Overweight and obesity have high morbidity and mortality throughout the life course, which is closely linked to diabetes, cardiovascular diseases, and psychological disorders. It is becoming the greatest threat to public health in the 21st century (Lowe et al., 2020). As of 2022, more than one billion people globally are suffering from obesity, with children and adolescents accounting for 159 million (Phelps et al., 2024). In China, the prevalence of childhood overweight is as high as 11.1% and the prevalence of childhood obesity is 7.9% (Pan et al., 2021). In obesogenic environments, food availability and calorie density are the main determinants of excess weight in school-age children(J. Zhang et al., 2024). Treatments for obesity have been sub-optimal, which is partly because understanding of its neurobiological correlates remains to be limited (Ochner et al., 2015; Wang et al., 2023). More importantly, childhood obesity has long been recognized as an increasingly essential predictor of adult obesity (Lister et al., 2023). Therefore, it is of great importance to examine the underlying neural basis of pediatric obesity and its relationship with future weight and eating behaviors during such a critical developmental period.
Hippocampus, a subcortical reward region, is intimately involved in regulating human food intake and obesity (Stevenson & Francis, 2017; Webber et al., 2009). Obesity and high-caloric diets have also been found to increase hippocampal neuroinflammation (Schmitt & Gaspar, 2023). The vicious circle model of obesity has proposed that Western diet (e.g., saturated fat and carbohydrate food) contributes to obesity, in part, by damaging the appetite regulation function of the hippocampus which ultimately induces further over-consumption of unhealthy food, weight gain, and obesity (Kanoski & Davidson, 2011). The appetite processing needs to integrate external food sensory information and internal energy-relevant signals from somatosensory cortex (e.g., postcentral gyrus [PoCG], frontal operculum, and insular) to evaluate when, what and how much to eat (Clasen et al., 2020; Eichenbaum, 2000; Kanoski & Grill, 2017; Quigley et al., 2021). A review study indicated that hippocampus and somatosensory cortex were activated in response to visual, olfactory and taste food stimuli (Huerta et al., 2014).
An emerging body of research has demonstrated that structural impairments and functional alterations in the hippocampus may be a neural vulnerability factor for obesity and future food approach behaviors (e.g., food responsiveness and overeating). Cross-sectional and two-year follow-up studies have consistently found that children with obesity exhibited hippocampal atrophy (i.e., smaller gray matter volume) (Bauer et al., 2015; Jiang et al., 2023; Mestre et al., 2017). Moreover, disrupted hippocampus activation in response to food stimuli has been shown to underlie childhood obesity (Bruce et al., 2010; Davids et al., 2010; Mestre et al., 2017; Samara et al., 2018). Such abnormal reactivity to food in the hippocampus was related to its changed resting-state activity (G. Li et al., 2021). Regarding resting-state functional synchrony, individuals with obesity displayed disrupted functional connectivity (FC) in the hippocampus and somatosensory cortex (PoCG and frontal operculum), which was linked with 12-month changes of eating behaviors and weight after bariatric surgery (Cerit et al., 2019; Geha et al., 2017; Lee et al., 2022). Notably, birth weight of children with overweight/obesity was associated with the FC between hippocampus and PoCG (Solis-Urra et al., 2023). However, existing resting-state fMRI studies have largely focused on the abnormal neurobiology of obesity in adults, rather than in children (for a review, see Syan et al., 2021). As such, the present study sought to determine whether and how the hippocampus-based FC was associated with childhood obesity.
Furthermore, abnormal effective connectivity (EC) between brain regions may act as the basis for functional synchrony, which could determine the directional causal interaction among different brain regions (Friston et al., 2014; Razi et al., 2015). Several methods have been proposed to examine the potential EC pattern, such as Granger causal analyses (GCA) and dynamic causal modeling (DCM) (Friston et al., 2003; Granger, 1969). Although previous studies using the GCA technology found aberrant strength of hippocampus-related directional connectivity in adults with obesity (Duan et al., 2020; Z. Zhang et al., 2022), this method cannot determine the valence of connections between regions (i.e., excitatory or inhibitory effect). Spectral DCM (spDCM), a DCM technique for resting-state data, exhibits higher computational efficiency and sensitivity when comparing group-between differences (Razi et al., 2015). This approach has been widely applied to reveal directional connections contribute to emotion intelligence, disordered eating, and bipolar disorder (Bajaj & Killgore, 2021; Chen et al., 2023; W. Li et al., 2024; Z. Zhang et al., 2022). However, the potential directional neural links involved in obesity are largely unknown. Only one study used the spDCM to examine adults’ obesity-related changes in the direction and valence of connections between key areas, and revealed that excess weight was associated with persistent top-down excitation of the hypothalamus, regardless of homeostatic state (Voigt et al., 2021).
Despite the hippocampus has been found to play a crucial role in food consumption and obesity, few studies focused on the neurofunctional alteration in the hippocampus among children with overweight/obesity. More importantly, it remains unknown whether and how pediatric obesity influences the direction of information exchange between key brain regions. Therefore, the current study represents an important first step in combining the FC and EC measurements to investigate the neural basis of pediatric obesity, with the hippocampus as region of interest (ROI). Based on previous findings of lesser FC of hippocampus with somatosensory cortex in obesity adults (Sun et al., 2018), we used the seed-based FC analysis to test the first hypothesis that children with overweight/obesity might show lower functional synchrony between hippocampus and somatosensory cortex. In addition, the spDCM technique was further employed to reveal the abnormal mechanism of information flow in relation to hippocampus (i.e., EC). Due to limited evidence, we hypothesized that the altered directional connections regarding the hippocampus might contribute to pediatric overweight/obesity. Furthermore, altered activity in appetitive brain regions have been shown to be a predictor for future eating behaviors and weight gain (Boswell & Kober, 2016). Regarding children with obesity, the neural vulnerability factors for future eating behaviors and body mass index (BMI) have been understudied. According to the vicious circle model of obesity, we assumed that abnormal hippocampal connections in children with obesity might be a prominent predictor for future eating behaviors and BMI. The current study will expand and deepen the understanding of neurofunctional substrates of pediatric obesity, which is essential for developing effective intervention strategies and for reducing long-term health-care costs associated with obesity (Lowe et al., 2020).