A cafeteria diet modifies the response to chronic variable stress in rats

Abstract

Stress is known to lead to metabolic and behavioral changes. To study the possible relationships between stress and dietary intake, male Sprague-Dawley rats were fed one of three diets for 6 weeks: high carbohydrate (HC), high fat (HF), or “Cafeteria” (CAF) (Standard HC plus a choice of highly palatable cafeteria foods: chocolate, biscuits, and peanut butter). After the first 3 weeks, half of the animals from each group (experimental groups) were stressed daily using a chronic variable stress (CVS) paradigm, while the other half of the animals (control groups) were kept undisturbed. Rats were sacrificed at the end of the 6-week period. The effects of stress and dietary intake on animal adiposity, serum lipids, and corticosterone were analyzed. Results showed that both chronic stress and CAF diet resulted in elevated total cholesterol, increased low-density lipoprotein (LDL), and lower high-density lipoprotein (HDL). In addition, increases in body weight, food intake, and intra-abdominal fat were observed in the CAF group compared with the other dietary groups. In addition, there was a significant interaction between stress and diet on serum corticosterone levels, which manifest as an increase in corticosterone levels in stressed rats relative to non-stressed controls in the HC and HF groups but not in the CAF group. These results show that a highly palatable diet, offering a choice of food items, is associated with a reduction in the response to CVS and could validate a stressor-induced preference for comfort food that in turn could increase body weight.

Keywords::

• Chronic-variable stress
• high-fat diet
• cafeteria diet
• food intake
• palatability
• corticosterone

Introduction

Stress is defined as a state of threatened homeostasis that is restored by a complex repertoire of physiological and behavioral adaptive responses (Chrousos 1998). Stresses can give rise to changes in eating behavior in humans as well as in animals. In humans, it was shown that stress can lead to hyperphagia (nibbling, bulimia) as well as hypophagia (Morley et al. 1983). In rodents, stress is known to inhibit food intake both acutely and chronically (Armario et al. 1984, 1990; Marti et al. 1994; Meerlo et al. 1996; Rybkin et al. 1997; Diane et al. 2008).

Chronic application of the same stressor induces habituation in rodents (Dal-Zotto et al. 2000; Marin et al. 2007; Stewart et al. 2008), leading to a diminution of behavioral and physiological responses to the stressor, including attenuated stress-induced hypophagia (Krahn et al. 1990). Chronic variable stress (CVS) is a model in which variable acute stress paradigms are applied unpredictably over several weeks. Traditionally, CVS has served as a model of depression in rodent studies (Katz 1982; Willner 2005). In addition, it is considered as a relevant stress model designed to be resistant to habituation, since it induces chronic hyperactivity of the hypothalamic–pituitary–adrenal (HPA) axis, as indicated by persistent elevated corticosterone levels (Rai et al. 2003; Marin et al. 2007). Furthermore, CVS is known to systematically decrease body weight gain (Ulrich-Lai et al. 2006; Marin et al. 2007). In rats or mice, CVS leads to anhedonia, characterized by a decrease in consumption of a palatable sweet solution (measured by a sucrose preference test) (Willner 1997).

Dietary characteristics can also influence the adaptation to chronic stress. Intake of palatable food is known to dampen stress responses (Adam and Epel 2007), especially when a choice of food items is offered (la Fleur et al. 2005). There appears to be a link between palatable “comfort” food and positive emotional feelings (sense of security, well-being). Indeed, it has been demonstrated that comfort foods improve stress symptoms in rats exposed to different types of acute or chronic stresses (Ulrich-Lai et al. 2007; Foster et al. 2009; Maniam and Morris 2010). For example, palatable diets were shown to improve anxiety and depression-like symptoms following an adverse early environment in rats (Maniam and Morris 2010). Similarly, in humans, increased consumption of sweet and fatty foods during stress was found to reduce the stress response (Epel et al. 2001; Zellner et al. 2006).

The objective of this study was to evaluate the effect of the availability of a palatable diet, as compared with monotonous high-fat or standard balanced diets, on the response to CVS in rats. We used a CVS model in which rats were exposed sequentially, over a period of weeks, to a variety of mild stressors. We compared three types of diet: a standard high-carbohydrate diet, a high-fat diet, and a cafeteria diet (Shafat et al. 2009). The ‘Cafeteria’ diet refers to the offering of a number of ‘cafeteria’ type foods (high fat, energy, and sugar contents) (Rothwell and Stock 1988; Shafat et al. 2009). We chose the cafeteria diet because of its greater palatability and its lack of monotony when compared with the other diets.

Materials and methods

Animals and diets

Forty-eight adult male Sprague-Dawley rats (Lebanese American University Stock) aged initially 8–10 weeks old were housed in groups of four in a temperature and humidity-controlled room under a 12:12 light/dark cycle (lights on at 08:00 h) during the whole length of the experiment. The animals were weight-matched and assigned to one of the three following dietary groups: standard high-carbohydrate (HC), high-fat (HF), or cafeteria diet (CAF; n = 16 per group). The cafeteria diet consists of the standard high-carbohydrate and a choice of highly palatable cafeteria-style foods with known caloric intake: chocolate, biscuits, and peanut butter (Table I). The diets were given ad libitum for 6 weeks. All experimental protocols were approved by the Animal Ethical Committee of the Lebanese American University, which complies with the Guide for the Care and Use of Laboratory Animals (National Research Council of the United States 2011). Food intake and body weight gain were monitored three times a week throughout the study. Food intake, corrected for spillage, including individual CAF diet items, was recorded at 8 a.m. by measuring the difference in food cup weight before and after presentation to the rats.

Table I.  Nutrient composition of the HC and HF diets and of cafeteria diet food items.

	Monotonous diets		Cafeteria diet food items
	HC^*	HF^†	Standard high carbohydrate^‡	“Kit Kat” chocolate^¶,§	“Digestive” biscuits^‖,§	Peanut butter^#,§
Protein (% wt)	19	16.5	19	7.6	6.3	23.6
Carbohydrates (% wt)	65.3	53	65.3	59.1	67.7	12.5
Sugars	9.2	7.5	9.2	43.7	13.6	6.3
Fat (% wt)	9.6	25	9.6	26.2	21.8	46.8
Fat breakdown
Saturated fat (%)	18	34^**	18	13	48	68
Monounsaturated fat (%)	29	38^**	29	46	0	0
Polyunsaturated fat (%)	47	34^**	47	41	52	32
Fiber (% wt)	4.3	3.6	4.3	3	3.5	6.2
Metabolizable energy (kJ/g)	17.7	21	17.7	21.1	20.7	24.5
Energy (protein %)	18	13	18	47	22	26
Energy (carbohydrate %)	62	42	62	13	68	59
Energy (fat %)	20	45	20	24	6	8

Notes: Food composition data are provided by the manufacturers' information or food tables. Data are not corrected for evaporation

^* Laboratory rodent starter diet no. 1, Hawa Chicken Co. (Safra, Lebanon)

^† Diet derived from the standard HC diet enriched with added oils (same procedure as Daher et al. 2006)

^‡ Hawa chicken rodent starter diet no 1, Lebanon

^¶ Nestle S.A. (Switzerland)

^§ Items shown to cause hyperphagia in rats fed cafeteria-type diets (Shafat et al. 2009)

^‖ McVitie's, United Biscuits (Hayes, Middlesex, UK)

^# Monarch (San Ramon, California, USA)

^** Values were chosen to match the average fatty acid composition of the cafeteria diet consumption for six rats during 5 days (separate preliminary study).

CVS procedure

After the first 3 weeks of diet, rats from each dietary group were again divided into two groups after being weight-matched (n = 8): control and stressed. Controls were kept undisturbed in their home cages during the following 3 weeks of treatment, and the others were stressed daily for the remaining period of the study using a CVS model. This stress model was modified from existing models (Duncko et al. 2001; Gamaro et al. 2003) and consisted of a 21-day variable stressor paradigm during which different individual stressors were applied each day for different lengths of time (one stressor per day). The following stressors were used: (a) cage tilt for 3 or 4 h (home cages were tilted to 30° from the horizontal); (b) space reduction for 4 or 5 h (eight rats were placed in a collective cage usually designed for four to five rats, dimensions 50 cm × 40 cm × 21 cm); (c) restraint for 1–3 h (rats were placed into a perforated plastic tube of 6 cm × 20 cm in which they could not turn over); (d) forced swimming for 10 min in cold water (five rats were placed together in a plastic tank, dimensions 100 cm height × 50 cm diameter containing 75 cm of water at 18°C) or in (e) warm water (same procedure at 26°C); (f) flashing light for 3 or 4 h (flashes at 40 W with frequency of 5 flashes/s); and (g) neighbor cages (move to an unclean unoccupied neighbor cage for 14 h). Animals remained on their diets throughout stress exposure. Stress application started at different times every day in order to minimize its predictability. Table II reports the detailed stressor schedule.

Table II.  Schedule of stressor agents used during the chronic stress treatment.

Day of treatment	Stressor used	Duration	Time of administration
1	Flashing light	4 h	12:00–16:00
2	Neighbor cage	14 h	17:00–07:00
3	Cage tilt	3 h	10:00–13:00
4	Space reduction	4 h	08:00–12:00
5	No stressor	x
6	Flashing light	4 h	14:00–18:00
7	Forced swimming (warm)	10 min	12:00–12:10
8	Restraint	1 h	09:00–10:00
9	Cage tilt	4 h	08:00–12:00
10	Flashing light	3 h	12:00–15:00
11	Neighbor cage	14 h	17:30–07:30
12	No stressor	x
13	Cage tilt	3 h	11:00–14:00
14	Restraint	3 h	10:00–13:00
15	Flashing light	4 h	14:00–18:00
16	Forced swimming (cold)	10 min	15:00–15:10
17	Restraint	2 h	12:00–14:00
18	Space reduction	5 h	08:00–13:00
19	No stressor	x
20	Restraint	2 h	09:00–11:00
21	Forced swimming (warm)	10 min	16:00–16:10

Sucrose preference test

A sucrose preference test was conducted every week to assess hedonic behavior and monitor the effectiveness of the CVS model (Willner 1997; Duncko et al. 2001). The test consists of a 12-h period of water deprivation, after which animals are exposed to two bottles: one containing water and the other a 1% sucrose solution. The amount of sucrose solution ingested during the subsequent 2 h, corrected for body weight, represents the hedonic behavior. The sucrose preference test was initiated for three sessions prior to the start of the stress procedure and was conducted in the animals' home cages.

Blood collection and abdominal fat dissection

The rats were fasted overnight at the end of the CVS procedure. The following morning they were deeply anesthetized with pentobarbital (100 mg/kg), blood was drawn from the inferior vena cava 5–6 min after the induction of anesthesia, and the rats were then euthanized by exsanguination. The intra-abdominal fat (epididymal, mesenteric, and retroperitoneal) was subsequently removed and weighed.

Serum lipid and serum corticosterone analysis

The blood samples were centrifuged at 2000g for 15 min. The collected serum was then stored at − 80°C for subsequent analysis. Blood lipid analysis (total cholesterol, high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, and triglycerides) was performed using Spinreact kits (Spinreact, Girona, Spain). Serum corticosterone levels were determined using the Double Antibody 125I Radioimmunoassay Kit for rats and mice (MP Biomedicals, LLC, Solon, OH, USA). All samples were run in duplicate and analyzed within the same assay.

Statistical analysis

Body weight gain and dietary intake data were analyzed separately using a three-way ANOVA, with treatment (no stress vs. stress) and diet (HC vs. HF vs. CAF) as the between-subject factors and time as the within-subjects factor.

Sucrose preference data, abdominal fat, serum corticosterone, and blood lipids were analyzed using a two-way ANOVA with treatment (no stress vs. stress) and diet (HC vs. HF vs. CAF) as factors. Descriptive data are presented as means ± standard error of the mean (SEM). Significant main effect differences were tested using Tukey–Kramer's post-hoc test for multiple comparisons. All data were analyzed using the SPSS 18 statistical package, with statistical significance defined as p < 0.05.

Results

Energy intake

There was a significant effect of diet on energy intake (F(2,47) = 4.25, p = 0.02). Animals fed the CAF diet had significantly higher energy intake normalized to their body weight (224.2 ± 9.9 kJ/100 g rat/day) compared with animals fed the HC (176.5 ± 6.8 kJ/100 g rat/day) or HF (152.5 ± 8.8 kJ/100 g rat/day) diets. Among stressed animals, energy intake was significantly affected by time (F(1,46) = 4.72, p = 0.035): stressed animals ate significantly less during the stress period (167.2 ± 8.5 kJ/100 g rat/day) compared with the baseline feeding period before stress (193.4 ± 7.1 kJ/100 g rat/day; Figure 1).

Figure 1.  Energy intake (kJ/rat/day) of rats fed a high-carbohydrate, high-fat, or cafeteria diet and either submitted to a CVS paradigm or not during weeks 3–6. Data are expressed as the mean ± SEM. *Statistical difference with regard to the first 3 weeks. ^aStatistical difference with regard to the high-carbohydrate dietary group in the same stress condition. p < 0.05 is considered significant. Note: there was no significant difference between the energy intake of stressed and non-stressed animals.

When comparing the average energy intake normalized to bodyweight from the cafeteria items, it appears that rats were consuming the biscuits the most, followed by HC food, chocolate, and peanut butter. There was no significant effect of stress on the consumption of any CAF item. However, within stressed animals, there was a significant effect of time on the consumption of peanut butter (F(1,5) = 34.7, p = 0.002), chocolate (F(1,5) = 7.6, p = 0.05), and standard HC diet (F(1,5) = 16.2, p = 0.013). Stressed animals consumed significantly less of these three items during the 3-week stress period compared with the 3-week baseline period (Table III).

Table III.  Energy intake (kJ/rat/day) from different cafeteria items of rats fed a cafeteria diet and either submitted to a CVS paradigm or not during weeks 4–6.

		Weeks 4–6
	Weeks 1–3	Non-stressed rats	Stressed rats
Peanut butter	30.1 ± 1.2^d	28.4 ± 1.5^c	21.4 ± 1.8^* ^,d
Chocolate	37.4 ± 2.1^c	34.9 ± 2.4^c	29.6 ± 1.7^* ^,c
Biscuits	87.4 ± 3^a	91.2 ± 2.1^a	84.3 ± 4.1^a
Standard HC	72.6 ± 2.9^b	75.9 ± 1.9^b	66.3 ± 2.2^* ^,b

Notes: Data are expressed as the mean ± SEM. Means within each column with different letters (a–d) differ significantly (p < 0.05)

* Statistical difference with regard to the first 3 weeks.

Sucrose preference

Stressed animals showed a decreased consumption of sucrose solution corrected for body weight (9.42 ± 1.3 ml/100 g body wt.) compared with non-stressed rats (14.7 ± 1.4 ml/100 g body wt.) F(1,35) = 4.1, p = 0.05. Similarly, sucrose preference, that is, the percentage of sucrose solution from the total liquid ingested, was significantly decreased in stressed animals compared with sucrose preference in controls (64.6% ± 1.3 vs. 70.1% ± 1.2). However, when looking at individual dietary groups, the percentage of sucrose solution ingested was only significantly lower in stressed animals of the HC (t(10) = 2.2, p = 0.04) and HF groups (t(10) = 2.4, p = 0.05) compared with their corresponding non-stressed groups (Figure 2). In contrast, sucrose preference was not significantly affected by diet nor was there a significant interaction effect between treatment and diet.

Figure 2.  Percentage of sucrose solution from the total liquid ingested during 2 h in rats fed a high-carbohydrate, high-fat, or cafeteria diet and either submitted to a CVS paradigm or not (n = 8). Data represent the average of three sucrose preference tests conducted during weeks 4–6 of the experiment. Results are expressed as mean ± SEM. *Statistical difference with regard to the non-stressed group having the same diet. p < 0.05 is considered significant.

Body weight and body composition

The final body weight of the rats was significantly affected by diet (F(2,47) = 3.9, p = 0.029), with CAF-fed animals having significantly higher body weights than both the HC and HF groups (401.5 ± 7.3 g for the CAF group vs. 376.2 ± 7.3 g for the HC, and 377 ± 7.3 g for the HF; Table IV). There was no effect of stress on the final body weight of rats. However, daily body weight gain of the stressed animals (all dietary groups together) was significantly affected by time (F(1,47) = 6.4, p = 0.015). Indeed, stressed rats had a significantly lower daily weight gain during the 3-week stress period (2.3 ± 0.2 g/rat/day) compared with the 3-week baseline period without stress (3.9 ± 0.2 g/rat/day).

Table IV.  Body weights, body weight gain, and abdominal fat for rats fed a standard chow, high-fat, or cafeteria diet and either submitted to a CVS paradigm or not (n = 8).

	High-carbohydrate diet		High-fat diet		Cafeteria diet		Statistical significance
	Non-stressed	Stressed	Non-stressed	Stressed	Non-stressed	Stressed	Effect of diet	Effect of stress	Interaction of diet with stress
Final body weight (g)	378 ± 4.1	374.5 ± 5	382.5 ± 11.1	371.5 ± 3.1	407.5 ± 12^*,†	395.6 ± 9.4^*,†	0.029	n.s.s.	n.s.s.
Total body weight gain (g)	130 ± 1.9	125.1 ± 2.1	124.9 ± 3.1	120.8 ± 2.9	158.5 ± 4.1^*,†	149.6 ± 3.7^*,†	0.017	n.s.s.	n.s.s.
Total weight gain in weeks 1–3 (g/day)	3.7 ± 0.1	3.9 ± 0.1	3.3 ± 0.2	3.3 ± 0.3	4.2 ± 0.2^*,†	4.4 ± 0.3^*,†	0.03	n.s.s.	n.s.s.
Total weight gain in weeks 4–6 (g/day)	3 ± 0.1	1.9 ± 0.2	3.5 ± 0.2	2.5 ± 0.2	4.1 ± 0.2^*,†	2.6 ± 0.3^*	0.04	0.024	n.s.s.
Abdominal fat percent (% body weight)	1.23 ± 0.01	1.33 ± 0.2	1.6 ± 0.1	1.6 ± 0.1	2.8 ± 0.4^*,†	2.5 ± 0.5^*,†	p < 0.001	n.s.s.	n.s.s.

Notes: Data are expressed as the mean ± SEM. p < 0.05 was considered significant. n.s.s., non-statistically significant (p>0.05)

* Statistical difference with regard to the standard chow dietary group in the same stress condition

^† Statistical difference with regard to the high-fat dietary group in the same stress condition.

Intra-abdominal fat weight was significantly affected by diet (F(2,47) = 12.1, p < 0.001). Animals fed the cafeteria diet had significantly higher intra-abdominal fat percent compared with high-carbohydrate-fed (t(30) = 4.5, p < 0.001) and high-fat-fed animals (t(30) = 3.4, p = 0.002). There was no significant effect of stress on intra-abdominal fat (F(1,47) = 0.03, p = 0.87).

Serum lipids

Total serum cholesterol levels were significantly affected by stress (F(1,43) = 7.6, p = 0.009) as well as diet (F(2,43) = 3.1, p = 0.05; Table V). Stressed animals had significantly higher serum cholesterol levels than non-stressed animals (112.6 ± 6.6 vs. 87.7 ± 6.2 mg/dl). Also, cholesterol levels of animals in the CAF group and in the HF group (107.6 ± 7.4 and 109.4 ± 7.4 mg/dl) were significantly higher than the level of HC-fed animals (83.5 ± 8.6 mg/dl), t(26) = 2, p = 0.05 and t(26) = 1.9, p = 0.05, respectively. There was, however, no significant interaction between diet and stress.

Table V.  Serum lipid parameters for rats fed a standard chow, high-fat, or cafeteria diet and either submitted to a CVS paradigm or not (n = 8).

	High-carbohydrate diet		High-fat diet		Cafeteria diet		Statistical significance
	Non-stressed	Stressed	Non-stressed	Stressed	Non-stressed	Stressed	Effect of diet	Effect of stress	Interaction of diet with stress
Total cholesterol (mg/dl)	76.4 ± 11.2	90.6 ± 10.5	95.4 ± 10.5	124.3 ± 10.58^*,†	92.2 ± 10.5	122.9 ± 13.3^*,†	p = 0.05	p = 0.009	n.s.s.
Triglycerides (mg/dl)	136.3 ± 16.5	123.9 ± 18.2	108 ± 15.4	136.1 ± 14.4	107.5 ± 15.4	155.5 ± 15.4	n.s.s.	n.s.s.	n.s.s.
HDL (mg/dl)	50 ± 3.1	46.4 ± 2.8	50.6 ± 4.5	41.5 ± 2.1	41.2 ± 3.1^†	37.9 ± 2.2^†	p = 0.034	n.s.s.	n.s.s.
LDL (mg/dl)	33.5 ± 7.5	44.2 ± 6.1	50.2 ± 14.2	87.9 ± 11.4^*,†	51 ± 10.2	84.9 ± 10.1^*,†	p = 0.023	p = 0.005	n.s.s.

Notes: Data are expressed as the mean ± SEM. p < 0.05 was considered significant. Values from the table may differ from text values because they refer to individual group values rather than clustered values for an effect. n.s.s., non-statistically significant (p>0.05)

* Statistical difference with regard to the non-stressed group having the same diet

^† Statistical difference with regard to the high-carbohydrate dietary group in the same stress condition.

LDL cholesterol levels were significantly affected by both diet (F(2,43) = 4.1, p = 0.023) and stress (F(1,43) = 8.9, p = 0.005). LDL was higher in CAF- and HF-fed animals (68 ± 7.5 and 69.1 ± 7.5 mg/dl) compared with the HC group (38.8 ± 8.8 mg/dl), t(26) = 2.23, p = 0.034 and t(26) = 2.34, p = 0.027, respectively. In addition, stressed animals had significantly higher LDL levels than non-stressed animals (72.4 ± 6.7 compared with 44.9 ± 6.3 mg/dl, p = 0.005). There was no significant interaction of diet and stress with regard to LDL levels. There was no significant effect of stress on HDL cholesterol levels. However, HDL was affected by diet F(2,40) = 3.7, p = 0.034, with CAF-fed animals having significantly lower HDL levels compared with HC-fed rats (39.6 ± 2.1 vs. 48.2 ± 2.6 mg/dl), t(25) = 2.2, p = 0.037.Serum triglyceride levels were not affected by dietary treatment (F(2,42) = 0.6, p = 0.80) or stress (F(1,42) = 0.2, p = 0.11).

Corticosterone levels

There was a significant interaction effect between stress and diet on rat serum corticosterone levels (F(2,47) = 8.9, p = 0.001; Figure 3). Stressed animals (all diets included) showed increased serum corticosterone (152 ± 4.9 ng/ml) levels compared with non-stressed animals (113.3 ± 4.9 ng/ml) (F(1,47) = 30.1, p < 0.001). However, the corticosterone levels were normalized in cafeteria-fed animals: there was no significant difference between the corticosterone levels of stressed compared with non-stressed animals fed the cafeteria diet.

Figure 3.  Serum corticosterone concentration (ng/ml) of rats fed a standard chow, high-fat, or cafeteria diet and either submitted to a CVS paradigm or not (n = 8). Data are expressed as the mean ± SEM. *Statistical difference with regard to the non-stressed group having the same diet. p < 0.05 is considered significant.

Discussion

The objective of this study was to investigate the influence of different diets on the response to stress. CVS and the CAF diet were associated with worsened lipid profiles. Moreover, a significant interaction was found between diet intake and stress, as demonstrated by decreased serum corticosterone levels in CVS animals on the CAF diet. These findings suggest a reduction in the response to CVS following palatable food consumption.

Animals fed the cafeteria diet had higher body weight and intra-abdominal fat percentages compared with the other dietary groups. Body weight and fat depot, particularly intra-abdominal fat, were already shown to increase after access to a palatable diet containing lard and sucrose (Foster et al. 2009). In our study, the CAF diet offers a choice of foods, thereby increasing its palatability and justifying the increase in body weight and fat. CAF- and HF-fed animals had higher total cholesterol and LDL cholesterol levels compared with the other animals. Similar effects of HF (Manting et al. 2011) and cafeteria diets (Sugatani et al. 2008) on blood lipids have previously been described. CAF-fed animals showed lower HDL cholesterol levels as well. The cafeteria diet therefore worsened the blood lipid profile of animals to a greater extent than the high-fat feeding. This difference is probably due to the higher overall consumption of the CAF diet, owing to its increased palatability as well as its higher saturated and trans-fat content.

Chronic exposure to mild unpredictable stress decreases the consumption of palatable sweet solutions (Willner et al. 1987; Willner 1997; Duncko et al. 2001; Wang et al. 2010). As expected, in our experiment, CVS induced anhedonia, manifested by decreased sucrose preference relative to non-stressed rats. However, when considering the CAF groups alone, the difference between the stressed and non-stressed groups was not significant. Similar results regarding sucrose preference were found in a study investigating the effects of palatable foods on stress in rats Maniam and Morris (2010). CVS models are known to induce chronic hyperactivity of the HPA axis, a conclusion supported by persistent elevated corticosterone levels (Rai et al. 2003; Marin et al. 2007). Increased corticosterone was also observed in our study, verifying the effectiveness of the stress regimen. In addition, among stressed animals, both food intake and body weight gain were lower during the stress period compared with the control period. Our results are in accordance with other studies that show a reduction in body weight gain and food intake in response to CVS (Gamaro et al. 2003; Solomon et al. 2010). Rats from the CAF groups were consuming biscuits the most, followed by standard HC food, chocolate, and finally peanut butter. Similar consumption patterns were found for these items in other studies (Shafat et al. 2009). Food preference was not based on sugar content, since the biscuits and HC diet are lower in sugar relative to chocolate. The preference for biscuits over chocolate could be due to their texture. In addition, stressed animals consumed significantly less of the standard HC diet, chocolate, and peanut butter during the 3-week stress period compared with the 3-week baseline period but maintained the same biscuit consumption, indicating a CVS-related change in preference.

Stressed animals had higher total cholesterol and LDL levels than non-stressed animals, with similar HDL and TG levels. These results agree with a recent study showing that chronic mild unpredictable stress across a 3-week period leads to an increase in rat LDL cholesterol and TG levels but no modification in HDL levels (Neves et al. 2009). In addition, a recent study investigating the effects of HF diets on blood lipid profiles (Manting et al. 2011) found that HF diets and chronic stress (once a day) act together and induce abnormalities in blood and tissue lipids referred to as the lipid metabolism disorder. Exposure to both a HF diet and chronic stress was found to induce even higher levels of cholesterol, TG, and liver TG than exposure to a HF diet only. Conversely, in a recent study by Paternain et al. (2011), a milder form of the chronic stress regimen used in our study (stressors administered three times a week) did not cause any changes in the plasma lipid profile of rats fed a standard or cafeteria diet. This could suggest the existence of a stress intensity threshold for CVS-related dyslipidemia.

There was a significant interaction between stress and diet on serum corticosterone levels. There was an increase in corticosterone levels of stressed rats compared with non-stressed rats in all the groups except the cafeteria-fed group. Therefore, CVS causes an increase in corticosterone, but this increase is attenuated by the CAF diet. These findings are also in agreement with the recent study by Maniam and Morris (2010) in which a cafeteria diet was shown to reduce anxiety and depression-like symptoms in rats following exposure to an adverse environment (maternal separation). Similarly, another study demonstrated that the consumption of a highly palatable and highly caloric diet (chocolate) prevented an increase in adrenal gland weight after exposure to chronic restraint stress in rats (Fachin et al. 2008). Finally, a study on mice revealed that the consumption of a high-fat and high-sugar diet was associated with decreased corticosterone levels in mice exposed to restraint stress (Kuo et al. 2008). The results of all these studies are in agreement with our own, suggesting that palatable food consumption reduces the overall impact of stress. The fact that the CAF diet was not only palatable but also offered a choice of items compared with the high-fat diet, which might be of great importance in our results. Similar results were found with acute restraint stress in rats. Indeed, it was shown that rats provided with a choice of lard or sucrose demonstrated an inhibition of ACTH and corticosterone responses, whereas rats on a monotonous high-fat diet did not experience such changes after a 1-week dietary exposure (la Fleur et al. 2005). It must be noted that the relatively high baseline corticosterone values obtained in our experiment may be due to both the pentobarbital injected [previously shown to increase serum corticosterone levels in rats (Vahl et al. 2005)] and the actual act of injecting it.

The mechanism by which there is an interaction between stress response and diet palatability could be through an attenuation of the HPA axis stress response. The HPA axis is a conductor for stress responses, but is also tightly intertwined with the endocrine regulation of appetite. Stress and palatable food can stimulate endogenous opioid release. It was proposed that in turn, opioid release could be part of a powerful defense mechanism protecting from the detrimental effects of stress by decreasing the activity of the HPA axis and thus attenuating the stress response (Adam and Epel 2007). In this context, it was shown in rats that a daily limited consumption of sugar or a non-caloric sweetener leads to a decrease in corticotropin-releasing hormone mRNA expression in the paraventricular nucleus of the hypothalamus. Furthermore, sucrose intake was found to attenuate restraint-induced c-fos mRNA expression in the basolateral amygdala, infralimbic cortex, and claustrum of rats (Ulrich-Lai et al. 2007). Further experiments measuring neurotransmitter levels need to be conducted in order to better understand the underlying mechanism that leads to decreased corticosterone levels and whether or not the CAF diet interferes with the sympathetic nervous system.

Evidence suggests that the time of food availability could also play a role in affecting food intake and corticosterone response. Indeed, a recent study in rat shows that intermittent palatable food access induces a different corticosterone response to acute restraint stress than that seen following regular access (Kinzig et al. 2008). Future work could therefore study the effects of administering the cafeteria diet items discontinuously during the CVS paradigm. Also, a high-sucrose dietary group could be included to account for the higher sucrose content of the CAF diet compared with the HC and HF diets.

This study showed a decreased response to CVS in rats fed a cafeteria diet compared with both a standard high-carbohydrate diet and a monotonous high-fat diet. A cafeteria diet seems to interact with chronic stress to prevent an increase in corticosterone levels. While chronic stress alone tends to lower adiposity and increase corticosterone levels, and while a palatable cafeteria-type diet alone does not lead to changes in corticosterone levels, the combination of both leads to an attenuation of the stress-induced corticosterone upsurge. This interaction could validate a stressor-induced preference for comfort food. Furthermore, these mechanisms, determined in rats, may explain some of the obesity epidemic occurring in our society, especially given the relationship between socioeconomic conditions and obesity, which may be related to social stress (Wardle et al. 2010). A review article by Dallman et al. (2003) has already suggested that people opt for comfort foods that are rich in fat and sugar in an attempt to reduce the activity of the chronic stress-response network. However, habitual use of these foods results in abdominal obesity, type II diabetes, cardiovascular disease, and stroke.

Declaration of interest The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.