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Keywords:

  • controlled caloric restriction;
  • automated feeder;
  • female and male rats;
  • adipose mass;
  • fat cell number and size

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. References

Objective: To investigate the effects of mild to moderate caloric restriction on parameters of body growth, fat mass, and adipose tissue cellularity in female and male Wistar rats.

Research Methods and Procedures: Three-month-old female and male Wistar rats were subjected to a chronic, mild to moderate caloric restriction paradigm (5%, 10%, or 20% reduction in caloric intake from ad libitum values) for 6 months. This was accomplished using a unique automated feeder system tailored to the food consumption levels of individual rats. Body weight and length, weight of lean organs, regional adipose mass, and adipose cellularity were measured before and after the diet restriction.

Results: Caloric restriction produced proportional decelerations in body weight increases in both genders, without significant changes in body length or lean organ mass. Marked and disproportional reductions in regional adipose tissue mass were produced at all levels of food restriction (even at 5% restriction). An unexpected finding was that in response to graded caloric restriction, female rats preserved adipose fat cell number at the expense of fat cell volume, whereas the converse was seen for male rats.

Discussion: These studies demonstrate a sexual dimorphism in the response to mild to moderate degrees of chronic caloric restriction. At low levels of caloric restriction, it is possible to affect regional adipose mass and cellularity while preserving lean organ mass.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. References

Extensive reports exist on the effects of severe reductions (∼30% to 50%) of ad libitum caloric intake in rodents and other nonhuman species (1, 2, 3, 4). Chronic and marked caloric restriction produces a significant reduction in body weight, due to decreases in both lean and fat mass (5, 6), and altered adipose tissue cellularity, due to decreases in mean fat cell volume (FCV)1 and suppression of fat cell proliferation (7). Although there is evidence that such a degree of caloric restriction may produce improvements in morbidity and mortality that are, in part, related to decreases in fat stores (7), this magnitude of sustained food deprivation is unrealistic in humans. A better understanding of adipose mass expansion in female and male animals and how this can be affected by mild degrees of energy restriction may lead to more effective weight loss treatments.

A systematic study of mild to moderate caloric restriction (i.e., 5% to 20% below ad libitum values) and its beneficial effects in animals would provide valuable information but has been difficult to achieve due to methodological limitations that are 3-fold: animals fed restricted diets change their pattern of food intake, becoming “meal eaters,” and rapidly gorge their daily food ration (8, 9); available food dispensing systems use pellets of fixed size and usually do not mimic natural food intake patterns (mostly nocturnal in rodents); and it is difficult, if not impossible, to tailor the desired amount of food dispensed to the animals’ individual spontaneous food consumption. These difficulties have been overcome by using an automated food dispenser, developed in our laboratory (10), that is capable of delivering specified amounts of granular chow to rodents at programmable intervals.

For the current investigation, we employed this automated feeder system to study the effects of graded, mild to moderate caloric restriction in female and male Wistar rats, from 3 to 9 months of age. Various parameters of body growth, lean and fat mass, and adipose depot composition and cellularity were measured at 7 weeks and at 3, 6, and 9 months. With mild to moderate caloric restriction, body length and lean body tissues were preserved even when food restriction led to sizable reductions in fat mass. Furthermore, both qualitative and quantitative gender differences existed in the response of adipose mass and composition to mild chronic caloric deprivation.

Research Methods and Procedures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. References

Animals

Six-week-old female (n = 70) and male (n = 70) Wistar rats were purchased from Charles River Laboratories (Wilmington, MA). After 1 week of acclimation in our animal facility (22 °C, light/dark = 7:00 am/7:00 pm), the rats were transferred to individual wire mesh bottom-hanging cages in racks equipped with an automated water dispenser and individual automated feeders that were programmed to dispense granular standard laboratory rodent chow (no. 5001; Purina, St. Louis, MO). All experimental procedures were approved by the Emory University Institutional Animal Care and Use Committee and were in accordance with Public Health Service guidelines.

Automated Feeders

The two major components of the automated feeder are the feeder unit and the computer module. Details of the feeder have been described previously (10). The computer module can be programmed to dispense any amount of granular chow at any designated time by varying both the time intervals and the number of replicated food portions. To preserve the nocturnal eating pattern of rodents, most of the food was programmed to reach each cage food cup in the evening and at night (7, 8, 9, 10, and 11 pm and 12, 1, 2, 4, and 6 am). Visual monitoring of feeder operation and animal well being was performed daily at one feeding programmed for 2 pm.

Before the start of the study, extensive testing of each feeder's average delivery (in grams) per one-shot delivery of food was conducted. The range of individual feeder performances (0.56 to 0.63 grams from feeder to feeder) allowed us to match feeders to the individual food intake levels of the rats so that each row of rats on a rack received the amount of chow appropriate (as 5%, 10%, or 20% reduction from ad libitum values) for each animal.

Experimental Design

A subset of seven ad libitum-fed rats of each gender, whose mean body weight and variance were the same as those of the remaining rats, was killed at 7 weeks of age for baseline terminal measures. From 7 to 12 weeks of age, the remaining rats were fed granular rodent chow ad libitum. Individual food consumption was measured (to 0.01 grams) three times weekly and averaged to determine daily consumption. Body weight was measured (to the nearest gram) biweekly. Particular attention was paid to individual rat food consumption for the 2 weeks preceding initiation of the caloric restriction paradigm (at 3 months of age). Each gender cohort was subdivided into 5 groups (of 14 to 16 rats) that were matched for body weight and percentage change in body weight between 7 weeks and 3 months of age.

One control group of rats (n = 7) was killed at 3 months. The other four groups were fed as follows for the next 6 months (ages 3 to 9 months): group A, fed ad libitum (i.e., food dispenser always produced an excess of food in the cup); group B, restricted to 95% of kilocalories consumed by group A (5% caloric deficit); group C, restricted to 90% of kilocalories consumed by group A (10% caloric deficit); and group D, restricted to 80% of kilocalories consumed by group A (20% caloric deficit).

The individual food intake data were used to determine the average ad libitum levels of calories for each gender, the best match of individual feeder performances to individual rats’ ad libitum caloric intake levels, and the amount of food that each rat in each of groups B to D should receive to achieve the appropriate group mean caloric deficit. Thus, all of the rats were fed at the same time intervals, but the number of one-shot deliveries per feeding time varied from group to group to conform to the programmed feeding modality for each group.

Terminal Measures

At each specified time-point (7 weeks and 3, 6, and 9 months of age), rats were anesthetized by intraperitone administration (0.151 mL/100 grams) of a mixture of xylazine (13 mg/kg) and ketamine (87 mg/kg). Blood was collected by cardiac puncture. Euthanasia was produced by exsanguination followed by cardiac excision under deep anesthesia. The following terminal measures were collected in seven to eight rats for each group and gender:

  • 1
    . Body weight and length of body and tail (nose to anus and anus to end of tail).
  • 2
    . Weights of selected lean organs (gonads, liver, kidneys, heart, spleen, and thymus). These organs were removed, blotted, and trimmed of visible fat and capsule, where appropriate. Carcass composition analysis was not performed on these animals. In a previous study of similar duration and slightly greater degree of food restriction, however, we found that lean body mass changes were proportionally similar to the changes in lean organ weight (11).
  • 3
    . Weights of four white adipose depots. Gonadal (female = parametrial; male = epididymal), retroperitoneal, mesenteric, and subcutaneous inguinal fat depots were harvested, rinsed with physiological saline, and blotted before weighing as described previously (12).
  • 4
    . Composition of four white adipose depots, gastrocnemius muscle, and liver. The major components (lipid, water, and fat-free dry mass) were determined for muscle and liver tissue and for each of the four adipose depots taken from each rat in each group (12). The data were expressed in absolute and relative (percentage of wet weight) terms.
  • 5
    . Cellularity [fat cell number (FCN) and FCV of four white adipose depots]. A modification of the method of Hirsch and Gallian (13) was used. In brief, for each depot from each rat, 40- to 50-mg tissue fragments of adipose tissue were fixed with osmium tetroxide (2% solution) and incubated for 7 d at 37 °C. The fixed cells were obtained by pushing the tissue fragments through a 250-μm Nitex mesh screen (Small Parts Inc., Miami Lakes, FL) and capturing the cells on a 25-μm mesh screen. The cells were resuspended in 0.9% saline and counted with a Coulter Counter (Model ZM; Beckman Coulter). An analogous tissue sample was processed for lipid content determination according to the method of Folch et al. (14).

Statistical Analysis

Data were subjected to ANOVA with post hoc analysis (Student-Newman-Keuls) where appropriate. Values of p < 0.05 were taken to indicate statistical significance.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. References

Performance and Efficiency of the Automated Feeder System

Using the automated feeder system, tailored to individual rats’ food consumption levels, we aimed to produce 5%, 10%, and 20% reductions in food intake for rats in groups B to D, relative to those in group A. Table 1 shows the actual and predicted food intake levels of the four groups, for each gender, over the 6 months of the restriction paradigm, and the actual percentage decreases in food consumed by groups B to D. Values shown are average grams per day for each group. The values representing observed percentage decreases from group A, for both genders, were not significantly different from the predicted decrements. This indicates that our feeding paradigm, combined with careful monitoring, was adequate to produce the desired nutritional stratification without the potentially confounding effects of “meal eating or gorging.”

Table 1. . Automated feeder efficiency and resulting effect of the feeding paradigm on parameters of lean and adipose growth rate
   % Decrease from group A
GroupPredicted intake (g)Observed intake (g)IntakeBody weightAdipose weight
  1. Predicted and observed changes in food intake during mild caloric restriction between 3 and 9 months of life. The relative decreases in body weight in both genders and the relative decreases in the sum of the four adipose depots studied are also shown. Values are means of observations in seven to eight animals in each group (SE is shown when appropriate). Group A was fed ad libitum. Groups B to D were fed, respectively, −5%, −10%, and −20% of the amount fed ad libitum to the control rats. Group averages were then calculated. See text for details.

Female rats     
 A22.1322.13 ± 0.11   
 B (−5%)21.0220.93 ± 0.085.425.2530.79
 C (−10%)19.9219.96 ± 0.139.819.5344.25
 D (−20%)17.7017.57 ± 0.1520.6118.5164.15
Male rats     
 A28.7928.79 ± 3.91   
 B (−5%)27.3526.39 ± 0.268.345.4022.87
 C (−10%)25.9125.63 ± 0.2411.009.9135.70
 D (−20%)23.0323.48 ± 0.2218.4414.7558.48

Effect of Graded Caloric Restriction on Body Weight

Table 1 shows that the programmed restriction in food intake (5%, 10%, and 20% reduction) produced analogous decreases in body weights relative to control group A in both female and male rats. The body weight curves of female and male rats over the duration of the study, under ad libitum feeding conditions and with various degrees of mild caloric restriction, are shown in Figure 1. Ad libitum-fed female rats (Figure 1) increased their body weights significantly from 7 weeks to 3 months of age (p < 0.05), and their growth curve plateaued from 3 to 9 months. Mild caloric restriction separated the group body weights according to degree of restriction. Only group D body weights (restricted by 20%) differed significantly (p < 0.05), at 9 months, from those of the ad libitum-fed group A.

image

Figure 1. Body weight curves for female and male rats over the duration of the study. The left panel shows the body weight curve for female rats in groups A (•, ad libitum fed = 100%), B (○, −5%), C (▾, −10%), and D (▿, −20%). The right panel shows the curves for their male counterparts. Data points represent mean values for each group ± SE. The caloric restriction paradigm was initiated at 3 months of age (day 42 on the horizontal axis). Both curves are plotted on the same scale to better illustrate gender differences in growth and response to chronic, mild, graded caloric restriction. See text for significant differences among groups.

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Ad libitum-fed male rats increased their body weights significantly from 7 weeks to 6 months (p < 0.05), and their weights plateaued from 6 to 9 months (Figure 1). Mild caloric restriction produced body weight separation between groups, but even at the highest level of restriction (20%, group D), body weight continued to increase significantly (p < 0.05) from 3 to 6 months. At 9 months of age, both groups C and D differed significantly (p < 0.05) from group A.

Effects of Graded Caloric Restriction on Measures of Lean Body Growth

Body length (both nose to anus and tail length) increased in both female and male rats from age 7 weeks to 3 months and in male rats from 3 to 6 months. Body length, however, was not affected to a significant degree by any level of food restriction in either gender (data not shown). In general, there was no significant effect of caloric restriction, in either gender, on the growth of the various lean body organs. Specifically, the weights of the gonads (ovaries or testes), the uterus, the spleen, the kidneys, and the heart were unaffected by any level of restriction (Table 2). Caloric restriction significantly decreased the weight of the liver (p < 0.05) but only in the male rats at the highest level of food restriction (group D vs. group A).

Table 2. . Cumulative effects of chronic, mild, graded caloric restriction on lean body organs
 Group AGroup BGroup CGroup D
  1. Weights (grams) ± SE of nonadipose organs are shown for female and male rats in groups A (ad libitum fed), B (−5%), C (−10%), and D (−20%) after 6 months of mild graded caloric restriction. Asterisk indicates significant decrease in liver weight for group D male rats relative to their group A counterparts (p < 0.05).

Female    
 Ovaries0.11 ± 0.010.12 ± 0.010.09 ± 0.010.12 ± 0.01
 Uterus0.73 ± 0.050.65 ± 0.070.70 ± 0.070.86 ± 0.06
 Heart1.06 ± 0.060.98 ± 0.050.92 ± 0.030.91 ± 0.03
 Liver12.23 ± 0.6811.07 ± 0.4611.61 ± 0.3410.25 ± 0.35
 Kidneys2.58 ± 0.112.54 ± 0.132.42 ± 0.042.47 ± 0.10
 Spleen0.87 ± 0.060.75 ± 0.040.75 ± 0.040.73 ± 0.04
 Thymus0.24 ± 0.020.26 ± 0.050.23 ± 0.050.21 ± 0.02
Male    
 Testes3.87 ± 0.143.64 ± 0.143.85 ± 0.143.90 ± 0.15
 Heart1.42 ± 0.061.31 ± 0.051.39 ± 0.041.27 ± 0.07
 Liver20.54 ± 1.1118.36 ± 0.9317.76 ± 0.6516.48 ± 0.62*
 Kidneys4.17 ± 0.153.92 ± 0.083.88 ± 0.123.60 ± 0.08
 Spleen1.10 ± 0.041.04 ± 0.061.06 ± 0.031.09 ± 0.07
 Thymus0.27 ± 0.020.22 ± 0.030.22 ± 0.020.20 ± 0.04

The lipid content of the liver, in either gender, was not significantly affected by any degree of caloric restriction (p = 0.30 for females; p = 0.06 for males). In contrast, muscle lipid was proportionally decreased from group A to groups B to D for the female rats and significantly lower for group D vs. group A (p < 0.05) (data not shown). The weight of the thymus in both genders showed evidence of involution with time and ad libitum feeding. Mild caloric restriction at any level, however, did not prevent this involution (Table 2).

Effects of Graded Caloric Restriction on Adipose Tissue Mass

The four adipose depots of ad libitum-fed animals of both genders generally showed progressive increases in mass, from age 7 weeks to age 9 months (Figures 2 and 3). For each depot of female rats, these increases were significant when depot weights from 7-week-old rats were compared with those of 9-month-old rats (p < 0.05 for all post hoc comparisons). The epididymal, retroperitoneal, and mesenteric depots of ad libitum-fed male rats all showed significant increases in mass from 7 weeks to 3 months to 6 months of age (p < 0.05 for all post hoc comparisons) and a plateau in mass from 6 to 9 months of age. The subcutaneous inguinal depot of ad libitum-fed male rats showed significant increases in mass at every measured time-point (p < 0.05).

image

Figure 2. Weights of adipose depots of female rats over the duration of the study. Data represent group mean values ± SE and are shown for each of the four adipose depots harvested at ages 7 weeks and 3, 6, and 9 months from rats in groups A (•, ad libitum fed = 100%), B (○, −5%), C (▾, −10%), and D (▿, −20%). The caloric restriction paradigm was initiated immediately after terminal measures were taken at 3 months. Data were plotted on the same scale as those for the male rats (see Figure 3) to better illustrate gender differences in growth and response to chronic, mild, graded caloric restriction. See text for significant differences among groups. wk, week(s); mo, month(s).

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image

Figure 3. Weights of adipose depots of male rats over the duration of the study. Data represent group mean values ± SE and are shown for each of the four adipose depots harvested at ages 7 weeks and 3, 6, and 9 months from rats in groups A (•, ad libitum fed = 100%), B (○, −5%), C (▾, −10%), and D (▿, −20%). The caloric restriction paradigm was initiated immediately after terminal measures were taken at 3 months. Data were plotted on the same scale as those for the female rats (see Figure 2) to better illustrate gender differences in growth and response to chronic, mild, graded caloric restriction. See text for significant differences among groups. wk, week(s); mo, month(s).

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Figures 2 and 3 show the absolute effects of graded caloric restriction on adipose depot growth at 6 and 9 months of age. In general, decelerations in adipose depot growth were stratified according to the degree of restriction for both genders. The parametrial and mesenteric adipose depots of 9-month-old female rats (after 6 months of caloric restriction) showed significant reductions in growth at every level of restriction, relative to ad libitum-fed controls. These reductions in growth were significant for the retroperitoneal and subcutaneous inguinal depots at the 10% and 20% restriction levels (Figure 2; p < 0.05 for all post hoc comparisons). At 9 months of age, the growth rates of the retroperitoneal and mesenteric depots of male rats were significantly reduced at every level of restriction, relative to ad libitum-fed controls, whereas that of the subcutaneous inguinal depot was significantly reduced at the 10% and 20% restriction levels and that of the epididymal depot was significantly affected only by 20% caloric restriction (Figure 3; p < 0.05 for all post hoc comparisons).

In general, adipose depots of female rats responded to caloric restriction with a greater relative reduction in mass than male rats. Table 1 shows that the programmed restriction in food intake (from 5% to 20%) produced greater cumulative decreases in fat for female than for male rats at 9 months of age at each level of food restriction. It is notable that caloric restriction reduced, but did not abolish, the growth of fat mass in the various depots of male rats, whereas in female rats, caloric restriction (particularly at the 10% and 20% levels) either halted or diminished the spontaneous growth that was observed at 3 months of age (see Figures 2 and 3). In 9-month-old female rats, the retroperitoneal, mesenteric, and inguinal depot masses were reduced by 52% to 67% relative to those of the ad libitum-fed controls; the parametrial depot mass, in contrast, was reduced by 75%. In male rats, the same three non-gonadal depots were reduced in mass by 57% to 68% relative to controls, whereas the epididymal depot was reduced by only 49%.

Relative Changes in Adipose Depot Growth and Cellularity

The relative (percentage) changes in adipose mass and adipose cellularity (FCV and FCN) that occurred from ages 3 to 9 months were determined for groups B to D and compared with those for group A. In general, the various degrees of mild caloric restriction (groups B to D) reduced the relative growth of adipose depots from female rats to a greater extent than that of depots from male rats. This gender-specific difference in relative growth of adipose depots reached statistical significance for the inguinal and mesenteric regions (p < 0.05).

Figure 4 shows a striking gender-specific pattern with regard to relative changes in adipose depot cellularity (FCV and FCN). Specifically, ad libitum-fed male rats showed a 2-fold or greater increase in their mean FCV in each depot, from ages 3 to 9 months, when compared with the analogous depots of the female rats. In addition, male rats in groups B to D showed relative increases in mean FCV in each depot. This is in sharp contrast to the female rats, which showed, with degrees of caloric restriction, either no change or even a decrease in FCV. Opposite findings were obtained for relative changes in FCN. In the gonadal depot, FCN was greater for female rats than for male rats at any level of caloric intake. This pattern was also observed for the retroperitoneal depot. Female rats defended FCN at the expense of FCV, whereas the male rats continued to increase FCV under conditions of food restriction but showed no increases in FCN. The particular sensitivity of the mesenteric depot in female rats to nutritional deprivation was evident even at the mildest level of caloric restriction (5%). The same analysis of the inguinal depot presents a break in the pattern with regard to FCN. Males continued to show relative increases in FCN, even under conditions of caloric restriction; the pattern for FCV, however, remained the same.

image

Figure 4. Effects of mild, graded caloric restriction on gender-specific modalities of growth in four adipose depots. The left panels show relative changes in FCV over the duration (6 months) of the caloric restriction paradigm in the four groups studied: groups A (ad libitum fed = 100%), B (−5%), C (−10%), and D (−20%). The right panels show relative changes in FCN during this time period. In each panel, asterisks indicate significant (p < 0.05) gender differences. See text for details. Grp, group.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. References

To our knowledge, this is the first systematic investigation of the effects of mild to moderate caloric restriction on the growth of lean tissue and adipose mass in female and male rodents over a significant portion of their life span. This study was made possible by the use of a versatile automated food dispenser capable of delivering granular chow in precise installments and at predetermined times. The results of this study show clear gender differences in the modalities of growth and in the quantitative and qualitative adaptation of adipose tissue mass and cellularity to moderate caloric restriction (5%, 10%, or 20% reduction from ad libitum feeding). In general, mild caloric restriction from ages 3 to 9 months markedly reduced the growth of the fat mass of the animals without significantly affecting the lean body mass (measured here by body length and by the weight of several key lean body organs). This indicates a capacity for nutrient partitioning, or, at the very least, it suggests a hierarchy of organs to be protected during caloric deprivation (11, 15, 16).

Nutritional intervention studies using rodents often provide the same absolute level of calories to all animals in a group, even though it is known that individual differences exist in the food intake levels and growth rates of animals born on the same day and kept in controlled litter size. The features of our automated feeder system have permitted an additional level of control. Specifically, by matching the individual food consumption of the animals, we have been able to observe the effects of a caloric deprivation tailored to the individual ad libitum food consumption of the animals. Table 1 shows close matching of the predicted and observed food intake levels.

It is of interest that female rats differed from male rats in the adaptation to caloric restriction during an active phase of growth. The growth and body weights of female rats fed ad libitum tended to plateau earlier than in male rats, whereas male rats continued to grow and expand not only their lean body mass but also their fat mass (see Figure 1). These inherent growth patterns may have contributed to the gender-specific differences in quantitative and qualitative body growth and adipose mass produced by equivalent caloric restriction (Table 1; Figure 1). Specifically, at the high end of this mild restriction (i.e., 20% from ad libitum ration), female rats maintained their body weights at levels observed at 3 months of age before caloric deprivation. In contrast, male rats continued to increase their body weight throughout the experimental period (Figure 1).

With respect to the adaptation of body fat mass to caloric deprivation, female rats reduced their body fat to a greater extent than male rats. Specifically, a 20% restriction for 6 months produced, in the various depots, either a weight plateau or a small reduction below the 3-month baseline (Figure 2). In contrast, male rats maintained (Figure 3), or continued to increase, their depot weights throughout the experiment. For example, the subcutaneous inguinal depot showed increased deposition of fat at all levels of food restriction. This may be due to late growth of the subcutaneous tissue, observed previously (12, 17). Of note, the gonadal depot of male rats showed, overall, the least sensitivity to caloric restriction, whereas the analogous depot of female rats showed the greatest sensitivity to this intervention.

One of the most interesting aspects of this study is summarized in the data of Figure 4. This figure depicts the gender differences in adipose tissue cellularity changes from ages 3 to 9 months. In addition to what was shown in Figures 2 and 3, namely a greater cumulative effect of caloric deprivation on fat mass in the female rats, an important qualitative gender difference, in relation to adaptive capacity of adipose cellularity to food deprivation, was manifest between female and male rats. In almost every depot, albeit with some regional differences, female rats sacrificed FCV to maintain the integrity of FCN. In marked contrast, male rats defended FCV with progressive caloric restriction and sacrificed increases in FCN.

The regulatory mechanisms involved in this differential adaptation of cellular modalities of growth to mild degrees of caloric deprivation are still unclear. Hormonal differences and genetic, neural, and paracrine factors likely play significant roles (18). We have recently reported gender differences in gene expression and adipose cellular content of leptin (19, 20). Some regional differences are seen, but the patterns observed in Figure 4 seem to be over and beyond regional responses.

Severe food restriction (30% to 50% below ad libitum levels) produces significant amounts of body weight loss in rats (6, 21). In our study of mild to moderate caloric restriction, there was a reduction in weight gain that was proportional to the degree of food restriction. Because body length and lean body organs were not affected even at the −20% level of restriction, it follows that the reduction in weight gain was prevalently due to changes in body fat mass. A limitation of this study is that carcass composition was not directly analyzed. In a previous study with a chronic caloric restriction of 25%, however, we measured carcass composition and observed a similar parallel diminution in measured lean body mass and in the individual weights of the same lean body organs measured in the 20% food restriction group of the current study (11).

In summary, we have developed a precise model of mild to moderate caloric restriction in rodents of both genders by the use of a versatile automated food dispenser system. Our study demonstrates that small reductions in daily caloric intake that do not significantly affect lean mass will produce marked and disproportional reductions in adipose mass. This observation could be applicable to humans, where excessive accumulation of adipose mass could be prevented or modified without negatively affecting the integrity of lean body mass or the lean organs. With caloric restriction, the four adipose depots of the female rats showed a greater relative change in FCV than did those of the male rats. In addition, with caloric restriction, the FCV of adipose depots of females was, in some cases, reduced below the 3-month level. In contrast, the rise in FCN from 3 to 9 months was significant for the female rats and was only partially affected by the caloric restriction, whereas the opposite was seen for the male rats. Thus, the female rats responded to caloric restriction with a preservation of FCN at the expense of FCV, whereas for male rats, the limitation in cell proliferation was greater than the reduction in FCV. This observation, if confirmed in humans and other species, suggests that therapeutic intervention to limit fat expansion could differ greatly between male and female animals and humans. Further studies will be necessary to clarify the mechanism(s) of this important gender difference in adaptation to mild energy deprivation.

Acknowledgment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. References

This work was supported in part by NIH R01 Grant DK47246-04 and R03 Grant AG16073-01. We acknowledge the expert technical assistance of Shea Fleming, Krishna Tagra, Hema Byrapuneni and Sudha Cheekati, and Sara Vignes in the various phases of this work. The secretarial assistance of Margaret Duello in the preparation of this manuscript is also gratefully acknowledged.

Footnotes
  • 1

    Nonstandard abbreviations: FCV, fat cell volume; FCN, fat cell number.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. References
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