Both infantile stimulation and exposure to sweet food lead to an increased sweet food ingestion in adult life

Coping with the experience of frustration throughout life: Sex- and age-specific effects of early life stress on the susceptibility to reward devaluation

Early life stress may lead to lifelong impairments in psychophysiological functions, including emotional and reward systems. Unpredicted decrease in reward magnitude generates a negative emotional state (frustration) that may be involved with susceptibility to psychiatric disorders. We evaluated, in adolescents and adult rats of both sexes, whether maternal separation (MS) alters the ability to cope with an unexpected reduction of reward later in life. Litters of Wistar rats were divided into controls (non handled - NH) or subjected to MS. Animals were trained to find sugary cereal pellets; later the amount was reduced. Increased latency to reach the reward-associated area indicates higher inability to regulate frustration. The dorsal hippocampus (dHC) and basolateral amygdala (BLA) were evaluated for protein levels of NMDA receptor subunits (GluN2A/GluN2B), synaptophysin, PSD95, SNAP-25 and CRF1. We found that adult MS males had greater vulnerability to reward reduction, together with decreased GluN2A and increased GluN2B immunocontent in the dHC. MS females and adolescents did not differ from controls. We concluded that MS enhances the response to frustration in adult males. The change in the ratio of GluN2A and GluN2B subunits in dHC could be related to a stronger, more difficult to update memory of the aversive experience.

The Effect of Early Neurological Stimulation on Puppy Welfare in Commercial Breeding Kennels

Throughout their lives, dogs may experience various stressful events. Early neurological stimulation (ENS), which was shown to alter stress responses beneficially in some animals, has not been fully explored in dogs. Seventy-six small-breed puppies from one commercial breeding kennel were divided into three treatment groups: ENS, held, and control. Puppies in the ENS group received 30 s of handling exercises for 21 days after birth; puppies in the held group were simply held for the same amount of time. Puppies in the control group were managed as per normal breeder practices (i.e., routine husbandry and physical health checks). Physical health was assessed weekly, and puppies were generally healthy and clean. Behavioral responses to stranger approach and isolation tests were evaluated pre- and post-ground-transportation to a distributor. Puppies were more affiliative toward a stranger post-isolation than pre-isolation (p < 0.001), and post-transport than pre-transport (p < 0.001). At the distributor, puppies in the isolation test spent less time in exploratory locomotion (p < 0.001) and vocalized more than at the breeder’s kennel (p = 0.011). Treatment did not affect these results. Overall, the results suggest that the type of ENS used in this study may not provide the purported benefits to puppies’ stress responses in commercial breeding populations.

Estimated causal effects of complementary feeding behaviors on early childhood diet quality in a US cohort

BACKGROUND

Complementary feeding (CF) provides an opportunity to shape children's future dietary habits, setting the foundation for good nutrition and health.

OBJECTIVES

We estimated effects of 3 CF behaviors on early childhood diet quality using inverse probability (IP) weighting of marginal structural models (MSMs).

METHODS

Among 1041 children from the Boston-area Project Viva cohort, we estimated effects on the mean Youth Healthy Eating Index (YHEI) score in early childhood of 1) delayed (≥12 mo) compared with early (<12 mo) introduction of sweets and fruit juice; 2) continued compared with ceased offering of initially refused foods; and 3) early (<12 mo) compared with late (≥12 mo) introduction of flavor/texture variety. Mothers reported CF behaviors at 1 y and completed FFQs for children in early childhood (median age: 3.1 y). We estimated average treatment effects (ATEs) using IP weighting of MSMs to adjust for both confounding and selection bias due to censored outcomes and examined effect modification by child sex and breastfeeding compared with formula feeding at 6 mo.

RESULTS

Twelve percent of mothers delayed introducing sweets/fruit juice, 93% continued offering initially refused foods, and 32% introduced flavor/texture variety early. The mean ± SD YHEI score was 52.8 ± 9.2 points. In adjusted models, we estimated a higher mean YHEI score with delayed (compared with early) sweets and fruit juice among breastfeeding children (ATE: 4.5 points; 95% CI: 1.0, 7.4 points), as well as with continued (compared with ceased) offering of refused foods among females (ATE: 5.4 points; 95% CI: 0.8, 9.1 points). The ATE for early (compared with late) flavor/texture variety was 1.7 points (95% CI: 0.3, 3.2 points) overall and stronger (2.8 points; 95% CI: 0.7, 5.1 points) among the formula-fed group.

CONCLUSIONS

Delayed introduction of sweets/juice, continued offering of refused foods, and early flavor/texture variety may all result in higher childhood diet quality. Effects may depend on child sex and infant breastfeeding status.

Life-course effects of early life adversity exposure on eating behavior and metabolism

Environmental variations in early life influence brain development, making individuals more vulnerable to psychiatric and metabolic disorders. Early life stress (ELS) has a strong impact on the development of eating behavior. However, eating is a complex behavior, determined by an interaction between signals of energy homeostasis, neuronal circuits involved in its regulation, and circuits related to rewarding properties of the food. Although mechanisms underlying ELS-induced altered feeding behavior are not completely understood, evidence suggest that the effects of ELS on metabolic, mood, and emotional disorders, as well as reward system dysfunctions can contribute directly or indirectly to altered feeding behavior. The focus of this chapter is to discuss the effects of ELS on eating behavior and metabolism, considering different factors that control appetite such as energy homeostasis, hedonic properties of the food, emotional and cognitive status. After highlighting classic studies on the association between ELS and eating behavior alterations, we discuss how exposure to adversity can interact with genetics characteristics to predict variable outcomes.

Sexually dimorphic behavioral effects of maternal separation in anorexic rats

Exposure to negative events during the neonatal period is one of the leading factors contributing to the development of psychiatric disorders, including anorexia nervosa. In this study, we investigated the effects of maternal separation (MS) on the development of anorexia in rodents using the mild-stress form of the activity-based anorexia (ABA) model (2 hr of free access to a running wheel and a 1-hr feeding test) in both male and female rats. We assessed anxiety-like and locomotor behavior and hyperactivity with the open field and elevated plus maze tests. Our results showed that ABA rats of both sexes displayed hyperactive behavior associated with reduced anxiety-like behavior when compared to controls. However, a sexually dimorphic effect of MS emerged in anorexic rats: while the females exposed to MS + ABA were hyperactive with diminished anxiety-related behaviors compared to females of the ABA group, MS in males attenuated or did not alter the effects of the ABA protocol. In conclusion, our data reveal that the synergistic effects of MS and ABA on physical activity and anxiety-like behavior act in opposite directions in the two sexes.

Promoting healthy food preferences from the start: a narrative review of food preference learning from the prenatal period through early childhood

The palatable, energy-dense foods that characterize modern environments can promote unhealthy eating habits, along with humans' predispositions to accept sweet tastes and reject those that are sour or bitter. Yet food preferences are malleable, and examining food preference learning during early life can highlight ways to promote acceptance of healthier foods. This narrative review describes research from the past 10 years focused on food preference learning from the prenatal period through early childhood (ages 2-5 years). Exposure to a variety of healthy foods from the start, including during the prenatal period, early milk-feeding and the introduction to complementary foods and beverages, can support subsequent acceptance of those foods. Yet development is plastic, and healthier food preferences can still be promoted after infancy. In early childhood, research supports starting with the simplest strategies, such as repeated exposure and modelling, reserving other strategies for use when needed to motivate the initial tasting necessary for repeated exposure effects to begin. This review can help caregivers and practitioners to promote the development of healthy food preferences early in life. Specific implementation recommendations, the role of individual differences and next steps for research in this area are also discussed.

Interaction between perceived maternal care, anxiety symptoms, and the neurobehavioral response to palatable foods in adolescents

Studies in rodents have shown that early life trauma leads to anxiety, increased stress responses to threatening situations, and modifies food intake in a new environment. However, these associations are still to be tested in humans. This study aimed to verify complex interactions among anxiety diagnosis, maternal care, and baseline cortisol on food intake in a new environment in humans. A community sample of 32 adolescents and young adults was evaluated for: psychiatric diagnosis using standardized interviews, maternal care using the Parental Bonding Inventory (PBI), caloric consumption in a new environment (meal choice at a snack bar), and salivary cortisol. They also performed a brain fMRI task including the visualization of palatable foods vs. neutral items. The study found a three-way interaction between anxiety diagnosis, maternal care, and baseline cortisol levels on the total calories consumed (snacks) in a new environment. This interaction means that for those with high maternal care, there were no significant associations between cortisol levels and food intake in a new environment. However, for those with low maternal care and who have an anxiety disorder (affected), cortisol was associated with higher food intake; whereas for those with low maternal care and who did not have an anxiety disorder (resilient), cortisol was negatively associated with lower food intake. In addition, higher anxiety symptoms were associated with decreased activation in the superior and middle frontal gyrus when visualizing palatable vs. neutral items in those reporting high maternal care. These results in humans mimic experimental research findings and demonstrate that a combination of anxiety diagnosis and maternal care moderate the relationship between the HPA axis functioning, anxiety, and feeding behavior in adolescents and young adults.

The early origins of food preferences: targeting the critical windows of development

The nutritional environment to which an individual is exposed during the perinatal period plays a crucial role in determining his or her future metabolic health outcomes. Studies in rodent models have demonstrated that excess maternal intake of high-fat and/or high-sugar "junk foods" during pregnancy and lactation can alter the development of the central reward pathway, particularly the opioid and dopamine systems, and program an increased preference for junk foods in the offspring. More recently, there have been attempts to define the critical windows of development during which the opioid and dopamine systems within the reward pathway are most susceptible to alteration and to determine whether it is possible to reverse these effects through nutritional interventions applied later in development. This review discusses the progress made to date in these areas, highlights the apparent importance of sex in determining these effects, and considers the potential implications of the findings from rodent models in the human context.

Neonatal handling: an overview of the positive and negative effects

As one of the first rodent models designed to investigate the effects of early-life experiences, the neonatal handling paradigm has helped us better understand how subtle changes in the infant environment can powerfully drive neurodevelopment of the immature brain in typical or atypical trajectories. Here, we review data from more than 50 years demonstrating the compelling effects of neonatal handling on behavior, physiology, and neural function across the lifespan. Moreover, we present data that challenge the classical view of neonatal handling as an animal model that results only in positive/beneficial outcomes. Indeed, the overall goal of this review is to offer the suggestion that the effects of early-life experiences-including neonatal handling-are nuanced rather than unidirectional. Both beneficial and negative outcomes may occur, depending on the parameters of testing, sex of the subject, and neurobehavioral system analyzed.

Early life stress is associated with anxiety, increased stress responsivity and preference for "comfort foods" in adult female rats

Chronic stress increases anxiety and encourages intake of palatable foods as "comfort foods". This effect seems to be mediated by altered function of the hypothalamic-pituitary-adrenal axis. In the current study, litters of Wistar rats were subjected to limited access to nesting material (Early-Life Stress group - ELS) or standard care (Control group) from postnatal day 2 to 9. In adult life, anxiety was assessed using the novelty-suppressed feeding test (NSFT), and acute stress responsivity by measurement of plasma corticosterone and ACTH levels. Preference for palatable foods was monitored by a computerized system (BioDAQ, Research Diets(®)) in rats receiving only regular chow or given the choice of regular and palatable diet for 30 days. ELS-augmented adulthood anxiety in the NSFT (increased latency to eat in a new environment; decreased chow intake upon return to the home cage) and increased corticosterone (but not ACTH) secretion in response to stress. Despite being lighter and consuming less rat chow, ELS animals ate more palatable foods during chronic exposure compared with controls. During preference testing, controls receiving long-term access to palatable diet exhibited reduced preference for the diet relative to controls exposed to regular chow only, whereas ELS rats demonstrated no such reduction in preference after prolonged palatable diet exposure. The increased preference for palatable foods showed by ELS animals may result from a habit of using this type of food to ameliorate anxiety.

The effects of ovariectomy on binge eating proneness in adult female rats

Ovarian hormones are associated with binge eating in women, however findings are limited by the lack of experimental control inherent in human studies. Animal research that manipulates ovarian hormone status and examines individual differences in extreme binge eating proneness is needed to model clinical phenotypes in humans and to confirm causal effects. The purpose of this study was to examine the effects of adult ovariectomy on overall binge eating risk and extreme binge eating phenotypes using the binge eating resistant (BER)/binge eating prone (BEP) rat model. We predicted that palatable food consumption would significantly increase after ovariectomy in all rats because ovarian hormones generally suppress food intake. If differences in responsiveness to ovarian hormones underlie BER/BEP phenotypes, then differences in binge eating between BER and BEP rats would be eliminated or diminished after ovariectomy. Changes in palatable food (PF) intake were compared in BER and BEP rats before and after ovariectomy in two samples of adult females. Findings were highly similar in the two samples. PF intake increased significantly following ovariectomy in all rats. However, BEP rats consistently consumed larger amounts of PF than BER rats, both before and after ovariectomy. The consistency of findings across two samples of rats provides strong support for activational effects of ovarian hormones on binge eating. However, the immunity of extreme binge eating phenotypes to ovarian hormone ablation suggests that other, earlier mechanisms (e.g., organizational hormone effects or hormone-independent effects) determine the expression of binge eating phenotypes.

Early life handling decreases serotonin turnover in the nucleus accumbens and affects feeding behavior of adult rats

In our previous studies, we reported that neonatally handled rats have an increased ingestion of sweet food but are resistant to the damaging effects of a chronic exposure to a highly palatable diet. Accumbal serotonin (5-HT) is important for feeding behavior and plays a role in the vulnerability to diet-induced obesity. Therefore, our hypotheses were (1) 5-HT turnover in the nucleus accumbens is altered in neonatally handled animals and plays a role in their differential feeding behavior and (2) if this is so, a chronic pharmacological treatment affecting 5-HT reuptake (chronic imipramine) would be able to revert the behavioral findings. Litters were divided into nonhandled and handled (10 min/day, Days 1-10 after birth). In Experiment 1, we demonstrated that a decreased 5-HT metabolism in the nucleus accumbens was observed in adult handled animals. In Experiment 2, the two previous groups were subdivided and assigned to receive imipramine diluted in water or water alone. After 30 days of treatment, we evaluated their weight gain and feeding behavior. Handled rats weighed less than nonhandled rats, and all imipramine-treated rats showed a reduction in weight gain after 60 days of treatment. Imipramine reverted the increased sweet food consumption seen in neonatally handled rats. We conclude that serotonin is involved in the altered feeding behavior of neonatally handled rats, and this protocol is an important tool for studying the mechanisms by which early life events have a long-term impact on feeding preferences.

Early life exposure to a high fat diet promotes long-term changes in dietary preferences and central reward signaling

Overweight and obesity in the United States continues to grow at epidemic rates in large part due to the overconsumption of calorically-dense palatable foods. Identification of factors influencing long-term macronutrient preferences may elucidate points of prevention and behavioral modification. In our current study, we examined the adult macronutrient preferences of mice acutely exposed to a high fat diet during the third postnatal week. We hypothesized that the consumption of a high fat diet during early life would alter the programming of central pathways important in adult dietary preferences. As adults, the early-exposed mice displayed a significant preference for a diet high in fat compared to controls. This effect was not due to diet familiarity as mice exposed to a novel high carbohydrate diet during this same early period failed to show differences in macronutrient preferences as adults. The increased intake of high fat diet in early exposed mice was specific to dietary preferences as no changes were detected for total caloric intake or caloric efficiency. Mechanistically, mice exposed to a high fat diet during early life exhibited significant alterations in biochemical markers of dopamine signaling in the nucleus accumbens, including changes in levels of phospho-dopamine and cyclic AMP-regulated phosphoprotein, molecular weight 32 kDa (DARPP-32) threonine-75, DeltaFosB, and cyclin-dependent kinase 5. These results support our hypothesis that even brief early life exposure to calorically-dense palatable diets alters long-term programming of central mechanisms important in dietary preferences and reward. These changes may underlie the passive overconsumption of high fat foods contributing to the increasing body mass in the western world.

Early life experience alters behavioral responses to sweet food and accumbal dopamine metabolism

Neonatal handling in rats persistently alters behavioral parameters and responses to stress. Such animals eat more sweet food in adult life, without alterations in lab chow ingestion. Here, we show that neonatally handled rats display greater incentive salience to a sweet reward in a runway test; however they are less prone to conditioned place preference and show less positive hedonic reactions to sweet food. When injected with methylphenidate (a dopamine mimetic agent), non-handled rats increase their sweet food ingestion in the fasted state, while neonatally handled rats do not respond. We did not observe any differences regarding baseline general ambulatory activity between the groups. A lower dopamine metabolism in the nucleus accumbens was observed in handled animals, without differences in norepinephrine content. We suggest that early handling leads to a particular response to positive reinforcers such as palatable food, in a very peculiar fashion of higher ingestion but lower hedonic impact, as well as higher incentive salience, but diminished dopaminergic metabolism in the nucleus accumbens.

Severe intrauterine growth restriction is associated with higher spontaneous carbohydrate intake in young women

Intrauterine growth restriction (IUGR) is associated with metabolic disorders in adulthood. In rats, an early adverse environment alters food preferences in adult life. We investigated whether IUGR is associated with spontaneous macronutrient preferences in humans. Two thousand sixty-three participants from a Brazilian birth cohort were evaluated at 24 y of age using a food frequency questionnaire, physical examination, anthropometric measurements, and biochemical assays (glucose, insulin, cholesterol, and triglycerides). IUGR was defined by the birth weight ratio (BWR = birth weight/mean weight for gestational age). Individuals were classified as non growth restricted (BWR > or =0.85), moderately growth restricted (0.85 > BWR > or = 0.75), and severely growth restricted (BWR <0.75). Severe IUGR women consumed a greater carbohydrate to protein ratio, even after controlling for social variables. There was a continuous association between growth restriction and later carbohydrate to protein ratio consumption in women. Women from both IUGR groups had a larger waist to hip ratio (WHR). The prevalence of metabolic syndrome was comparable between the groups. IUGR women preferred carbohydrates to protein in their regular diet, suggesting that spontaneous food choices may precede the appearance and contribute to the risk for metabolic diseases in this group.