The Cafeteria Diet: a standardized protocol and its effects on behavior. Jaume F. Lalanza, Eelke M.S. Snoeren. Neuroscience & Biobehavioral Reviews, December 10 2020. https://doi.org/10.1016/j.neubiorev.2020.11.003
Rolf Degen's take: https://twitter.com/DegenRolf/status/1337087523336712195
Highlights
• Cafeteria (CAF) diet has high construct validity.
• CAF diet induces hyperphagia and metabolic syndrome better than other diets.
• A CAF protocol should include different nutrients, tastes, textures, etc.
• A CAF protocol should rotate and be voluntary.
• CAF diet alters reward preferences and tends to reduce stress and spatial memory.
Abstract: Obesity is a major health risk, with junk food consumption playing a central role in weight gain, because of its high palatability and high-energy nutrients. The Cafeteria (CAF) diet model for animal experiments consists of the same tasty but unhealthy food products that people eat (e.g. hot dogs and muffins), and considers variety, novelty and secondary food features, such as smell and texture. This model, therefore, mimics human eating patterns better than other models. In this paper, we systematically review studies that have used a CAF diet in behavioral experiments and propose a standardized CAF diet protocol. The proposed diet is ad libitum and voluntary; combines different textures, nutrients and tastes, including salty and sweet products; and it is rotated and varied. Our summary of the behavioral effects of CAF diet show that it alters meal patterns, reduces the hedonic value of other rewards, and tends to reduce stress and spatial memory. So far, no clear effects of CAF diet were found on locomotor activity, impulsivity, coping and social behavior.
Keywords: Cafeteria DietWestern DietJunk FoodObesityAnimal ModelSystematic ReviewFood PreferenceStressMemoryReward System
4.6. Mood: Anxiety- and Depression-like Behaviors
Food and mood are closely related. While stress in some cases induces a reduction in the amount of food consumed, in others, stress increases eating, and often the eating of highly palatable food. This phenomenon is colloquially referred to as eating “comfort food”, and it applies to both humans and rodents (Dallman et al., 2003; Pecoraro et al., 2004). Comfort food can be defined as food that is eaten as an attempt to reduce anxiety and for its well-being effects rather than its nutritional value, owing to humans and other animals obtaining pleasure due to emotional relief (Dallman, 2010; Dallman et al., 2003).
Consistently, stress is considered a risk factor for obesity (Sinha and Jastreboff, 2013), and epidemiological and longitudinal studies have found a bidirectional relationship between mood disorders and obesity (Gariepy et al., 2010; Luppino et al., 2010). A recent meta-analysis found that the prevalence of anxiety and depressive symptoms were significantly higher in overweight and obese children and adolescents than in the non-overweight/obese (Wang et al., 2019). This comorbidity is, however, not fully understood yet and it could be strongly affected by genetic predisposition and environmental factors (Mansur et al., 2015).
If there is a causal relationship between junk food and mood in humans, one would expect that exposure to the CAF diet should also affect anxiety- and depression-like behaviors in rodents. Regarding gestational designs, only a few studies assessed the effects of dams receiving CAF diet on stress responses in their pups. Superficially, these studies seem to have reported opposing effects – two studies finding reduced anxiety levels in pups fed a CAF diet in the open field (Speight et al., 2017; Wright et al., 2011a), while a third study found a decrease in the amount of time spent in the center, suggesting an increase in anxiety (Ramírez-López et al., 2016). However, a more detailed examination of the methods employed appear to offer an explanation of the differences observed. Specifically, the studies differed in how strongly the open field environment in which anxiety-related behavior was illuminated – light levels being a key determinant of how anxiogenic the environment is. Hence, Wright et al. and Speight et al., used high (i.e. strongly anxiogenic) levels of illumination above the test set-up (130 and 70 lux, respectively) compared to Ramírez-López et al. (30 lux). Illumination in an open arena could be a critical factor for the visual system and as discussed before, albino rats may experience more disturbances than pigmented strains. Nevertheless, all these three studies used the same albino strain Wistar, so the strain does not seem the cause of this discrepancy. Therefore, we hypothesize that the low illumination was insufficient to induce significant anxiety, at least in the open field test.
Regarding tests of anxiety-related behavior in the elevated plus maze, the lack of effects seen in the study by Speight et al, can be explained at least by the age at testing, since they looked at rats on PND23, 2 days after weaning, while the rats in the other studies were tested 9-10 weeks after weaning (Ramírez-López et al., 2016; Wright et al., 2011a). Ramírez-López et al. (2016) saw increased levels of anxiety-like behavior in the elevated plus maze, whereas Wright et al. (2011a) found the decreased levels in the same test in males. It should be noted though that Wright et al. (2011a) compared pre-gestational, gestational and lactation periods, and the reductions in anxiety were not generalized to all experimental groups.
Finally, increased levels of anxiety-like behaviors were also seen in the light-dark test, as mice pups from dams on a CAF diet entered the dark (and less anxiogenic) compartment faster and more often from the light (and more anxiogenic) compartment (Ribeiro et al., 2018).
Therefore, indirect CAF diet exposure via their mothers may result in both an increase and a reduction in anxiety-related behavior. As mentioned before, pups showed more anxiety when their dams were fed CAF diet in some studies (Ramírez-López et al., 2016; Ribeiro et al., 2018), however these studies administered a little varied CAF diet compared to the studies that found reduced anxiety-like behavior. In addition, it is possible that increased stress levels in pups were not produced directly by gestational CAF diet exposure but by abnormal maternal care, because the CAF diet can alter maternal care behaviors (Ribeiro et al., 2018; Speight et al., 2017) (see Social Behavior section below). Therefore, after considering these methodological differences, we tentatively conclude that indirect CAF diet exposure via their mothers may result in a reduction in anxiety-related behavior, even though more studies are required in order to further validate this conclusion.
In classical design studies, where subjects were fed CAF diet themselves, the effects of this diet on behavioral and neurophysiological measures of anxiety levels also remain unclear. When we look at effects in male rats and mice, four studies found reduced anxiety levels, in either the open field or elevated plus maze test (de Oliveira et al., 2019; Lalanza et al., 2014; Leffa et al., 2015; Pini et al., 2017), while four studies found no changes in anxiety (Beilharz et al., 2018; Ferreira et al., 2018; Sack et al., 2017), or an increase upon CAF diet consumption (Ferreira et al., 2018; Warneke et al., 2014). Again, all studies conducted with rats chose an albino strain (Wistar or Sprague-Dawley), so the albinism of the animals cannot explain these differences. In female rats, two studies found a reduction in anxiety levels (Lalanza et al., 2014; Warneke et al., 2014) with more pronounced effects in adult females (Warneke et al., 2014), while two studies found no changes (Cigarroa et al., 2016; da Costa Estrela et al., 2015). Since illumination levels during behavioral tests were not properly reported in most of these classical studies, we cannot determine whether this might have contributed to the different findings. Social isolation is neither an alternative factor, as single and grouped-housing protocols are found in both outcomes. It is, thus, difficult to draw any conclusions on whether CAF diet has an effect on anxiety.
Interestingly, CAF diet exposure shortly after weaning did not, at first, reduce anxiety in the elevated plus maze and in the light/dark test, but it did significantly attenuate the anxiogenic effect of maternal separation (Maniam and Morris, 2010a). Attenuating effects on stress levels due to maternal separation were also seen in the dams (Maniam and Morris, 2010b). These results might suggest that CAF diet, and junk food in general, exerts significant anxiolytic effects in stressed subjects.
The hypothesis of junk food consumption reducing stress is supported by the effects of the CAF diet on neurophysiological measures. Maniam and Morris (2010a, 2010b) also found that a CAF diet reduced the expression of glucocorticoid receptors (GR) and corticotropin-releasing hormone (CRH) after maternal and litter separation. Similarly, the CAF diet also reduced ACTH (adrenocorticotropic hormone) and corticosterone after chronic stress exposure (Gustaityte et al., 2019; Zeeni et al., 2015b; Zeeni et al., 2013) and CAF diet in rats reduced adrenal gland weight gain following restrain stress (Macedo et al., 2015). Finally, it was also shown that switching from STD chow to CAF diet exposure reduced corticosterone levels after restraint-induced stress (South et al., 2012) and also reduced amygdala GR receptors mRNA expression compared to chronic (15 weeks) CAF diet (Martire et al., 2014).
Overall, we conclude that CAF diet consumption can attenuate stress responses. This clear effect of a CAF diet on the neurophysiology of stress is not a surprise, given that the neural circuits mediating stress responses, reward processing and those regulating feeding behavior and metabolism are intimately connected (for a review, Meye and Adan, 2014; Rabasa and Dickson, 2016; Sinha, 2018). There is overlap and mutual influence between these systems, for instance, the paraventricular nucleus of the hypothalamus is a hub connecting feeding and stress by regulating food intake and activation of the HPA axis, these actions being sensitive to both glucocorticoids and insulin (for a review, Koob and Schulkin, 2019; Morris et al., 2015).
The association between CAF diet consumption and depression-like behaviors has also been assessed. In male rats, it was shown that a fixed CAF diet does not induce depression-like behavior as assayed by the forced swim test and the sucrose preference test (Ferreira et al., 2018; Macedo et al., 2015). In male mice, though, a rotated CAF diet reduced immobility time in both the forced swim and the tail suspension test, suggesting a reduction of certain aspects of depressive-like behavior (Leffa et al., 2015). This latter finding is in line with the observation of increased levels of the neurotrophins BNDF (brain-derived neurotrophic factor) and NGF (nerve growth factor) in the hippocampus, both which are considered as anti-depression markers (for a review, Björkholm and Monteggia, 2016).
In female rats, a rotated CAF diet has also been shown to reduce some signs of depression-like behavior in the forced swim test (Maniam and Morris, 2010b). These results highlight the importance of considering both the specie – mouse is not a small rat – and the sex of experimental subjects, especially when studying mood disorders which display pronounced sex biases (for a review, Altemus et al., 2014).
Interestingly, in male rats exposed to stressors (either maternal and litter separation or chronic variable stress), the CAF diet compared to STD chow reduced depression-like behaviors (Maniam and Morris, 2010a, 2010b;) and tend to reduce them (Zeeni et al., 2013). This would suggest that a CAF diet exerts its antidepressant effects on animals already suffering from a mood disorder, as we noted in the stress sub-section. On the other hand, however, da Costa Estrela et al. (2015) found the opposite effect, whereby the CAF diet reduced the climbing time in the forced swim test (climbing is considered a non-depressive-like behavior) in female rats previously exposed to restraint-induced stress. However, in the STD chow group, the restraint-induced stress did not induce the expected depression-like behaviors, which might indicate that animals were, indeed, not depressed.
To conclude, the CAF diet has been shown to have anxiolytic and antidepressant-like effects on animals that were placed in higher stressful states. For non-stressed animals, the effects of a CAF diet on anxiety and depression have been less clear, but there seems to be a trend towards an anxiolytic effect overall (Table 4). Thus, the idea of eating junk food as a comfort food to relieve negative mood states may be valid. Nevertheless, more research is needed for a better understanding of the bidirectional relationship between junk food eating and negative mood states.
4.7. Coping Behavior
The aforementioned behavioral tests, such as the elevated plus maze, light/dark test and the forced swim test are all assays based on unconditioned responses, and are said to involve passive responses. That is, there is not an active solution for the animals to take in order to avoid the stressor (Bourin et al., 2007). The elevated plus maze, for example, has a safe zone, the closed arms, and there is no need for the animal to move to the rest of the maze beyond its natural instinct to explore. So, the animal can avoid the stressful situation passively. The same principle applies to the forced swim test, where there is not an active approach that would be beneficial to the rodent. In the real world, however, rodents and humans face complex problems that can usually be solved with active, complex and brave actions. For example, giving a speech as introvert is a hard situation that requires an active confrontation with one’s own fears. In other words, one needs to cope with it.
Two studies have investigated the effect of the CAF diet on this kind of coping behavior in rodents. Mucellini et al. (2019) used the step-down inhibitory avoidance task to study the effects of gestational exposure at puberty and adult age, while Cigarroa et al. (2016) used the two-way avoidance task in a classical design.
The step-down inhibitory avoidance task consists of an arena with a glass platform elevated above the floor. This platform triggers an anxiety-like response in rats. Consequently, the natural urge is to leave the platform and return to the floor. However, since the floor is electrified, the rat has to fight against its fear of highness in order to avoid the electroshock. The latency to get down is therefore a measure of aversive memory or coping. Compared with a more passive light-dark test with the absence of punishment for entering the comfortable zone, this inhibitory avoidance test requires a coping strategy from the animal. As we have discussed, the CAF diet has no effects on locomotor activity, which could impair a coping response due to excessive or deficit motion. On the other hand, CAF tended to reduce stress, mainly in stressed animals, so CAF diet could be beneficial for facing stressful situations calmly. However, CAF diet did not affect coping in the gestational study and reduced coping in the classical study. Methodological differences in both studies might explain these distinct results.
It was found that exposure to CAF diet did not affect coping behavior in step-down inhibitory avoidance task. The latency to get down significantly increased from training to test session, which means that rats learned to cope with the fear of heights. However, this effect was found in both STD chow and CAF diet groups at both ages, puberty and adulthood (Mucellini et al., 2019). Therefore, indirect CAF diet exposure does not affect coping behavior in rats.
Cigarroa et al. (2016) tested adolescent female rats that were directly exposed to a CAF diet in the two-way avoidance task. This task makes use of a fear-mediated conflict between a tendency to freeze and a tendency to escape from an electroshock. The box is composed of two compartments separated by an open door. In order to escape or avoid the upcoming foot shock, signaled by a light and/or a tone, the animal must learn to change between the two compartments. So, when the light/tone signals the incoming electroshock, the animal should come back to the compartment where previously the animal received an electroshock. As there is not a safe compartment, the animal has to face its fears of returning to the compartment where it previously received a punishment in order to avoid the following electroshock (Lalanza et al., 2015). Interestingly, and different from the absence of gestational effects, when female rats were exposed to the CAF diet directly, they displayed impaired coping behavior. CAF diet fed rats had a smaller decrease in the number of avoidances and a smaller increase in the escape latencies.
Two studies are not enough to draw general conclusions, but they indicate that a CAF diet does not improve coping behavior and one suggests it may do the contrary. The differences found between the gestational and the classical study could simply be caused by the timing of CAF diet exposure, but the sex of the subjects could also have played a role, because the gestational study looked at male offspring, while the classical study examined females. Future studies will have to shine more light on this topic.
4.8. Cognition and Memory
Human studies suggest that diets rich in fat and sugar are a risk factor for cognitive impairment in adults, for increasing vulnerability to dementia and, even, for slowing the development of children’s cognitive skills (Davidson et al., 2019; Morris et al., 2015). Consistently, similar impairment effects were also found in rodents exposed to HF/HS diets (Cordner and Tamashiro, 2015). Therefore, it is expected that the CAF diet will impair cognitive function. Memory could be investigated with different approaches. In this review, based on the types of behavioral tests used, we differentiate between spatial and contextual memory, recognition memory and executive function.
In gestational studies, to the best of our knowledge, the only assay so far deployed has been the object recognition test. It was found that indirect CAF diet exposure via their dams reduced recognition memory in female pups, although for male pups this impairment was less robust (Moreton et al., 2019; Mucellini et al., 2019; Wright et al., 2014). There is a need for more studies that further characterize these sex differences, and we strongly encourage researchers to go deeper into the question of how perinatal exposure to a CAF diet might affect the cognitive abilities of offspring.
In classical studies, the CAF diet has been found to impair spatial and contextual memory, but no effects on recognition memory or executive function have been observed. Spatial memory is a hippocampus-dependent memory that relies on remembering a location with the help of environmental cues. In three studies that assessed spatial memory in the Morris water maze, two found that the CAF diet reduced spatial memory (Ferreira et al., 2018; Lewis et al., 2019), whereas the other found no effect compared to STD chow and HF/HS diets (Pini et al., 2017). However, this apparent discrepancy might be explained by methodological differences. Pini et al. (2017) applied a shorter and more intense learning phase (2 days, 12 trials per day) in the Morris water maze, compared to the standard procedure of 4 to 6 days with only 4 to 5 trials per day (Bromley-Brits et al., 2011; Vorhees and Williams, 2006). Furthermore, in that study, the CAF diet did not induce obesity indicating that their CAF diet protocol was not an effective obesogenic model, maybe because CAF diet was limited only for one-hour a day. In addition, the rats were single-housed while were grouped in the first studies (Ferreira et al., 2018; Lewis et al., 2019). Spatial memory can also be assessed by the Barnes maze, which is a less stressful version of the Morris water maze, without water. Interestingly, the CAF diet did not affect spatial memory in such a Barnes maze, but as the authors reported, this lack of effect could be explained by the lower levels of stress. It is known that stress can reduce the motivation to learn, which could then explain the differences in results between the Barnes and Morris water mazes (Gomez-Smith et al., 2016). All of these studies were carried out with albino strains, what could have interfered with rat’s performance in visuo-behavioral measurements like the Barnes and Morris water mazes (Prusky et al., 2002). Further studies using pigmented strains will confirm the detrimental effects of CAF diet on spatial memory without the inconveniences of visual acuity.
The CAF diet also reduced contextual memory as assessed by both the object recognition test and the fear conditioning test (Beilharz et al., 2016, 2014; Reichelt et al., 2015). Interestingly, these behavioral tests are able to assess different types of memories depending on the exact protocol used. In the object recognition test, for example, one can assess contextual memory by changing the positions of the objects instead of the objects themselves. Applying this paradigm, it was found that consuming a CAF diet reduced the time spent exploring the object placed in a new position (Beilharz et al., 2016, 2014), Thus, and similar to the Morris water maze, animals fed with the CAF diet did not distinguish between different locations. In another experiment, the same researchers surprisingly found no effects of CAF diet on contextual memory compared to STD chow using the same protocol for the object recognition test. However, and like the previous studies, the exploration ratio between the old and the new position was very close to 50% in the CAF group, which indicates low levels of contextual memory. The exploration ratio indicates the amount of time spent exploring the two different objects or positions, so a ratio of 50% means that both objects/positions were explored a similar amount of time, without preferences for the new one. Interestingly, when the CAF diet was combined with probiotics (evidence suggests that the gut-brain axis and the microbiota could affect the central nervous system), the exploration time of the object placed in the new position actually increased (Beilharz et al., 2018). This interaction between CAF diet and probiotics suggests that CAF diet could have reduced the exploration time of the object in the new position even though the comparison with the STD chow group did not reach significance.
The fear conditioning test can be used to assess recognition memory with an associative learning paradigm or contextual memory, using a contextual learning paradigm. One study found that the CAF diet impaired contextual memory in the fear conditioning test, as shown by CAF diet-fed rats displaying reduced freezing, compared to STD chow controls, in the context associated with the electroshock (Reichelt et al., 2015). However, a second study found no effects of a CAF diet in the contextual version of the fear conditioning test (Ferreira et al., 2018). However, in this second case, only the training context without the conditioned stimulus was used to test contextual memory, which could explain the lack of significant difference between the diet groups.
More consistently, the CAF diet has not been found to impair recognition memory using the “new object” version of the object recognition or the associative learning paradigm of the fear conditioning test in multiple studies (Beilharz et al., 2018, 2016, 2014; Ferreira et al., 2018; Leffa et al., 2015; Reichelt et al., 2015). Surprisingly, in a study comparing two CAF diet groups, one with free-access to cola-based soft drink and another with orange-based soft drink, opposite effects were found on recognition memory (Feijó et al., 2019). While the CAF + cola group increased the time exploring the new object, the CAF + orange reduced it compared to the control group. However, the soft drink groups were not compared to each other and the control group was different for the CAF + cola and CAF + orange. Therefore, this result must be taken carefully and does not represent a clear evidence of CAF diet impairing recognition memory.
Finally, executive function was only tested in one study using a puzzle box test, which consists of a subdivided arena with different obstacles to make progress more difficult (e.g. an underpass filled with sawdust) that change each trial. The animal needs to overcome the obstacles to leave the anxiogenic compartment and get into the small and covered safe compartment. The general analysis of the puzzle box, based on the latency to reach the save compartment, showed that executive function was not affected by the CAF diet, even though increased CA1-CA3 volume (Sack et al., 2017).
At a neurophysiological level, the hippocampus is a logical candidate for the impairment of spatial and/or contextual memory caused by the CAF diet, since this brain region also regulates eating behavior (Davidson et al., 2019). It has been shown that a CAF diet increases neuroinflammation and reduces neurogenesis in the hippocampus, which, in most of the studies, correlated with a spatial/contextual memory impairment at a behavioral level (Beilharz et al., 2016, 2014; Ferreira et al., 2018; Gomez-Smith et al., 2016; Reichelt et al., 2015). Interestingly, a gestational study did not found impairment of maternal CAF diet on pups, but neither effects on hippocampal BNDF (Mucellini et al., 2019). Similar results regarding impairment of spatial memory and disruption of the hippocampal formation have also been found with HF/HS diets (Boitard et al., 2014; Molteni et al., 2002; Stranahan et al., 2008).
In aggregate, we conclude that direct exposure to a CAF diet seems to be a risk factor for spatial and contextual memory deficits, while indirect exposure via dams may impair recognition memory, which was not affected by direct exposure to CAF diet in classical studies. Although preliminary studies indicate a potential role for changes to the hippocampus in mediating these cognitive declines (Table 4), further work is needed to more fully reveal the brain mechanisms underlying the changes in spatial/contextual and recognition memory produced by a junk food diet.
4.9. Social behavior
The concept of social behavior is complex and it includes many types of interactions, including behaviors such as social approach, social play behavior, sexual behavior, and maternal care. A few of the studies we reviewed tested the effects of the CAF diet on some of these social behaviors.
Sexual behavior is itself a multifaceted social behavior (for a review of sexual behaviors, Heijkoop et al. (2018)) consisting of numerous behavioral components that are under regulated by complex hormonal systems, including the estrous cycle. High-fat diets and obesity are able to affect reproductive functions, and obesity is a risk factor for infertility in both women and men (Broughton and Moley, 2017; Kahn and Brannigan, 2017). Therefore, it is possible that the CAF diet also affects sexual behavior.
In female rats it was indeed found that eating a CAF diet affected the estrous cycle: the CAF diet reducing ovulation rates and release of luteinizing hormone, while also increasing the frequency of the diestrus phase and the release of prolactin (Sagae et al., 2012). Interestingly, however, these changes to the estrous cycle did not have consequences for sexual behavior itself, the CAF diet not altering sexual paracopulatory or receptive behaviors, i.e. hops, darts, ear wiggling, solicitations and lordosis.
The effects of the CAF diet on male sexual behavior, on the other hand, have only been investigated in a gestational study. This study showed that CAF exposure altered male rat sexual behavior with less intromission behavior. This change in behavior was potentially due to decreased production of reproductive hormones (luteinizing hormone, follicle-stimulating hormone, and testosterone) (Jacobs et al., 2014). More research is needed to determine whether the CAF diet affects male and female sexual behavior differently, or whether these apparent differences were caused by the timing and/or way of administration.
Social play behavior is a rewarding and widespread social activity among mammals (for a review, Vanderschuren et al., 2016). As social play is crucial during the early phases of development, the two studies applying CAF diet and assessing social play were carried out on young animals. A gestational study found a reduction in social play in pups of dams fed with CAF diet (Ribeiro et al., 2018). Whereas a classical study with adolescent rats found that a CAF diet resulted in more social play compared to STD chow, with females being more playful than males (Lalanza et al., 2014).
This discrepancy in effects on social play could actually also be explained by changes in maternal care behavior. As mentioned above, maternal care might have more influence on pups than the diet itself, because abnormal maternal care is a risk factor for social dysfunction. For example, in a recent meta-analysis, Bonapersona et al. (2019) found that aberrant maternal care behavior reduced social behavior mainly in males. In other words, when a CAF diet is administered to dams, this could influence pups’ behavior through changes in maternal care. Surprisingly, only two studies analyzed the effects of a CAF diet on maternal care, and both found alterations in maternal care, including increased licking/grooming (Speight et al., 2017) and arched nursing and nesting (Ribeiro et al., 2018).
Maternal CAF diet exposure is, therefore, a risk factor for the pups as has been discussed in this review. High fat diets and obesity are also risk factors for the abnormal fetal development (Wentzel et al., 2019) and human studies have also found evidence that maternal obesity is a risk factor for future neurodevelopmental and psychiatric disorders in offspring during adolescence and adulthood (for a review, Baker et al., 2017; Rivera et al., 2015).
5. Limitations and Conclusions
The aim of this review was twofold. First, we have aimed to establish a standardized CAF diet protocol that would help the field avoid the current wide-ranging disparities in CAF diet protocols. Second, we have evaluated the current state of knowledge regarding the behavioral effects of exposure to a CAF diet on rodents.
The large amount of inconsistency in results has made it difficult to draw firm conclusions on the effects of this diet on behavioral outcomes. However, overall, we believe we can reasonably conclude that a CAF diet: 1) does not change locomotor activity, 2) increases snacking behavior, and 3) reduces the hedonic value of other rewards such as sucrose and ethanol, a principle similar to the hypofunction or deficiency of the reward and dopaminergic system (Blum et al., 2014; Johnson and Kenny, 2010). In addition, exposure to a CAF diet 4) impairs spatial and contextual memory, and 5) tends to have an anxiolytic and antidepressant effects mainly in animal previously exposed to stress. We are, however, very cautious in drawing these general conclusions, because different studies have used widely varying methodologies. Even leaving aside the specifics of how a CAF diet was given, studies varied in terms of age, species, and behavioral tests, all of which could have affected the outcomes. Such differences could well be the reason that we did found insufficient evidence to draw clear conclusions regarding impulsivity, coping behavior and social behavior. The findings suggest that CAF diet altered these behaviors as well, but more research is needed to control for the methodological variations, which hope would lead to more consistent conclusions.
Furthermore, a limitation of this review and the obesogenic diets in general is the difficulty of distinguishing the neuropsychophysiological effects caused by palatable food intake from those derived from the consequently induced overweight/obesity. To illustrate this problem, a negative correlation has been found between the amount of dopaminergic receptors and body weight (de Weijer et al., 2011; Michaelides et al., 2012; Wang et al., 2001). Taking into account that palatable diet in the form of junk and ultraprocessed food is a major cause of obesity (Rosenheck, 2008), it is likelythat the food‒body weight interaction is a vicious circle phenomena, because long-term consumption of junk food also affects the dopaminergic signaling (as mentioned above). For instance, women who gained weight during the last six months had a lower striatal response towards palatable food compared to stable-weigh women. This could increase the vulnerability for overeating (as a compensatory mechanism of stimulating a hypofunctional reward system) and result in increases in body weight (Stice et al., 2010).
The large variation in the feeding protocols used to study the effects of CAF diet exposure could also have caused the different behavioral outcomes. In order to improve coherence between experiments, and thereby allow comparisons between studies, we propose that researchers will use a standardized protocol. Therefore, we propose that future CAF protocols should: 1) combine different tastes, textures and nutrients; 2) include salty and sweet products as well as products containing chocolate elements; 3) rotate and vary the diet each day, with the menu structure as an excellent tool; 4) give animals ad libitum access to the CAF diet; and 5) offer a healthy (or standard) alternative, since a CAF diet must be available voluntarily and chosen due to its palatability.
Nowadays, junk food is highly prevalent in Western societies and plays a key role in the epidemic of overweightness and obesity, which underlies the corresponding epidemic in non-communicable diseases such as diabetes and cardiovascular disease. Basic animal research is necessary to understand not only the physiological and behavioral effects of junk food, but also to comprehend the triggers and risk factors for overeating these kinds of food products. They will also inform the development of potential treatments for obesity. As we have argued, the CAF diet is an excellent model for recapitulating current problematic human eating patterns due to its high construct validity. The standardized CAF diet protocol proposed herein would enhance comparisons between research laboratories, thereby aiding the understanding of the obesity epidemic and developing ways to mitigate it.
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