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Objective: The aims of this study were to investigate some features of the metabolic profile and the body composition of male Lou/C rats and to examine whether these characteristics are strictly related to the food-intake reduction.
Research Methods and Procedures: Fourteen-week-old male Lou/C rats were compared with age-matched male Wistar rats fed ad libitum (WAL) and another group of male Wistar rats whose food was chronically restricted (WFR) to the same amount as the Lou/C rats from weeks 3 to 14.
Results: Food intake and body weight were significantly (p < 0.01) reduced in Lou/C compared with WAL rats, whereas these reductions were perfectly reproduced in WFR rats. Lou/C rats demonstrated lower relative weights of retroperitoneal (0.97 ± 0.07 vs. 1.67 ± 0.16 and 1.88 ± 0.15 g/100 g body) and epididymal (1.01 ± 0.02 vs. 1.62 ± 0.12 and 1.80 ± 0.11 g/100g body) fat depots than did the two other groups and no decrease in the percentage of carcass proteins, which was observed in the WFR rats. In addition, compared with the WFR group, the Lou/C rats showed lower plasma glucose levels (3.65 ± 0.14 vs. 4.72 ± 0.15 and 4.7 ± 0.19 mM); a tendency (p < 0.1) for lower liver glycogen concentrations; and similar levels of glycerol, free fatty acids, and β-hydroxybutyrate concentrations. Epinephrine and the relative weight of the adrenal glands were significantly (p < 0.01) lower in the Lou/C rats than in the WAL rats and the two other groups, respectively.
Discussion: The ability of the Lou/C rats to accumulate less body fat than their equally food-restricted Wistar counterparts (WFR) suggests a difference in basal metabolism in this strain of rats that resembles obesity-resistant rats.
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Obesity, or elevated body weight, is associated with several metabolic disturbances and constitutes one of most prevalent risk factors for common chronic diseases in the Western world. No efficacious long-term treatment has yet been identified to permanently decrease excess weight. There are a number of metabolic factors that tend to drive post-obese individuals back to their previously high body weight (1). However, it has been shown that caloric restriction can lead to improvement of the risk factor profiles for several diseases associated with obesity (2). Moreover, it has been reported in numerous studies over the last 60 years that restriction of energy intake in rodents, without malnutrition, increases lifespan (3) and delays the onset of various age-related diseases (4). It is not surprising, therefore, that effects of chronic food restriction have been extensively studied in humans and rodents (5).
Lou/C rats, an inbred strain of rats of Wistar origin, were first bred at the University of Leuven, Belgium (6). Lou/C rats exhibit low-caloric intake (∼40% of the energy intake of Wistar rats fed ad libitum) and no development of obesity with age compared with male Wistar rats (7). Nevertheless, few laboratories have specifically investigated this strain of rats for its spontaneous low-caloric intake (7, 8, 9, 10). Lou/C rats exhibit a lighter body weight and their percentage of fat has been shown to be stable throughout life: ∼13% in male and 11% in female rats (8). In addition, Lou/C rats exhibit a particular preference for fat at the expense of carbohydrates during aging (7). Surprisingly, however, specific data on the metabolic profile and body composition of this strain of rats are almost nonexistent. The study of the metabolic profile and body composition of Lou/C rats in connection with their spontaneous reduced food intake and body weight constitutes a different approach to the phenomena of energy restriction and its action on age-related changes in body weight in rats. Such knowledge could in turn contribute to a new approach to prevent obesity. The aim of this study was to investigate some features of the metabolic profile and the body composition of 14-week-old male Lou/C rats and to examine whether these characteristics are strictly related to the food-intake reduction. To accomplish this aim, Lou/C rats were compared with Wistar rats whether fed ad libitum (WAL) or chronically (11 weeks) calorie-restricted (WFR) to the same amount of food intake as the Lou/C rats. The main results of this study indicate that Lou/C rats accumulate less body fat than their Wistar counterparts, despite a similar food intake, most likely at the expense of an increased fat use.
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Lou/C rats displayed a spontaneous low-food intake (p < 0.01) compared with normally fed Wistar rats (Figure 1A). This food intake reduction was ∼14% at 4 weeks of age, reaching a plateau of ∼40% at 8 weeks of age. The imposed restriction of food intake in the other group of Wistar rats (WFR) resulted in a pattern of food intake that was perfectly matched with the Lou/C group (Figure 1A). As expected, body weight increased with aging in all groups of rats (Figure 1B). During the first 3 weeks, body weight was significantly (p < 0.01) lower in Lou/C rats than in the two other groups of rats. On and after the fourth week, there were no significant (p > 0.05) differences in body weight between Lou/C and WFR rats, the two of them being significantly (p < 0.01) lower than body weight of WAL. Food efficiency, calculated as the ratio of body weight gain per food intake, decreased significantly (p < 0.01) with aging in all groups of rats (Figure 1C). Food efficiency was the same for all groups throughout the experiment with the exception of the fourth, fifth, and ninth weeks, where significant (p < 0.01) differences were found among WAL and the two other groups of rats.
Figure 1. Food intake (A), body weight (B), and food efficiency (C; ratio of the body weight gain to food intake) throughout the experiment in Wistar rats either fed ad libitum (WAL) or food-restricted (WFR) and in Lou/C rats. Values are means ± SE of n = 6 to 8 rats per group. Small SEs do not appear on the graphs. *Significantly different from the two other groups (p < 0.01).
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Body weight and length, measured at the end of the experiment, were significantly (p < 0.01) and similarly reduced in Lou/C and WFR rats compared with WAL rats (Table 1). Alternatively, relative weights of soleus, plantaris, and red gastrocnemius muscles were significantly (p < 0.01) higher in Lou/C rats than in the two other groups of rats (Table 1). Liver and adrenal gland relative weights were higher and lower (p < 0.01), respectively, in Lou/C rats than in the two other groups of rats (Table 1). The relative weight of white adipose tissue from the retroperitoneal and epididymal areas was significantly (p < 0.01) lower in Lou/C rats than in the two other groups (Figure 2, A and B). There were no significant (p > 0.05) differences between WFR and WAL in the relative weights of the white adipose tissues. The percentage of lipids in carcass was not significantly different among the three groups (Figure 2C). The percentage of proteins in carcass was, however, significantly (p < 0.01) lower in WFR rats than in WAL, Lou/C, and WAL rats having similar percentages of carcass proteins (Figure 2D). Blood glucose levels were significantly (p < 0.01) lower in Lou/C rats than in the two other groups of rats (Figure 2E). Liver glycogen content showed a tendency (p < 0.1) to be lower in Lou/C rats than in the other groups (Figure 2F).
Table 1. Final body weight and length, organ weight, and muscle weight in Wistar rats either fed ad libitum (WAL) or food-restricted (WFR) and in Lou/C rats*
| ||WAL||Lou/C||WFR||F||p <|
|Body weight (g)||361 ± 7.7†||264 ± 3.4||275 ± 4.0||103.7||0.0001|
|Body length (cm)||42.7 ± 0.32†||40.2 ± 0.26||40 ± 0.41||21.3||0.0002|
|Muscle weight (mg/100 g body)|| || || || || |
|Red gastrocnemius||522 ± 5||570 ± 7†||512 ± 8||19.83||0.0001|
|Plantaris||87 ± 1.1||118 ± 1.4†||89 ± 1.8||135.4||0.0001|
|Soleus||40 ± 0.7||46 ± 1.2†||41 ± 0.7||11.7||0.0004|
|Liver weight (g/100 g body)||3.13 ± 0.06||3.4 ± 0.07†||3.19 ± 0.06||5.45||0.012|
|Adrenal gland weight (mg/100 g body)||17.8 ± 0.9||14.2 ± 0.4†||17.3 ± 0.5||10.1||0.001|
Figure 2. The relative weight of white adipose tissue (WAT) in the retroperitoneal (A) and the epididymal area (B), percentage of carcass lipids (C) and carcass proteins (D), plasma glucose (E), and liver glycogen concentrations (F) in Wistar rats either fed ad libitum (WAL) or food-restricted (WFR) and in Lou/C rats. Values are means ± SE of n = 7 to 8 rats per group. **Significantly different from the two other groups at p < 0.01. **Significantly different from WAL at p < 0.01. The F and p values for the ANOVA are WAT retroperitoneal, 12.7 and <0.0002; WAT epididymal, 18.8 and <0.0001; carcass lipids, 0.44 and <0.64; carcass proteins, 4.03 and <0.03; glucose, 14.1 and <0.0001; and liver glycogen, 2.59 and <0.1.
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There were no significant differences among the three groups of rats in plasma glycerol, free fatty acids, and β-hydroxybutyrate concentrations (Table 2). Liver TG concentrations were similar in WAL and Lou/C rats, values of the Lou/C being significantly (p < 0.01) higher than concentrations measured in WFR rats (Table 2). Liverβ-hydroxybutyrate levels were not statistically different among the three groups of rats (Table 2). Lou/C rats compared with WAL and WFR rats showed a statistically (p < 0.01) lower HK activity in liver, plantaris, and soleus muscles. There were, however, no significant differences in CPT and HAD activities in liver or muscle tissues among the three groups of rats (Table 2).
Table 2. Plasma and liver variables of lipid metabolism and liver and muscle enzyme activities in Wistar rats either fed ad libitum (WAL) or food-restricted (WFR) and in Lou/C rats*
|Glycerol (mM)||0.15 ± 0.02||0.13 ± 0.01||0.16 ± 0.01||1.16||0.33|
|Free fatty acids (mM)||0.48 ± 0.05||0.5 ± 0.04||0.5 ± 0.07||0.06||0.94|
|Plasma β-hydroxybutyrate (mM)||0.40 ± 0.07||0.55 ± 0.08||0.41 ± 0.09||0.86||0.43|
|Liver triglyceride (μmol/g)||16.19 ± 1.4||18.18 ± 1.26||12.96 ± 0.55§||5.98||0.009|
|Liver β-hydroxybutyrate (μmol/g)||0.86 ± 0.17||1.27 ± 0.21||1.05 ± 0.12||1.51||0.24|
|Hexokinase (μmol/g/min)|| || || || || |
|Liver||0.73 ± 0.05||0.51 ± 0.05‡||0.60 ± 0.04||5.28||0.013|
|Plantaris||1.21 ± 0.08||0.69 ± 0.02†||1.14 ± 0.05||24.6||0.0001|
|Soleus||1.02 ± 0.07||0.81 ± 0.02‡||0.98 ± 0.05||4.84||0.018|
|Carnitine palmitoyl transferase (μmol/g/min)|| || || || || |
|Liver||0.71 ± 0.04||0.66 ± 0.03||0.69 ± 0.03||0.46||0.63|
|Red gastrocnemius||0.20 ± 0.01||0.22 ± 0.02||0.23 ± 0.02||0.79||0.46|
|3-OH-CoA dehydrogenase (μmol/g/min)|| || || || || |
|Liver||89.4 ± 4.5||87.6 ± 3.4||92.5 ± 4.8||0.34||0.71|
|Plantaris||7.66 ± 0.49||6.35 ± 0.19||7.62 ± 0.54||2.67||0.09|
|Soleus||15.01 ± 1.07||15.20 ± 0.75||15.08 ± 0.82||0.01||0.98|
Basal insulin, glucagon, and norepinephrine levels were similar in all groups of rats (Table 3). Plasma epinephrine levels were, however, significantly (p < 0.01) lower in Lou/C than in WAL rats (Table 3).
Table 3. Plasma hormone levels in Wistar rats either fed ad libitum (WAL) or food-restricted (WFR) and in Lou/C rats*
| ||WAL||Lou/C||WFR||F||p <|
|Insulin (pM)||348.4 ± 48.52||392.6 ± 38.07||336.6 ± 36.72||0.456||0.64|
|Glucagon (pM)||50.01 ± 4.58||53.38 ± 2.59||52.41 ± 3.22||0.289||0.78|
|Epinephrine (mM)||2.24 ± 0.44†||0.94 ± 0.14||1.88 ± 0.37||4.281||0.029|
|Norepinephrine (mM)||2.65 ± 0.28||2.36 ± 0.17||3.13 ± 0.24||2.504||0.108|
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It has been reported that Lou/C rats exhibit a spontaneous 40% reduction of caloric intake resulting in a reduced growth rate and no development of obesity with age (8, 10). Comparisons of the present data on food intake in Lou/C and normally fed Wistar rats after 8 weeks of age revealed a difference that agrees perfectly with the reported 40% reduction in food intake in Lou/C rats. At 1 week after weaning (4 weeks of age), a significant 14% reduction in food intake was measured in Lou/C rats. This suggests that the mechanism responsible for the food intake reduction in Lou/C rats takes place very early in life. Although it is now well-established that Lou/C rats exhibit a spontaneous reduction in growth rate (8, 10), direct comparisons of changes in body weight over a period of time between Lou/C rats and their Wistar counterparts have never been reported. Concomitantly with the low food intake, Lou/C rats exhibited a reduced growth rate, so that adult Lou/C rats weighed approximately two-thirds of the weight of normally fed Wistar rats. It is interesting to observe that the difference in body weight between Lou/C and WAL rats, which took place very early in life (see insert Figure 1B), was most likely linked to the food intake reduction phenomena. One of the premises of this study was to match food intake in a group of Wistar rats (WFR) to that of the Lou/C rats. Figure 1A shows that this manipulation resulted in a nearly identical amount of food ingested in WFR and Lou/C rats. The first effect of the food restriction in WFR rats was that their body weight and length were reduced to exactly the same extent as in the Lou/C rats (Table 1). In most of the studies, a 30% to 50% reduction of caloric intake early in the lifespan (1 to 3 months of age) has been shown to result in growth retardation (18). Altogether, these data are perfectly in line with the conclusion that the lower body weight in Lou/C rats compared with normally fed Wistar rats is due to the spontaneous low food intake in Lou/C rats. This interpretation is supported by the observation that food efficiency throughout the weeks was, with a few exceptions, similar for the three groups of rats.
One of the main findings of this study was that several characteristics of body composition were different between the Lou/C and WFR rats, although food intake and the resulting reduction in body weight throughout the weeks were perfectly matched between both groups. The most striking feature of these differences in body composition was that the relative weight of the retroperitoneal and epididymal fat depots was much more reduced in Lou/C than in WFR rats. Although carcass adiposity was not different, the fat pad weights assessed here do represent a large proportion of total adipose stores. Chronic caloric restriction in male Sprague–Dawley rats has been reported to result in larger fat reduction (33%; visceral fat) than what was measured in the present WFR rats, but for a longer period (12 months) (19). In contrast, the larger reduction in fat depots in Lou/C rats than in WFR rats, with the same amount of food ingested, clearly indicates that Lou/C rats have specific metabolic adaptations to caloric restriction that result in less fat accumulation.
Unlike the WFR rats, carcass proteins in Lou/C rats were not decreased by the low-caloric intake. This suggests that the decrease in body weight in WFR rats is, to a certain extent, explained by a decrease in lean body mass. Furthermore, compared with the two other groups, Lou/C rats showed higher relative weights for various skeletal muscles and the liver. These data indicate that not only do Lou/C rats exhibit a spontaneous low-food intake resulting in a reduction of body weight, but they are also able, contrary to WFR rats, to manipulate the lower amount of ingested nutrients to preserve and even increase body proteins and lower body fat accumulation. In recent years, a rat model of diet-induced obesity has been used to investigate how rats that are genetically predisposed to become obese on a high-fat diet differ from those that are diet-resistant on their weight gain patterns and their weight-loss patterns during prolonged caloric restriction (20, 21). Similarly, these data indicate that the same amount of caloric ingestion in Lou/C and WFR rats resulted in a different weight gain pattern that is characterized by less fat accumulation in the Lou/C rats and most likely less lean body mass accumulation in WFR rats.
In addition to the above-mentioned changes in body composition, Lou/C rats also exhibited some specific metabolic characteristics. One of the interesting metabolic features of Lou/C rats is that they exhibit a lower level of plasma glucose and a tendency for lower liver glycogen content than the WFR rats. Plasma glucose and insulin levels have been reported to be generally decreased by caloric restriction (22) and hepatic gluconeogenic enzymes are generally increased (23). The lower plasma glucose level in Lou/C rats compared with WFR rats might be indicative of the presence of a specific metabolic adaptation to caloric restriction in the relatively young Lou/C rats that is not yet seen or absent in the WFR rats. In contrast, insulin and glucagon levels were similar in all three groups of rats. This suggests that the lower levels of plasma glucose in Lou/C rats were not caused by an acute metabolic disturbance of glucose metabolism. In addition to lower plasma glucose levels, Lou/C rats were also characterized by a lower level of plasma epinephrine. Interestingly, the relative weights of the adrenal glands were also lower in Lou/C rats than in the two other groups. It is interesting to recall that there is electrophysiological evidence of neural links between the liver, possibly involving the glycogen content, and the response of the adrenal glands (24). The lower levels of liver and muscle HK activity in Lou/C rats also suggest some adaptations of glucose metabolism in Lou/C rats. Altogether, these metabolic data support the idea that Lou/C rats exhibit some metabolic adaptations that are compatible to the ones seen in rodents adapted for prolonged caloric restriction. It can be postulated that Lou/C rats can cope with low-food intake by using spontaneous metabolic adaptations that are generally found in rats adapted to chronic food restriction.
The fact that Lou/C rats exhibit lower fat accumulation over the 11 weeks suggests a different regulation of lipid metabolism in these rats. Surprisingly, plasma glycerol and free fatty acid concentrations as well as activities of enzymes involved in fatty acid mitochondrial transport (CPT) and lipid oxidation (HAD) were not different among Lou/C rats and the two other groups. Notwithstanding these data, liver and plasma β-hydroxybutyrate levels were ∼20% to 30% higher(although not significant) in Lou/C rats than in WFR rats. This observation, along with the fact that liver TG concentration was not reduced in Lou/C rats, as it was in the WFR rats, suggest that, indeed, Lou/C rats may have metabolized more lipids than their Wistar counterparts. One has to recall that Lou/C rats exhibit a particular preference for fat (80%) at the expense of carbohydrate during aging without any modification of the evolution of body weight (7). There is also evidence that obesity-resistant rats, compared with obesity-prone rats, oxidized fat in a greater relative proportion in response to a high-fat diet (25). It is, thus, interesting to draw a parallel between Lou/C rats and obesity-resistant rats, both of them showing proportionally less accumulation of fat, most likely at the expense of an increased fat use.
In summary, the results of this study have shown that the spontaneous low-food intake (40%) in male Lou/C rats results in an approximately one-third reduction in body weight over a 14-week period. Although the same food intake–body weight reduction pattern can be reproduced in their male Wistar counterparts, Lou/C rats still exhibit reduced adipose tissue and higher muscle weights. These data, together with lower levels of plasma glucose in Lou/C rats, compared with food intake- and body weight-matched Wistar rats, indicate a regulation of basal metabolism in the former resembling that of rodents submitted to caloric restriction for a long period or to rats that are diet-resistant to become obese in a model of diet-induced obesity. Whether Lou/C rats have a spontaneously different profile of gene expression, as it has been recently reported in caloric-restricted mice (26), remains to be explored.