The psychological basis of hunger and its dysfunctions

This article describes a new and emerging psychological perspective on hunger, together with the implications of that perspective, which is based upon learning and memory. Hunger is a psychological state characterized by a desire to eat. Historically, conceptions of hunger have largely been expressed in terms of physiology (eg, biological process X causes hunger). However, physiology neither offers a psychological account of hunger nor explains why memory impairment can eliminate hunger. Two forms of hunger are identified - specific and general. Specific hunger is for particular palatable foods. It involves recollecting episodic memories of eating that food, when an associated cue is encountered (eg, an advert). General hunger is a desire to eat triggered by temporal (eg, it is lunchtime) or interoceptive (eg, tummy rumble) cues. It involves semantic memory retrieval, which then augments the expected - remembered - pleasure for any food. Both hungers are supported by the medial temporal lobe memory system. Damage to this system can occur from eating a Western-style diet and, longer-term, from obesity and its consequences. Medial temporal lobe memory damage may cause deficits in specific hunger, but most especially in general hunger, resulting in little motivation to eat foods that the individual considers to be of low-to-moderate palatability, such as fruit and vegetables. The implications of this account for teaching people hunger, for how hunger is affected by diet, for public education, and pharmaceutical intervention, are discussed. Psychological concepts of hunger are widely used in nutritional practice. This article provides a new and emerging perspective on the psychological basis of hunger and its implications.

Time-restricted feeding does not prevent adverse effects of palatable cafeteria diet on adiposity, cognition and gut microbiota in rats

Time-restricted feeding (TRF) is a popular dietary strategy whereby daily food intake is limited to a <12h window. As little is known about the effects of TRF on cognitive and behavioral measures, the present study examined the effects of time-restricted (8h/day; zeitgeber time [ZT]12-20) or continuous access to a high-fat, high-sugar cafeteria-style diet (Caf; Caf and Caf-TRF groups; n=12 adult male Sprague-Dawley rats) or standard chow (Chow and Chow-TRF groups) on short-term memory, anxiety-like behavior, adiposity and gut microbiota composition over 13-weeks with daily food intake measures. TRF significantly reduced daily energy intake in Caf- but not chow-fed groups. In Caf-fed groups, TRF reduced the proportion of energy derived from sugar while increasing that derived from protein. Caf diet significantly increased weight gain, adiposity and fasting glucose within 4 weeks; TRF partially reduced these effects. Caf diet increased anxiety-like behavior in the Elevated Plus Maze in week 3 but not week 12, and impaired hippocampal-dependent place recognition memory in week 11; neither measure was affected by TRF. Global microbiota composition differed markedly between chow and Caf groups, with a small effect of TRF in rats fed chow. In both chow and Caf diet groups, TRF reduced microbiota alpha diversity measures of Shannon diversity and evenness relative to continuous access. Results indicate only limited benefits of TRF access to an obesogenic diet under these conditions, suggesting that more severe time restriction may be required to offset adverse metabolic and cognitive effects when using highly palatable diets.

Habitual intake of fat and sugar is associated with poorer memory and greater impulsivity in humans

The vicious cycle model of obesity suggests that repeated habitual intake of a diet high in fat and sugar (HFS) results in impairment in hippocampal function which in turn increases impulsive behaviours, making it harder to resist unhealthy diet choices. Evidence from studies with rodents consistently show switching to a HFS diet impairs performance on hippocampally-sensitive memory tasks. The limited literature in humans also suggest impaired memory and increased impulsivity related to higher habitual HFS intake. However, these changes in memory and impulsivity have been looked at independently. To investigate how these effects are inter-related, three experiments were conducted where relative HFS intake was related to measures of memory and impulsivity. In Experiment 1 (90 female participants), HFS was associated with higher scores on the Everyday Memory Questionnaire-revised (EMQ), and higher scores on the total, Attention (BISatt) and Motor (BISmot) sub-scales of the Barratt Impulsiveness Scale (BIS11). Experiment 2 (84 women and 35 men), replicated the association between HFS and EMQ, and also found HFS related to poorer performance on the hippocampally-sensitive 4 mountain (4MT) memory task. The association between HFS intake and the BISatt replicated, but there were no significant associations with other BIS11 measures or delay-discounting for monetary rewards. Experiment 3 (199 women and 87 men) replicated the associations between DFS and 4MT and EMQ, and also found an association with overall recall, but not response inhibition, from a Remembering Causes Forgetting task: HFS was also significantly associated with BIS total, BISatt and BISmot. In all three studies these associations remained when potential confounds (BMI, age, gender, hunger state, restrained and disinhibited eating) were controlled for. Mediation analysis found that the effect of HFS on memory at least part mediated the relationship between HFS and impulsivity in Experiments 1 and 3, but not 2. Overall these data provide some support for the vicious cycle model, but also suggest that trait impulsivity may be a risk factor for poor dietary choice.

High-Fat-Sugar Diet is Associated with Impaired Hippocampus-Dependent Memory in Humans

Overconsumption of high-fat and high-sugar (HFS) diet may affect the hippocampus, and consequently, memory functions. Yet, converging evidence is needed to demonstrate that the type of memory affected by HFS diet consumption is indeed hippocampus dependent. Moreover, the extent to which HFS diet can also affect executive functioning, and indirectly affect memory requires further examination. In this online study, we asked 349 young adults to report their HFS diet consumption and complete a word memory task, the Everyday Memory Questionnaire, and importantly two memory tasks that have been shown to robustly engage the hippocampus, i.e., the Pattern Separation and Associative Memory Tasks. Participants also completed two executive functioning tasks, the Trail Making Task (TMT) and the Stroop Task. These measures assess attention/cognitive flexibility and the ability to inhibit cognitive interference, respectively. After controlling for confounding variables, we found that participants who reported higher level consumption of a HFS diet performed worse on the Pattern Separation Task and that higher HFS intake was significantly associated with poorer TMT task performance and longer Stroop average reaction time (RT). TMT and Stroop RT scores indicative of reduced executive function also partially mediated the relationship between HFS diet and memory performance on the pattern separation task. Taken together, our results provide converging evidence that HFS diet may impair hippocampus-dependent memory. HFS diet may also affect executive functioning and indirectly impair memory function. The findings are consistent with human subject and animal studies and call for further investigations on the psychological and neural mechanisms underlying the dietary effects on cognitive processes.

Eating behavior as a new frontier in memory research

The study of memory is commonly associated with neuroscience, aging, education, and eyewitness testimony. Here we discuss how eating behavior is also heavily intertwined-and yet considerably understudied in its relation to memory processes. Both are influenced by similar neuroendocrine signals (e.g., leptin and ghrelin) and are dependent on hippocampal functions. While learning processes have long been implicated in influencing eating behavior, recent research has shown how memory of recent eating modulates future consumption. In humans, obesity is associated with impaired memory performance, and in rodents, dietary-induced obesity causes rapid decrements to memory. Lesions to the hippocampus disrupt memory but also induce obesity, highlighting a cyclic relationship between obesity and memory impairment. Enhancing memory of eating has been shown to reduce future eating and yet, little is known about what influences memory of eating or how memory of eating differs from memory for other behaviors. We discuss recent advancements in these areas and highlight fruitful research pursuits afforded by combining the study of memory with the study of eating behavior.

Comparable metabolic effects of isocaloric sucrose and glucose solutions in rats

Much of the global increase in sugar intake is attributable to rising consumption of sugar-sweetened beverages (SSBs). Because people compensate poorly for liquid calories, SSB consumption increases total energy intake, raising the risk of harmful metabolic effects in addition to possible effects of sugars per se. Glucose and fructose, the constituent sugars in sucrose, can exert distinct effects on metabolism and also differ in their satiating properties, suggesting that compensation for the calories in these sugars may also vary. In light of claims that the fructose within sucrose is particularly harmful, the present study compared the effects of giving rats access to either a sucrose or an isoenergetic glucose solution. Adult male rats were fed standard chow and water supplemented with 95 ml of 10% glucose (Glucose group; n = 10), 9% sucrose solution (Sucrose group; n = 10) or water only (Control group; n = 10) daily for 7 weeks. Sugar-fed groups had higher total energy intakes than the Control group, but the extent of this incomplete compensation did not vary between Sucrose and Glucose groups. In a short-term compensation test, sugar groups were less sensitive to the effects of a sweet pre-meal, with no differences between the Glucose and Sucrose groups. Relative to water, both sugars reduced insulin sensitivity after 4 weeks on the diets and elevated fat mass at 7 weeks. Results suggest that sucrose and glucose induce comparable metabolic impairments and alter the homeostatic regulation of food intake even under conditions where daily access is capped.

Hippocampal silent infarct leads to subtle cognitive decline that is associated with inflammation and gliosis at twenty-four hours after injury in a rat model

Silent infarcts (SI) are subcortical cerebral infarcts that occur in the absence of clinical symptoms commonly associated with ischemia and are linked to dementia development. Little is known about the pathophysiology underlying the cognitive dysfunction associated with SI, and few studies have examined the early cellular responses and neurobiological underpinnings. We induced SI in adult male Sprague-Dawley rats using an infusion of endothelin-1 in the CA1 dorsal hippocampus. Twenty-four hours later, we assessed cognition using the hippocampal-dependent object place recognition task. We also examined whether the resulting cognitive effects were associated with common markers of ischemia, specifically cell and synapse loss, gliosis, and inflammation, using histology and immunohistochemistry. Hippocampal SI led to subtle cognitive impairment on the object place recognition task 24 -hs post-injury. This was characterized by a significant difference in exploration proportion relative to a pre-injury baseline and a positive association between time spent with both the moved and unmoved objects. SI did not result in any detectable cell or synaptophysin loss, but did increase apoptosis, gliosis and inflammation in the CA1. Principal component analysis indicated the main variables associated with hippocampal SI included increased time spent with the unmoved object, gliosis, apoptosis and inflammation as well as decreased exploration proportion and CA1 cells. Our data demonstrate that hippocampal SI can lead to cognitive dysfunction 24 -hs after injury. Further, this appears to be driven by early degenerative processes including apoptosis, gliosis and inflammation, suggesting that these may be targets for early interventions treating hippocampal SI and its cognitive consequences.

Chemogenetic silencing of hippocampus and amygdala reveals a double dissociation in periadolescent obesogenic diet-induced memory alterations

In addition to numerous metabolic comorbidities, obesity is associated with several adverse neurobiological outcomes, especially learning and memory alterations. Obesity prevalence is rising dramatically in youth and is persisting in adulthood. This is especially worrying since adolescence is a crucial period for the maturation of certain brain regions playing a central role in memory processes such as the hippocampus and the amygdala. We previously showed that periadolescent, but not adult, exposure to obesogenic high-fat diet (HFD) had opposite effects on hippocampus- and amygdala-dependent memory, impairing the former and enhancing the latter. However, the causal role of these two brain regions in periadolescent HFD-induced memory alterations remains unclear. Here, we first showed that periadolescent HFD induced long-term, but not short-term, object recognition memory deficits, specifically when rats were exposed to a novel context. Using chemogenetic approaches to inhibit targeted brain regions, we then demonstrated that recognition memory deficits are dependent on the activity of the ventral hippocampus, but not the basolateral amygdala. On the contrary, the HFD- induced enhancement of conditioned odor aversion specifically requires amygdala activity. Taken together, these findings suggest that HFD consumption throughout adolescence impairs long-term object recognition memory through alterations of ventral hippocampal activity during memory acquisition. Moreover, these results further highlight the bidirectional effects of adolescent HFD on hippocampal and amygdala functions.

Sugar consumption and behavioural inhibition in the rat

The metabolic effects of sugary drinks have been extensively studied, whereas the effects on psychological processes have received relatively limited attention. Several studies have found that high-sugar diets can produce impaired performance by rats on tests assessing spatial learning and memory. In contrast, despite claims that weakened inhibitory control underlies many sugar-induced deficits, evidence supporting this proposal has been limited. The aim of the present study was to assess the impact of high-sugar diets on response inhibition, as measured by rats' performance on a differential reinforcement of low rates schedule (DRL) in Experiments 1 and 2 and on a Pavlovian discrimination reversal task in Experiment 3. In all three experiments a 30-day diet stage, in which Sugar groups were given unrestricted access to 10% sucrose solution and Control groups had access to water only, was followed by behavioural tests. In Experiment 1 the Sugar group performed poorly on a spatial memory task, but no difference was detected between the performances of the two groups in the DRL test. In Experiment 2 longer DRL training was given and post-diet performance was assessed both before and after access to sugar was withdrawn. Null results were obtained under both conditions. In Experiment 3 rats' performance on a discrimination learned prior to the diet intervention was not affected by the high-sugar diet, but neither was performance once the discrimination was reversed. The implications of these results for understanding of sugar-induced psychological deficits are discussed.

High fat diet-induced obesity causes a reduction in brain tyrosine hydroxylase levels and non-motor features in rats through metabolic dysfunction, neuroinflammation and oxidative stress

Obesity is a health problem that has been associated with neuroinflammation, decreased cognitive functions and development of neurodegenerative diseases. Parkinson's disease (PD) is a chronic neurodegenerative condition characterized by motor and non-motor abnormalities, increased brain inflammation, α-synuclein protein aggregation and dopaminergic neuron loss that is associated with decreased levels of tyrosine hydroxylase (TH) in the brain. Diet-induced obesity is a global epidemic and its role as a risk factor for PD is not clear. Herein, we showed that 25 weeks on a high-fat diet (HFD) promotes significant alterations in the nigrostriatal axis of Wistar rats. Obesity induced by HFD exposure caused a reduction in TH levels and increased TH phosphorylation at serine 40 in the ventral tegmental area. These effects were associated with insulin resistance, increased tumor necrosis factor-α levels, oxidative stress, astrogliosis and microglia activation. No difference was detected in the levels of α-synuclein. Obesity also induced impairment of locomotor activity, total mobility and anxiety-related behaviors that were identified in the open-field and light/dark tasks. There were no changes in motor coordination or memory. Together, these data suggest that the reduction of TH levels in the nigrostriatal axis occurs through an α-synuclein-independent pathway and can be attributed to brain inflammation, oxidative/nitrosative stress and metabolic disorders induced by obesity.

Roux-en-Y gastric bypass increases GABA-A receptor levels in regions of the rat brain involved in object recognition memory and perceptual acuity

Roux-en-Y gastric bypass surgery (RYGB), one of the most common and successful procedures for combatting obesity, is associated with post-surgery substance use disorder (SUD) and other addictive behaviors in a subset of patients. We investigated the effects of RYGB on GABA-A receptor levels in the rat brain to identify potential mechanisms of this behavior. The GABAergic system is affected in addiction and has been implicated in the pathology of obesity. We assigned male Sprague-Dawley rats to four groups: standard, low fat diet with sham surgery (control), ad libitum HFD with sham surgery (Sham), calorie restricted HFD with sham surgery (Sham-FR), or HFD with RYGB surgery. Surgery was performed after 8 weeks on the control or HFD diet. Rats maintained their respective diets for 9 weeks post-surgery, then were sacrificed for GABA-A receptor autoradiography using the [³H] Flunitrazepam ligand. We identified increased GABA-A binding in the perirhinal cortex of ad-libitum HFD fed rats compared to normal diet controls. RYGB surgery increased GABA-A in the ectorhinal cortex compared to normal diet controls, and increased binding in the jaw region of the primary somatosensory cortex compared to food-restricted rats that received sham surgery. Hypothalamus GABA-A was also negatively correlated with body weight in the RYGB group, where GABA signaling may play a role in obesity regulation. These results suggest that HFD and RYGB modulate GABA signaling in regions important for object recognition memory, and that increased GABA-A levels in the jaw's perceptual field cortex arise from the surgery itself, independent of caloric restriction.

Hippocampal-dependent appetitive control is impaired by experimental exposure to a Western-style diet

Animals fed a Western-style diet (WS-diet) demonstrate rapid impairments in hippocampal function and poorer appetitive control. We examined if this also occurs in humans. One-hundred and ten healthy lean adults were randomized to either a one-week WS-diet intervention or a habitual-diet control group. Measures of hippocampal-dependent learning and memory (HDLM) and of appetitive control were obtained pre- and post-intervention. HDLM was retested at three-week follow-up. Relative to controls, HDLM performance declined in the WS-diet group (d = 0.43), but was not different at follow-up. Appetitive control also declined in the WS-diet group (d = 0.47) and this was strongly correlated with HDLM decline (d = 1.01). These findings demonstrate that a WS-diet can rapidly impair appetitive control in humans-an effect that could promote overeating in consumers of a WS-diet. The study also suggests a functional role for the hippocampus in appetitive control and provides new evidence for the adverse neurocognitive effects of a WS-diet.

The Cognitive Control of Eating and Body Weight: It's More Than What You "Think"

Over the past decade, a great deal of research has established the importance of cognitive processes in the control of energy intake and body weight. The present paper begins by identifying several of these cognitive processes. We then summarize evidence from human and nonhuman animal models, which shows how excess intake of obesity-promoting Western diet (WD) may have deleterious effects on these cognitive control processes. Findings that these effects may be manifested as early-life deficits in cognitive functioning and may also be associated with the emergence of serious late-life cognitive impairment are described. Consistent with these possibilities, we review evidence, obtained primarily from rodent models, that consuming a WD is associated with the emergence of pathophysiologies in the hippocampus, an important brain substrate for learning, memory, and cognition. The implications of this research for mechanism are discussed within the context of a “vicious-cycle model,” which describes how eating a WD could impair hippocampal function, producing cognitive deficits that promote increased WD intake and body weight gain, which could contribute to further hippocampal dysfunction, cognitive decline, and excess eating and weight gain.

Dietary patterns are associated with attention-deficit/hyperactivity disorder (ADHD) symptoms among preschoolers in mainland China

BACKGROUND/OBJECTIVES

To identify the major dietary patterns among preschoolers and assess the relation of major dietary patterns to attention-deficit/hyperactivity disorder (ADHD).

SUBJECTS/METHODS

A cross-sectional survey was conducted comprising a large sample of 14,912 children aged 3-6 years old in Ma'anshan city of Anhui Province in China. A semi-quantitative food frequency questionnaire and the 10-item Chinese version of the Conners' abbreviated symptom questionnaire were administered to assess usual dietary intakes and ADHD. Social-demographic information was also collected. Major dietary patterns were identified by principal components analysis. Logistic regression was employed to explore the association of dietary patterns with ADHD.

RESULTS

Five dietary patterns were identified explaining 47.96% of the total diet variation. The "processed" dietary component was the principal pattern that explained the higher percentage of variability, 24.69%. After controlling for potential confounders, preschoolers in the top quintile of the "processed" (odds ratio (OR) = 1.56, 95% confidence interval (CI): 1.31-1.86) and "snack" (OR = 1.76, 95% CI = 1.49-2.07) dietary pattern score had greater odds for having ADHD symptoms compared with those in the lowest quintile. Lowest scores on the "vegetarian" were associated with significantly a higher likelihood of indications of ADHD symptoms (OR = 0.67, 95% CI = 0.56-0.79).

CONCLUSIONS

This study showed that preschoolers tend to choose unhealthy dietary pattern in Ma'anshan, China. "Processed" and "snack" dietary patterns were significantly and positively correlated with ADHD symptoms, while "vegetarian" dietary patterns were negatively correlated with ADHD symptoms.

Dietary influences on cognition

Obesity is a world-wide crisis with profound healthcare and socio-economic implications and it is now clear that the central nervous system (CNS) is a target for the complications of metabolic disorders like obesity. In addition to decreases in physical activity and sedentary lifestyles, diet is proposed to be an important contributor to the etiology and progression of obesity. Unfortunately, there are gaps in our knowledge base related to how dietary choices impact the structural and functional integrity of the CNS. For example, while chronic consumption of hypercaloric diets (increased sugars and fat) contribute to increases in body weight and adiposity characteristic of metabolic disorders, the mechanistic basis for neurocognitive deficits in obesity remains to be determined. In addition, studies indicate that acute consumption of hypercaloric diets impairs performance in a wide variety of cognitive domains, even in normal non-obese control subjects. These results from the clinical and basic science literature indicate that diet can have rapid, as well as long lasting effects on cognitive function. This review summarizes our symposium at the 2017 Society for the Study of Ingestive Behavior (SSIB) meeting that discussed these effects of diet on cognition. Collectively, this review highlights the need for integrated and comprehensive approaches to more fully determine how diet impacts behavior and cognition under physiological conditions and in metabolic disorders like type 2 diabetes mellitus (T2DM) and obesity.

Adverse effects of consuming high fat–sugar diets on cognition: implications for understanding obesity

There is increasing evidence for important roles of key cognitive processes, including attention, memory and learning, in the short-term decision making about eating. There is parallel evidence that people who are overweight or obese tend to perform worse on a variety of cognitive tasks. In this review, the evidence for these two ideas is summarised and then the idea that overconsumption of Western-style high-fat (HF)–high-sugar diets may underlie the association between obesity and poorer cognitive performance is explored. In particular, evidence in animals and human subjects that repeated consumption of HF or HF and sugar (HFS) diets leads to specific impairments in the functioning of the hippocampus, which underpin the consequent changes in cognition is summarised. These findings lead into the vicious cycle model (VCM), which suggests that these cognitive changes have knock-on negative effects for future appetite control, and evidence that altered hippocampal function is also associated with impaired appetite control is explored. The review concludes that there is consistent evidence in the animal literature and emerging evidence from human studies that supports this VCM. It is also noted, however, that to date studies lack the nutritional specificity needed to be able to translate these basic research findings into clear nutritional effects, and concludes that there is an urgent need for additional research to clarify the precise nature of the apparent effects of consuming HFS diets on cognition.

A four-day Western-style dietary intervention causes reductions in hippocampal-dependent learning and memory and interoceptive sensitivity

In animals, a Western style diet-high in saturated fat and added sugar-causes impairments in hippocampal-dependent learning and memory (HDLM) and perception of internal bodily state (interoception). In humans, while there is correlational support for a link between Western-style diet, HDLM, and interoception, there is as yet no causal data. Here, healthy individuals were randomly assigned to consume either a breakfast high in saturated fat and added sugar (Experimental condition) or a healthier breakfast (Control condition), over four consecutive days. Tests of HDLM, interoception and biological measures were administered before and after breakfast on the days one and four, and participants completed food diaries before and during the study. At the end of the study, the Experimental condition showed significant reductions in HDLM and reduced interoceptive sensitivity to hunger and fullness, relative to the Control condition. The Experimental condition also showed a markedly different blood glucose and triglyceride responses to their breakfast, relative to Controls, with larger changes in blood glucose across breakfast being associated with greater reductions in HDLM. The Experimental condition compensated for their energy-dense breakfast by reducing carbohydrate intake, while saturated fat intake remained consistently higher than Controls. This is the first experimental study in humans to demonstrate that a Western-style diet impacts HDLM following a relatively short exposure-just as in animals. The link between diet-induced HDLM changes and blood glucose suggests one pathway by which diet impacts HDLM in humans.