiment (Fig. 5, left). Seven of the 17 animals subjected to the more severe weight losses in the replication showed signs of severely compromised energy production as manifested by hypothermia, unconsciousness, and whole body tremors; this effect occurred when body weight was a mean of 74% of initial (range 69-81%). Five of these animals were revived with glucose injections and a warm environment. We did not, however, attempt weight losses in excess of 30%.
-Y 120 t1 II0 (3 c;l 100 > 0
90
FIG. 5. Left: weight change in female hamsters subjected to 0% (0, n = 7), 8% (A, n = 8), 16% (0, n = 5), and 22% weight loss (A, n = 8), and receiving sunflower seeds during refeeding. Right: weight change and food intake of female hamsters subjected to 0% (0, n = 61, 22% (A, n = 6), and 29% (D, n = 11) weight loss, refed pellets only.
0
m 80 -CD 70 w w 1L a 12 IzIO n 0 0
L
8 I
1
0 IO FAST
1
1
I
0 IO 20 RECOVERY
I
1 1
I
0 IO FAST (days
0
1
I
IO 20 RECOVERY
1
30
1
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El10 Despite partial or complete weight gains seen in this experiment, the underweight hamsters did not increase their food intake after food was again made avialable ad libitum. The food intakes of control (11.0 t 0.3 g/ day), 22% loss (10.9 t 0.6), and 29% loss (11.2 t 0.3) groups were indistinguishable during the entire period of refeeding. In this experiment we have shown that hamsters regain all of the weight loss during a fast but do not always catch up to the growing control weight level. The data suggest that 22% weight loss is approximately the maximum for which survival is guaranteed although full weight recovery starts to be compromised (Fig. 5, lefi). The data in Fig. 5 suggest that the rate of weight recovery immediately after the return to uninterrupted access to food is related to the degree of weight loss during fasting. We have examined this relationship by pooling all of the data from this and all of the preceding experiments (with the exception of the 24-h group of experiment 1, which showed an unusual failure to gain weight). The individual data were combined in weight loss categories differing by 5%, and the rate of weight gain during the first 7 days of refeeding was computed for each category. The lleast squares linear regression curve so obtained (Fig. 6) was highly significant (r = 0.943). Weight recovery, thus, is faster after larger weight losses and occurs at a rate of 0.9 g/day for every 10% weight loss incurred (Fig. 6).
BORER
ET
AL.
TABLE 2. Meal parameters during ad libitum and restricted feeding Intake,
Ad libitum Restricted
12.1
5.0
g
No. of Meals
Duration Meal,
13.3” (8-18) 7.8 (5-10)
of 1st min
Mean Duration of Other Meals, min
4.8 (l-10) 10.7t (6-20)
4.7 (2-10)
Restricted feeding, animals restricted to 5 g/day. * Means (ranges) for n = 4 in each condition. t Significantly different from other meals (x2 = 61.5, df = 1, P < 0.0001). This analysis was performed according to the number of meals longer or shorter than 10 min.
ule for 3 weeks, during which time these meal patterns were completely stable, despite continued weight loss. During refeeding of animals that underwent 4 days of total starvation (net weight loss 17%), the meal patterns were not consistently altered from the predeprivation ad libitum patterns in the same animals (data not shown). In some cases there were changes in meal size and frequency, but these occurred in both directions. As before, there was no net hyperphagia: predeprivation food intake was 12.1 g, whereas the mean intake on the first 5 days of refeeding was 12.2 g. Predeprivation body weight was attained af?er 13 days. This experiment shows that the regular pattern of meal taking by the hamster is not profoundly altered by Experiment 6. Mea,! patterns in ad libitum fed and food rationing or after a fast. The rat tends to take very postfasting hamsters. The results shown in Table 2 large, frequent meals after a fast (22), and shows a indicate that hamsters fed ad libitum eat about 13 large increase in meal size when placed on restriction meals per day, each of about 5-min duration and aver- schedules (20, 27). The hamster, like the guinea pig age size 0.9 g. These meals are equally spaced through(l3), shows only a modest increase in the size (duration) out the 24-h period. During food rationing to 5 g/day, of the first meal after a fast and little or no tendency to hamsters ate about 8 meals per day of 0.6 g within 9.4 adapt that pattern with experience (19,32): meal taking (range 8-11) h & r introduction of the food. The first is relatively refractory to changes in energy status (31). meal of the day during restricted feeding (i.e., after about 14-h fast) was the longest, 11 min, and possibly DISCUSSION larger than the remaining meals, which averaged 5 min GENERAL duration. These animals were maintained on this schedIn this study, we have examined the influence of fasting, weight loss, and the composition of the diet on )r 0 the feeding responses and weight recovery in adult hamsters. We have determined that hamsters respond to starvation in two distinctive ways. First, hamsters fail to appreciably modify their ad libitum pattern of meal taking if their access to laboratory chow is limited to less than 24 h/day. The ad libitum pattern of feeding in adult hamsters consists of 5-min episodes of intake of about 0.9-1.3 g separated by about 2 h of fast. Hamsters maintain this pattern of feeding when they are challenged with caloric restriction in the form of food rationing (experiment 6, Table 2) or with restricted access to food (experiments 1 and 3). Under either regime of caloric restriction hamsters continue to ingest only about 1.1 g of food during a2 N= 36 4 3 24 32 12 3 l-2 h of exposure to food even when their meals are I I 1 . 1 I I artificially separated by intervals of 5-11 h. In striking 0 5 IO 15 20 25 30 35 contrast to rats, which respond vigorously to experimenPERCENT WEIGHT LOSS tal spacing of their meals (20) and to caloric deficits due FIG. 6. Relationship between magnitude of weight loss and rate of compensatory weight gain during first 7 days after return of ad to restricted access to food (2, 28) by increasing both the libitum food. size and the frequency of their meals, hamsters persist Downloaded from www.physiology.org/journal/ajpendo by ${individualUser.givenNames} ${individualUser.surname} (129.081.226.078) on January 10, 2019.
STARVATION
IN
THE
GOLDEN
El11
HAMSTER
in taking standard-size meals followed by an obligatory fast of about 2 h, without any adjustments to compensate for the intervening periods of fast. As a consequence hamsters lose weight on all the schedules of restricted access to food at rates that are proportional to the duration of fast and inversely proportional to the number of opportunities for eating meals of standard size and frequency. Hamsters on schedules of restricted access to food maintain this nonadaptive pattern of feeding in spite of continuous weight losses (19, 31 and experiments 1 and 3) and severe depletions of body fat reserves (33). Although in some studies (34) young hamsters were reported capable of adjusting to schedules of restricted access to food over periods of many weeks through modest increases in total food intake, we have been unable to replicate this phenomenon in adult, or in juvenile, 6-wk-old hamsters (unpublished data). Although hamsters fail to overeat after starvation, this failure may not stem from their inability to increase daily caloric intake (“rate limiting step” hypothesis, 30). During forced anabolism by chronic insulin treatment they show 50% increases in daily food intake (28). Similarly, during voluntary exercise hamsters increase their caloric intake independent of diet (4, 8). They are also able to handle more bulk because they compensate for dietary dilution (31). The reason for the differences in feeding responses of hamsters relative to these different forms of energy loss is not understood and deserves further study. In particular, the apparent obligatory intermeal interval after a small meal may imply the existence of an omnipotent peripheral satiety mechanism and that exercise and insulin hasten absorption of a meal and the dissipation of such a signal. This speculation, which is similar to the rate-limiting hypothesis of Silverman (31), awaits empirical investigation. The second characteristic response of hamsters to starvation is a full recovery from weight deficits of 20% or less under the conditions of continuous access to food. Weight recovery can proceed without an increase in the quantity of food consumed daily (Figs. 1, 2, 4, and 5). When the data of experiments l-5 were combined, a significant linear correlation was found between the rate of weight gain in the first 7 days of refeeding and the magnitude of weight loss (Fig. 6). That is, hamsters gain back weight at 0.9 g/day for each 10% of body weight lost. This contrasts with findings in the rat, which increases the duration, but not the rate of compensatory weight gain after different weight losses (11, 23). Compensatory increases in the rate of weight gain in underweight hamsters presumably take place through physiological mechanisms of energy conservation. Hamster, like other hibernators, has efficient mechanisms for generating and reducing body heat which may contribute to the efficient compensation for weight losses. Finally, this study contributes to an understanding of the biology of feeding in the adult hamster in five additional ways. a) Hamsters, like gerbils (17) and
guinea pigs (13) do not show a circadian rhythm to their feeding, but take regularly spaced meals throughout the 24 h. b) During 24 h of fasting, the reductions in serum insulin concentration and free fatty acid mobilization associated with ketogenesis after 4-8 h of fast are indicative of a switch away from glucose catabolism, much as has been described in the rat. c) Intermeal intervals of 24-h duration appear to fall outside the physiological range compatible with full weight recovery. Animals subjected to 24-h fasts flanked by 24-h of access to food ingested lower quantities of food than would be expected on the basis of duration of food access and expected meal size and frequency. In addition, such animals continued to feed and gain weight at substandard rates during the return to continuous access to food. This debilitation may be related to the anorexia seen in rats after prolonged starvation (11) or to decimation of bacteria and protozoa in the forestomach that participate in hamster digestion (3, 16) as is known to occur during prolonged starvation in true ruminants (26). d) Weight losses in excess of 20% resulted in incapacitation of many animals (experiment 5), are outside the physiological range compatible with full weight recovery (31), and in fact exceed the proportion of body fat available for mobilization in the hamster (18). e) The ability of some refed groups to attain the absolute weights of control animals, whereas other groups showed a less complete recovery, may have two explanations. First, control groups showed variable ponderal growth rates (cf. Fig. 5, left and right). Second, adult hamsters are apparently unable to monitor and correct a cumulative deficit in the slow ponderal growth. When they are refed after a prolonged exposure to moderate caloric deficit, a residual deficit in body weight and body length corresponds closely to the amount of slow growth that would have taken place during that period in nondeprived animals (6). In summary, the hamster has an excellent fuel regulatory system. Its natural feeding cycle of about 2 h will cause it to feed 6 h before it would be confronted with possible problems of carbohydrate shortage. Further, the hamster is a prodigious hoarder, gathering food rather than eating even when deprived (e.g., l), and the creation of this extracorporeal store is a buffer against the necesssity of extraordinary ingestive feats in the face of starvation. Ecologically the hamster may not be programmed for extreme fasts, and laboratory studies should use accordingly mild deprivations in this species. We thank E. Valenstein, A. E. Fisher, and E. M. Stricker for the use of facilities: M. Markovs, A. C. Tsai, B. Peng, and J. Bach for the measurement of insulin and blood metabolites; and M. Root (Ely Lilly Research Laboratories) for the gift of rat insulin. This study was supported in part by National Institute of Mental Health and National Science Foundation Grants R03 MH29877-01 and PCM 7807626 to K. T. Borer. Address requests for reprints to: K. T. Borer, Dept. of Physical Education, Univ. of Michigan, Ann Arbor, MI 48109. Received
3 April
1978; accepted
in final
form
18 September
1978.
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REFERENCES 1. ANDERSON, M. C., AND S. T. SHETTLEWORTH. Behavior adaption to fixed-interval and fixed-time food delivery in gold hamster. J. Exp. Anal. Behav. 25: 33-49,1977. 2. BAKER, R. A. The effects of repeated deprivation experiences on feeding behavior. J. Comp. Physiol. Psychol. 48: 37-42,1955. 3. BANTA, C. A., R. G. WARNER, AND J. B. ROBERTSON. Protein nutrition of the golden hamster. J. Nutr. 105: 38-45, 1975. 4. BORER, K. T. Absence of weight regulation in exercising hamsters. Physiol. Behav. 12: 589-597,1974. 5. BORER, K. T., AND L. R. KAPLAN. Exercise-induced growth in golden hamsters: effects of age, weight, and activity level. Physiol. Behav. 18: 29-34,1977. 6. BORER, K. T., AND R. P. KELCH. Increased serum growth hormone and somatic growth in exercising adult hamsters. Am. J. PhysioZ. 234: E611-E616, 1978 orAm. J. Physiol.: EndocrinoZ. Metab. Gastrointest. Physiol. 3: E611-E616, 1978. 7. BORER, K. T., AND A. A. KOOI. Regulatory defense of the exercise-induced weight elevation in hamsters. Behav. BioZ. 13: 301-310, 1975. 8. BROWNE, S. A. H., AND K. T. BORER. The basis for the exerciseinduced hyperphagia in adult hamster. Physiol. Behav. 20: 553557,1978. 9. DUNCAN, D. B. Multiple range and multiple F tests. Biometrics 11: l-42,1955. 10. FRIEDMAN, M. I., AND E. M. STRICKER. The physiological psychology of hunger: A physiological perspective. Psychol. Rev. 83: 409-431,1976. 11. HAMILTON, C. L. Problems of refeeding after starvation in the rat. Ann. NY Acad. Sci. 157: 1004-1017,1969. 12. HERVEY, G. R. Regulation of energy balance. Nature 222: 629631, 1969. 13. HIREH, E. Some determinants of intake and patterns of feeding in the guinea pig. Physiol. Behav. 11: 687-704, 1973. 14. HO, R. J. Radiochemical assay of long-chain fatty acids using 63Ni as tracer. AnaZ. Biochem. 36: 105-113,197O. 15. Ho, R. J., AND H. C. MENG. A simple and ultrasensitive method for determination of free fatty acid by radiochemical assay. Anal. Biochem. 31: 426-436, 1969. 16. HOOVER, W. H., D. L. MANNINGS, AND H. E. SHEERIN. Observations on digestion in the golden hamster. AnimaZ Sci. 28: 349352,1969. 17. KANAREK, R. B., J. D. OGILBY, AND J. MAYER. Effects of dietary caloric density on feeding behavior in Mongolian gerbils. Physiol. Behav. 19: 49’7-501, 1977. 18. KODAMA, A. M. In vivo and in vitro determinations of body fat and body water in the hamster. J. AppZ. Physiol. 31: 218-222, 1971. 19. KUTSCHER, C. L. Species differences in the interaction of feeding and drinking. Ann. NY Acad. Sci. 157: 539-552, 1969.
20. LAWRENCE, D. H., AND W. A. MASON. Food intake in the rat as a function of deprivation intervals and feeding rhythms. J. Comp. Physiol. Psychol. 48: 267-271, 1955. 21. LEVITSKY, D. Feeding patterns of rats in response to fasts and changes in environmental conditions. Physiol. Behav. 5: 291300, 1970. 22. LEVITSKY, D., AND G. COLLIER. Effect of diet and deprivation on meal eating behavior in rats. PhysioZ. Behav. 3: 137-140, 1968. 23. LEVITSKY, D. A., I. FAUST, AND M. GLASSMAN. The ingestion of food and the recovery of body weight following fasting in the naive rat. Physiol. Behav. 17: 575-580,1976. 24. MAYER, J., N. B. MARSHALL, J. J. VITALE, J. H. CHRISTENSEN, M. B. MASHAYEKHI, AND F. J. STARE. Exercise, food intake and body weight in normal rats and genetically obese adult mice. Am. J. Physiol. 177: 544-548, 1954. 25. MCGARRY, J. D., J. M. MEIER, AND D. W. FOSTER. The effect of starvation and refeeding on carbohydrate and lipid metabolism in vivo and in the perfused rat liver. J. Biol. Chem. 248: 270278, 1973. 26. MEISKE, J. C., R. L. SALSBURG, J. A. HOEFER, AND R. W. LUECKE. Effect of starvation and refeeding on some activities of rumen microorganisms in vitro. J. AnimaZ Sci. 17: 774-781, 1958. 27. ROWLAND, N. Feeding patterns in rats on restricted access schedules: palatability, bulk, and other determinants of intake. BUZZ.Psychon. Sot. 5: 306-308, 1975. 28. ROWLAND, N. Effects of insulin and 2-deoxy-n-glucose on feeding in hamsters and gerbils. PhysioZ. Behav. 21: 291-294,1978. 29. SCHEFFE, H. The AnaZysis o/Variance. New York: Wiley, 1959. 30. SILVERMAN, J. J. Failure of 2-deoxy-n-glucose to increase feeding in the golden hamster. Physiol. Behav . In press. 31. SILVERMAN, H. J., AND I. ZUCKER. Absence of postfast food compensation in the golden hamster (Mesocricetus auratus). Physiol. Behav. 17: 271-285, 1976. 32. SIMEK, V. Influence of short-term and long-term intermittent fasting on morphological changes of the digestive system of the golden hamster (Mesocricetus auratus). Acta Sot. ZooZ. BohemosZov. 33: 151-161, 1969. 33. SIMEK, V. Effect of sex and season on the production and deposition of lipid and glycid reserves in the intermittently starving golden hamster. Physiol. Bohemoslov. 24: 183-X&1975. 34. SIMEK, V., AND R. PETRASEK. The effect of two time-different feeding regimens on food intake, growth rate and lipid metabolism in golden hamster (Rodentia). Acta Sot. ZooZ. Bohemoslov. 38: 152-159, 1976. 35. YOUNG, D. A. B., AND A. E. RENOLD. A fluorimetric procedure for the determination of ketone bodies in very small quantities of blood. CZin. Chim. Acta 13: 791-793, 1966.
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and behavioral responses in the golden hamster
KATARINA TOMLJENOVIC BORER, NEIL ROWLAND, ARWIN MIROW, ROBERT C. BORER, JR., AND ROBERT P. KELCH Departments of Physical Education and Pediatrics, University of Michigan, Ann Arbor, Michigan 48109; and Department of Psychology, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 BORER, KATARINA TOMLJENOVI~, NEIL ROWLAND, ARWIN MIROW, ROBERT C. BORER, JR., AND ROBERT P. KELCH. Physiological and behavioral responses to starvation in the golden hamster. Am. J. Physiol. 236(2): ElOkE112, 1979 or Am. J. Physiol.: Endocrinol. Metab. Gastrointest. Physiol. S(2): El05E112, 1979.-Physiological and behavioral re-
sponsesof adult hamsters to starvation were studied by measuringfood intake, weight recovery, serumconcentrations of glucose, insulin, free fatty acids and P-hydroxybutyrate, and ketonuria in animals subjectedto different vveight losses, diets, and durations of fast. Hamsters were debilitated by fasts longer than 12 h or leading to greater than 20%weight loss. Hamsters’ feeding patterns were unmodified by fasts ranging between 5 and 12 h and showed no circadian periodicity. Hamsters predominantly recovered from weight losses without increasing their food consumption (unlessthey were offered a diet of pellets and seeds)and without changing their meal patterns, at a rate of weight gain proportional to the magnitude of preceding weight loss if provided with uninterrupted accessto food. By 8 h of fast, blood metabolites were indicative of mobilization of body fat. Hamsters are thus behaviorally unresponsiveto duration of fast, but compensate physiologically for weight losseswith proportional increases in the rate of weight gain. meal patterns; hyperphagia; weight recovery; glucose;insulin; free fatty acids; @hydroxybutyrate; weight loss;diet composition
golden hamster. Although this rodent is able to increase its food intake during dietary dilution (3l), chronic insulin treatment (28), and exercise with concomitant increased linear growth and rapid weight gain (4-6, 8), they are apparently unable to adjust to schedules of restricted food availability. When food is given for only a fraction of a day, there is little or no compensatory overeating (19,31,34) compared to the rat, which learns to take very large meals (20, 27). Silverman (30, 31) proposed a “rate limiting step” hypothesis according to which the hamster is unable to eat, digest, absorb, or anabolize food fast enough to support high food intakes and rapid weight gains. However another study that differs from that of Silverman and Zucker (31) in both diet and the method of restriction has reported overeating and full weight recovery after starvation in golden hamsters (7). In the present experiments we have further investigated the phenomenon of poststarvation feeding in the hamster, with special reference to this discrepancy. We have systematically examined the role of meal frequency during food restriction on the postfast meal patterns and weight gain. We have also investigated the role of degree of weight loss and of diet composition on postfast eating and weight gain. MATERIALS
PHYSIOLOGICAL
ADAPTATIONS
regulating the availability
of metabolic fuels (e.g., lo), along with behavioral strategies such as hoarding or hyperphagia, enable animals to survive in their environment of fluctuating food supply. Periodic famine has demanded the evolution of efficient mechanisms of energy conservation during, and rapid anabolism after, the fast. However, the relative contributions of physiological mechanisms and behavioral changes in refeeding after starvation are unclear. Thus although the much-studied rat is capable of increasing daily food intake after a fast (2, 20-22), the increase in food consumption is neither proportional to the duration of the fast nor is it essential to weight gain (23). Starvation-induced weight loss in the rat leads to recovery to normal weight during ad libitum refeeding, the duration of which is proportional to the weight deficit (11, 23). This response does not seem to be the case in the 0363-6100/79/0000-0000$01.25
AND METHODS
Animals. Golden hamsters (Mesocricetus auratus Waterhouse) were purchased from Engle Laboratory Animals, Farmersburg, IN or from Charles River Laboratories, Lakeview, NJ. Both males and females with initial weights of about 100 g were used. Data have been combined because no sex or age differences were evident in the results. Housing and general procedures. Hamsters were housed singly in suspended wire cages with Purina Formulab chow (pellets or powder) available as described below and with tap water available from bottles with stainless steel nozzles. In addition, for experiment 6, unshelled sunflower seeds were also available. To prevent the loss of pellet fragments and of sunflower seeds through the wire floor in the cages of animals whose food was rationed, food boxes (10 x 16 cm x 5 cm high) with close grain wire floor and acrylic sides were provided. Powdered diets were avilable in small porce-
Copyright 0 1979 the American Physiological Society
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BORER
lain dishes. All food measurements were made to to. 1 g after extracting any food from the pouches and correcting for spillage. Animals were assigned to experimental groups on a matched-weight basis. Recording of meal patterns. A wire cage (18 x 24 cm) with an acrylic top was fitted on one wall with a removable &cm deep recess. The bottom of the recess was. shaped as a cup and served as the container for powdered food. Food intake was determined by removing and weighing recess plus contents. A red light and photocell detector were placed in opposite walls of the cup so that each time the animal’s head entered the recess, the photobeam was interrupted and the signal registered on Esterline Angus event recorder with chart speed of 7.6 cm/h. A few hamsters used the recess as a nest and were discarded. Most hamsters produced clearly identifiable meals defined as a minimum of lmin duration flanked by at least 10 min of no activity. This criterion readily eliminated random exploratory entries from analysis, and individual patterns so analyzed were stable from day-to-day. We assume, but do not prove, that meal size is proportional to meal duration. Meal pattern data were collected in Pittsburgh, whereas most of the food restriction studies were carried out in Ann Arbor. In both laboratories the conditions were comparable, with controlled temperature (22°C) and a light dark cycle (12L:12D). Data analysis. Data were expressed as mean t SE. Differences in food intakes were averaged for the first 7 days of refeeding after weight loss and examined by analysis of variance (ANOVA) and Duncan’s multiple range test (9). All weight changes were expressed as percentage of change from the initial value. Rates of weight gain were determined from slopes of the least square linear regressions of body weight as a function of time during the first 7 days of refeeding or the last 7 days of the experiment. Differences in these rates of weight gain were examined with ANOVA and Scheffe simultaneous inference procedure (29). Deprivation and recovery data are presented separately so that the 1st day of recovery coincided in all the animals despite slight within-group variations in the length of time required to attain a target weight loss. Experi ment 1. Refeeding and recovery from a 20% weight loss incurred by food rationing access to food. Five groups of hamsters
or restricted
were used in this experiment. Six male animals were restricted to one 5-g ration daily (approximately one-half ad libitum intake) until their body weight had declined to 80% of ‘ihe starting level. At this time food was restored ad libitum. A control group (n = 6, male) had food ad libitum throughout. A third group (n = 6, female) was given restricted access to food according to a schedule of alternating 12-h periods of feed and fast; each phase comprised 6 h light and 6 h dark. The fourth group (n = 13, female) was restricted according to alternating 24h period of feed and fast. Both restriction schedules were in effect until 20% weight loss was achieved. The f&h group was another ad libitum fed control (n = 6, female). In each case, food (pellets) and body weight were measured daily during restriction and during 14-
ET
AL.
26 days of ad libitum refeeding. Experiment 2. Role of diet in refeeding afier starvation. Four weight-matched groups of six females were used in this experiment. Two groups were subjected to 20% weight loss by daily rations of 5-g pellets, whereas the other two groups were fed pellets ad libitum. In the refeeding period, one of the fasted groups was given pellets alone (a high carbohydrate diet, 3.5 kcal/g), whereas the other group was given a choice for the first 12 days of refeeding between pellets and sunflower seeds (a high-fat, low-carbohydrate source, 6.8 kcalig). The ad libitum fed control groups were similarly partitioned between dietary conditions. Experiment 3. Influence of intermeal intervals during food restriction on subsequent food intake and recovery of body weight. A 20% weight loss was induced in four
experimental groups of female hamsters by rationing their food intake to 5 g/day. This food ration was equally divided among the scheduled meals. The 5-h group (n = 6) received four l-h meals, each separated by 5 h of fast; the 7-h group (n = 6) received three l-h meals, each separated by 7 h of fast; the 11-h group (n = 7) received two l-h meals separated by 11-h fasts. These groups thus differed both in intermeal interval as well as in the total time available for feeding (4, 3, and 2 h, respectively). An additional group, the 10-h group (n = 6), was given two 2-h meals daily to control for the variable duration of food presentation. Thus 5-h and loh groups both had 4 h of food access but differed in intermeal interval. Food intake and body weight were noted daily during restriction and in the subsequent ad libitum refeeding period. Experiment 4. Effects of duration of fast on serum concentrations of glucose, insulin, free fatty acids and ketone bodies, and on ketonuria. Female hamsters (n = 22) that had fully recovered from previous deprivation experiments and males (n = 70) without prior history of
deprivation were used in this experiment. Blood metabolite levels were measured after 0, 4, 8, 12, 16, 20, and 24 h of fast (with water available). Blood was collected by heart puncture under ether anesthesia (one sample in females) or by decapitation (all males; second sample in females) , and the collected blood was chill .ed on ice. Serum was separated by centrifugation (5,000 rpm, 4°C) and stored at -20°C for subsequent assay of glucose, free fatty acids, P-hydroxybutyrate, and insulin. Urine was aspirated from the bladder after decapitation, and one drop applied to Acetest reagent tablets (Ames) for semiquantitative colorimetric determination of ketonuria. Serum glucose concentration was determined by the glucose oxidase method (Glucostat , Worthington). Free fatty acid concentrations were determined by a radiochemical method (14, 15). Serum P-hydroxybutyrate was determined with a fluorimetric enzymatic method (35). Insulin concentrations were determined with a double antibody radioimmunoassay using rat insulin as standard 1251-labeled porcine insulin, and porcine insulin antiserum developed in the guinea pig. The curve obtained with serial dilutions of hamster sera was parallel to the inhibition curve with rat insulin. Experiment
5.
Effects
of differing
magnitudes
of
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STARVATION
IN
THE
GOLDEN
weight loss on refeeding hamsters. Four groups
El07
HAMSTER
and weight
recovery
in fasted
of female hamsters were assigned to weight losses of 0% (n = 7), 8% (n = 8), 17% h = 5), or 22% (n = 8), the reductions in weight being achieved through daily restriction of food to a 5-g ration. During the refeeding phase, these hamsters received sunflower seeds in addition to chow pellets. The experiment was replicated with three groups of female hamsters maintained on pellets only; subjected to 0% (n = 6), 22% (n = 6), and 29% (n = 11) weight loss. as above. Daily weight measures were taken throughout, but food intake was measured only in the second replication.
Experiment 6. Meal patterns in ad libitum fed and postfasting hamsters. Meal patterns of powdered chow
were recorded continuously in three groups of male hamsters. Four were ad libitum fed controls. Four sustained a 20% weight loss during restriction to 5 g of food daily, given as a single ration in the morning. Eight hamsters were completely fasted for four days, during which time a 17% weight loss occurred. Meal patterns were studied during ad libitum refeeding and weight gain. RESULTS
AND
DISCUSSION
Experiment 1. Refeeding and recovery fFom a 20% weight loss incurred by food rationing or restricted access to food. The results of the food rationing experiment (groups 1 and 2) are shown in Fig. 1. The 20%
weight loss was achieved in A7 days. Sometimes the hamsters did not consume the whole ration of 5 g. When free feeding was restored, food intake rose to control levels, and rapid weight gain ensued so that body weight was in the control range after 12 days. Control animals maintained the slow rate of ponderal growth characteristic of adult hamsters (5). -o\” 5 II01
(3 w 100 3L I > 90 n g
80 1
The results of the restricted access experiment are shown in Fig. 2. Hamsters subjected to alternating 12-h periods of feeding and fast lost weight at -0.9 t 0.2 gl day and required 14-33 days to reach the target 20% weight loss. In contrast, the 24-h group lost weight at -2.3 t 0.1 g/day and had lost 20% of initial weight in 711 days. Consistent with their slower weight loss, the 12-h group consumed more food, 6.7 t 0.2 g/day, than did the 24-h group, 8.3 g/2 days or 4.1 t 0.2 g/day (t = 7.512, P c 0.001). When returned to ad libitum feeding, the 12-h group, but not the 24-h animals, significantly increased their rate of weight gain over that of the control hamsters, which were gaining weight at moderately rapid rates (2.2 t 0.2, 1.3 t 0.1, and 1.0 t 0.2 g/ day, respectively, F = 10.0, P c 0.001). During the last week of the experiment, the respective weight gains were 0.7 t 0.1, 0.3 t 0.1, and 0.0 t 0.3 g/day (F = 4.98, P < 0.02), th a t is, the 24-h group was now gaining weight at a subnormal rate. On the last experimental day, weight of both the 12- and 24-h groups was below control levels (F = 86.38, P c O.OOl), consistent with their slowed growth rate. This experiment shows that rationed animals regain the control weight without overeating (Fig. l), whereas time-restricted animals show recovery of predeprivation, but not of control weights (Fig. 2). We thus replicate the findings of Silverman and Zucker (31) who found absence of postfast hyperphagia and incomplete weight recovery after fasting. The failure of the 24-h group to recover norm .a1 rates of weight gain, however, suggested that some feature of the restricted access schedule is disruptive to the refeeding phase. In experiment 2 we examined whether the absence of postfast hyperphagia persisted under a different dietary regime, and in experiments 3 and 4 we looked at the variation in intermeal intervals as a possible source of feeding disturbances and of metabolic changes.
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; x
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05 FAST
T I6 RECOVERY ( days) FIG. 1. Weight change and food intake in female hamsters subjected to 20% weight loss by restriction of food intake to 5 g/day (0, n = 6) compared with ad libitum fed controls (0, n = 6).
12 IO 8
0”
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BORER ET AL.
Experiment 2. Role of diet in refeeding after starvation. The 20% weight loss was completely recovered to control levels within 14 days of refeeding in both dietary
Experiment 3. Influence of intermeal intervals during food restriction on subsequent food intake and recovery of body weight. The spacing of the daily ra-
conditions (Fig. 3). There were differences in food intake over the first 7 days of refeeding which could be attributed to dietary (F = 162.03, P < 0.001) and weight loss @’ = 88.83, P c 0.001) variables. Mean daily food intake during the first 7 days of recovery was 46.5 t 0.8 kcal in underweight hamsters fed pellets and seeds, 40.1 t 0.8 in control ha .msters fed pellets and seeds, 37 .9_t 0.4 in underweight hamsters fed pellets only, and 31.7 t 0.5 in control hamsters eating pellets. There was no change in the percentage of sunflower seeds eaten by underweight hamsters (74.5 t 3.3) when compared to nondeprived hamsters (75.6 t 3.3). ’ There -is clear (25%) hyperphagia in both the starved and the normal controls fed the sunflower seed choice that confirms the previous report by Borer and Kooi (7). A high-fat diet thus promoted rapid weight gain in ad libitum fed hamsters and a very rapid weight recovery in starved animals. In this experiment, in contrast to experiment 1, we found a 26% postfast hyperphagia in animals fed -a pellet diet alone. The strength of this finding is, however, mitigated by the low intakes (31.7 kcal or 9.0 g) of the control animals relative to other ad libitum fed groups reported in this study.
tioned food affected the food consumption during the restriction period (Fig. 4). Only the 5-h group consumed most of their 5-g allotment (4.9 t 0.2 g). The food intake of the other groups was reduced in proportion to the intermeal interval. The respective intakes of 7-h, 11-h, and 10-h groups were 3.8 t 0.3, 2.3 t 0.3, and 2.2 t 0.2 g/day, with no trend to increase the proportion consumed with days on the schedule. The differences in food intake as a function of duration of the intermeal interval were significant (F = 26.5, P < 0.01) in all comparisons except between the lo- and 11-h groups. As expected from these differences in food intake, the rates of weight loss were significantly different (J’ = 7.5, P < 0.01): -2.2 t 0.3, -2.4 t 0.2, -4.5 t 0.6, and -3.7 t 0.3 g/day for 5-, 7-, ll-, and 10-h groups, respectively. The length of the intermeal interval had no effect on the food intake and weight gain when food was again available ad libitum. Daily food intake of all five groups was indistinguishable, and all of the restricted groups regained body weights that were above their predeprivation weight, but less than the weights of ad libitum fed controls (F = 2.536, P < 0.05). This experiment shows that the weight loss is a function of meal spacing, and that the food intake is a function of duration of food access. Hamsters ingested about 1.1 g during the l-2 h of access to food regardless of the duration of intervening fast. In addition, hamsters ate 1.1 g of food during one or two consecutive
)- 120 x ‘3 I IO
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0 a
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40 30
n 0 0 IL
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I
0 IO DEPRIVATION (days)
I
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1
0 IO 20 RECOVERY
FIG. 3. Weight change and food intake of female hamsters subjected to 20% weight loss by rationing of food intake to 5 g/day, as a function of dietary choice during first 10 days of refeeding. One group of fasted hamsters (0, n = 6) received pellets and sunflower seeds, and other (H, n = 6), pellets only. Matched nondeprived groups of hamsters were given either pellets and sunflower seeds (0, n = 6) or pellets only (0, n = 6). Arrows indicate times sunflower seeds were introduced and removed.
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I
I
IO 20 30 RECOVERY
(days) FIG. 4. Weight change and food intake of hamsters subjected to 5 g/day food rationing with food available at intervals of 5 h (A, n = 6), 7 h Co, n = 6), 11 h (0, n = 7), or 10 h (m, n = 6) compared with ad libitum fed controls (0, n = 6). Duration of access to food was four lh periods for 5-h group, two 2-h periods for 10-h group.
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STARVATION
IN
THE
GOLDEN
El09
HAMSTER
hours of access to food (10-h group) but multiples of that amount during hourly exposures to food that were spaced at intervals of 5-11 h. This result indicates that, after a food intake of about 1.1 g, there is an obligatory fast of about 2 h before hamsters will initiate another meal regardless of the duration of intervening fast. This important observation, which might be consistent with the “rate limiting hypothesis” (30), will be considered later. Experiment 4. Effects of duration of fast on serum concentrations of glucose, insulin, free fatty acids and ketone bodies, and ketonuriu. The results are summa-
rized in Table 1. Blood glucose fell only slightly during the fast, reflecting excellent glucostasis. Insulin levels fell to a basal level within 8 h. Free fatty acids (FFA) and ketone bodies showed a large increase by the 8th h of fast and remained elevated thereafter. Six out of ten males showed moderate to severe ketonuria at 24 h. These data indicate that the hamster, like other mam1. Blood metabolite
TABLE
durations
levels at selected
of food deprivation Duration
Serum
Concn
Glucose,
mg/dl
(10 d)
Insulin,
mg/ml
(10 CT)
FFA, nM/ml
(6 Q)
Ketones,
(6 Q)
nM/ml
of Fast, h
ANOVA
of 0
4
118.1 +6.1 2.6 20.8 17.0 k4.4 37.2 26.8
118.3 210.6 1.7 20.7 12.7 22.7 56.8 27.7
8
16
24
F
P
0.05, Fig. 5, left). In the replication (Fig. 5, right), there were significant weight differences on the last day of the experiment (F = 6.04, P < 0.04) due to the failure of the 22% weight loss group to attain more than 87% of final control weights (P < 0.001) although they did recover their predeprivation weight. Ponderal growth rate of the control hamsters in the experimental replication (Fig. 5, right) was higher than in the control hamsters used in the original experiment (Fig. 5, left). Seven of the 17 animals subjected to the more severe weight losses in the replication showed signs of severely compromised energy production as manifested by hypothermia, unconsciousness, and whole body tremors; this effect occurred when body weight was a mean of 74% of initial (range 69-81%). Five of these animals were revived with glucose injections and a warm environment. We did not, however, attempt weight losses in excess of 30%.
-Y 120 t1 II0 (3 c;l 100 > 0
90
FIG. 5. Left: weight change in female hamsters subjected to 0% (0, n = 7), 8% (A, n = 8), 16% (0, n = 5), and 22% weight loss (A, n = 8), and receiving sunflower seeds during refeeding. Right: weight change and food intake of female hamsters subjected to 0% (0, n = 61, 22% (A, n = 6), and 29% (D, n = 11) weight loss, refed pellets only.
0
m 80 -CD 70 w w 1L a 12 IzIO n 0 0
L
8 I
1
0 IO FAST
1
1
I
0 IO 20 RECOVERY
I
1 1
I
0 IO FAST (days
0
1
I
IO 20 RECOVERY
1
30
1
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El10 Despite partial or complete weight gains seen in this experiment, the underweight hamsters did not increase their food intake after food was again made avialable ad libitum. The food intakes of control (11.0 t 0.3 g/ day), 22% loss (10.9 t 0.6), and 29% loss (11.2 t 0.3) groups were indistinguishable during the entire period of refeeding. In this experiment we have shown that hamsters regain all of the weight loss during a fast but do not always catch up to the growing control weight level. The data suggest that 22% weight loss is approximately the maximum for which survival is guaranteed although full weight recovery starts to be compromised (Fig. 5, lefi). The data in Fig. 5 suggest that the rate of weight recovery immediately after the return to uninterrupted access to food is related to the degree of weight loss during fasting. We have examined this relationship by pooling all of the data from this and all of the preceding experiments (with the exception of the 24-h group of experiment 1, which showed an unusual failure to gain weight). The individual data were combined in weight loss categories differing by 5%, and the rate of weight gain during the first 7 days of refeeding was computed for each category. The lleast squares linear regression curve so obtained (Fig. 6) was highly significant (r = 0.943). Weight recovery, thus, is faster after larger weight losses and occurs at a rate of 0.9 g/day for every 10% weight loss incurred (Fig. 6).
BORER
ET
AL.
TABLE 2. Meal parameters during ad libitum and restricted feeding Intake,
Ad libitum Restricted
12.1
5.0
g
No. of Meals
Duration Meal,
13.3” (8-18) 7.8 (5-10)
of 1st min
Mean Duration of Other Meals, min
4.8 (l-10) 10.7t (6-20)
4.7 (2-10)
Restricted feeding, animals restricted to 5 g/day. * Means (ranges) for n = 4 in each condition. t Significantly different from other meals (x2 = 61.5, df = 1, P < 0.0001). This analysis was performed according to the number of meals longer or shorter than 10 min.
ule for 3 weeks, during which time these meal patterns were completely stable, despite continued weight loss. During refeeding of animals that underwent 4 days of total starvation (net weight loss 17%), the meal patterns were not consistently altered from the predeprivation ad libitum patterns in the same animals (data not shown). In some cases there were changes in meal size and frequency, but these occurred in both directions. As before, there was no net hyperphagia: predeprivation food intake was 12.1 g, whereas the mean intake on the first 5 days of refeeding was 12.2 g. Predeprivation body weight was attained af?er 13 days. This experiment shows that the regular pattern of meal taking by the hamster is not profoundly altered by Experiment 6. Mea,! patterns in ad libitum fed and food rationing or after a fast. The rat tends to take very postfasting hamsters. The results shown in Table 2 large, frequent meals after a fast (22), and shows a indicate that hamsters fed ad libitum eat about 13 large increase in meal size when placed on restriction meals per day, each of about 5-min duration and aver- schedules (20, 27). The hamster, like the guinea pig age size 0.9 g. These meals are equally spaced through(l3), shows only a modest increase in the size (duration) out the 24-h period. During food rationing to 5 g/day, of the first meal after a fast and little or no tendency to hamsters ate about 8 meals per day of 0.6 g within 9.4 adapt that pattern with experience (19,32): meal taking (range 8-11) h & r introduction of the food. The first is relatively refractory to changes in energy status (31). meal of the day during restricted feeding (i.e., after about 14-h fast) was the longest, 11 min, and possibly DISCUSSION larger than the remaining meals, which averaged 5 min GENERAL duration. These animals were maintained on this schedIn this study, we have examined the influence of fasting, weight loss, and the composition of the diet on )r 0 the feeding responses and weight recovery in adult hamsters. We have determined that hamsters respond to starvation in two distinctive ways. First, hamsters fail to appreciably modify their ad libitum pattern of meal taking if their access to laboratory chow is limited to less than 24 h/day. The ad libitum pattern of feeding in adult hamsters consists of 5-min episodes of intake of about 0.9-1.3 g separated by about 2 h of fast. Hamsters maintain this pattern of feeding when they are challenged with caloric restriction in the form of food rationing (experiment 6, Table 2) or with restricted access to food (experiments 1 and 3). Under either regime of caloric restriction hamsters continue to ingest only about 1.1 g of food during a2 N= 36 4 3 24 32 12 3 l-2 h of exposure to food even when their meals are I I 1 . 1 I I artificially separated by intervals of 5-11 h. In striking 0 5 IO 15 20 25 30 35 contrast to rats, which respond vigorously to experimenPERCENT WEIGHT LOSS tal spacing of their meals (20) and to caloric deficits due FIG. 6. Relationship between magnitude of weight loss and rate of compensatory weight gain during first 7 days after return of ad to restricted access to food (2, 28) by increasing both the libitum food. size and the frequency of their meals, hamsters persist Downloaded from www.physiology.org/journal/ajpendo by ${individualUser.givenNames} ${individualUser.surname} (129.081.226.078) on January 10, 2019.
STARVATION
IN
THE
GOLDEN
El11
HAMSTER
in taking standard-size meals followed by an obligatory fast of about 2 h, without any adjustments to compensate for the intervening periods of fast. As a consequence hamsters lose weight on all the schedules of restricted access to food at rates that are proportional to the duration of fast and inversely proportional to the number of opportunities for eating meals of standard size and frequency. Hamsters on schedules of restricted access to food maintain this nonadaptive pattern of feeding in spite of continuous weight losses (19, 31 and experiments 1 and 3) and severe depletions of body fat reserves (33). Although in some studies (34) young hamsters were reported capable of adjusting to schedules of restricted access to food over periods of many weeks through modest increases in total food intake, we have been unable to replicate this phenomenon in adult, or in juvenile, 6-wk-old hamsters (unpublished data). Although hamsters fail to overeat after starvation, this failure may not stem from their inability to increase daily caloric intake (“rate limiting step” hypothesis, 30). During forced anabolism by chronic insulin treatment they show 50% increases in daily food intake (28). Similarly, during voluntary exercise hamsters increase their caloric intake independent of diet (4, 8). They are also able to handle more bulk because they compensate for dietary dilution (31). The reason for the differences in feeding responses of hamsters relative to these different forms of energy loss is not understood and deserves further study. In particular, the apparent obligatory intermeal interval after a small meal may imply the existence of an omnipotent peripheral satiety mechanism and that exercise and insulin hasten absorption of a meal and the dissipation of such a signal. This speculation, which is similar to the rate-limiting hypothesis of Silverman (31), awaits empirical investigation. The second characteristic response of hamsters to starvation is a full recovery from weight deficits of 20% or less under the conditions of continuous access to food. Weight recovery can proceed without an increase in the quantity of food consumed daily (Figs. 1, 2, 4, and 5). When the data of experiments l-5 were combined, a significant linear correlation was found between the rate of weight gain in the first 7 days of refeeding and the magnitude of weight loss (Fig. 6). That is, hamsters gain back weight at 0.9 g/day for each 10% of body weight lost. This contrasts with findings in the rat, which increases the duration, but not the rate of compensatory weight gain after different weight losses (11, 23). Compensatory increases in the rate of weight gain in underweight hamsters presumably take place through physiological mechanisms of energy conservation. Hamster, like other hibernators, has efficient mechanisms for generating and reducing body heat which may contribute to the efficient compensation for weight losses. Finally, this study contributes to an understanding of the biology of feeding in the adult hamster in five additional ways. a) Hamsters, like gerbils (17) and
guinea pigs (13) do not show a circadian rhythm to their feeding, but take regularly spaced meals throughout the 24 h. b) During 24 h of fasting, the reductions in serum insulin concentration and free fatty acid mobilization associated with ketogenesis after 4-8 h of fast are indicative of a switch away from glucose catabolism, much as has been described in the rat. c) Intermeal intervals of 24-h duration appear to fall outside the physiological range compatible with full weight recovery. Animals subjected to 24-h fasts flanked by 24-h of access to food ingested lower quantities of food than would be expected on the basis of duration of food access and expected meal size and frequency. In addition, such animals continued to feed and gain weight at substandard rates during the return to continuous access to food. This debilitation may be related to the anorexia seen in rats after prolonged starvation (11) or to decimation of bacteria and protozoa in the forestomach that participate in hamster digestion (3, 16) as is known to occur during prolonged starvation in true ruminants (26). d) Weight losses in excess of 20% resulted in incapacitation of many animals (experiment 5), are outside the physiological range compatible with full weight recovery (31), and in fact exceed the proportion of body fat available for mobilization in the hamster (18). e) The ability of some refed groups to attain the absolute weights of control animals, whereas other groups showed a less complete recovery, may have two explanations. First, control groups showed variable ponderal growth rates (cf. Fig. 5, left and right). Second, adult hamsters are apparently unable to monitor and correct a cumulative deficit in the slow ponderal growth. When they are refed after a prolonged exposure to moderate caloric deficit, a residual deficit in body weight and body length corresponds closely to the amount of slow growth that would have taken place during that period in nondeprived animals (6). In summary, the hamster has an excellent fuel regulatory system. Its natural feeding cycle of about 2 h will cause it to feed 6 h before it would be confronted with possible problems of carbohydrate shortage. Further, the hamster is a prodigious hoarder, gathering food rather than eating even when deprived (e.g., l), and the creation of this extracorporeal store is a buffer against the necesssity of extraordinary ingestive feats in the face of starvation. Ecologically the hamster may not be programmed for extreme fasts, and laboratory studies should use accordingly mild deprivations in this species. We thank E. Valenstein, A. E. Fisher, and E. M. Stricker for the use of facilities: M. Markovs, A. C. Tsai, B. Peng, and J. Bach for the measurement of insulin and blood metabolites; and M. Root (Ely Lilly Research Laboratories) for the gift of rat insulin. This study was supported in part by National Institute of Mental Health and National Science Foundation Grants R03 MH29877-01 and PCM 7807626 to K. T. Borer. Address requests for reprints to: K. T. Borer, Dept. of Physical Education, Univ. of Michigan, Ann Arbor, MI 48109. Received
3 April
1978; accepted
in final
form
18 September
1978.
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El12
BORER ET AL.
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