Overweight and obesity problems represent a serious threat to public health in which the physical and psychological dimensions of health are involved, as well as psychological, social, and environmental aspects [1]. In recent decades, rapid growth in overweight and obesity rates in various age groups has been detected globally [2]. In Latin America and the Caribbean, there are estimates that at least 3 out of every ten children and adolescents between 5 and 19 years of age are overweight [3]. According to the World Health Organization [4], the prevalence of obesity in children and adolescents in the world is 6.8% [6.1% − 7.6%]; this data in Mexico amounts to 14.8% [10.6%-19.3%], placing Mexico among the top countries in the world for obesity. Similarly, over 40% of Mexican children do not get the required moderate-to-vigorous physical activity (MVPA) of 60 minutes daily [5]. Obesity in children causes chronic inflammation, dyslipidemia, hypertension, endothelial dysfunction, and hyperinsulinemia [6]. Children with obesity experience mental health issues such as depression, anxiety, low self-esteem, and social problems such as bullying [7]. In addition, the impact of SARS-CoV-2 on sedentary lifestyles and eating habits was responsible for an increase in childhood overweight and obesity [8].
Obesogenic urban environments of the twenty-first century promote higher levels of sedentary behavior, bad dietary choices, and higher levels of stress, all of which are associated with higher obesity rates [9]. Green areas and public space design could be used as upstream public health interventions to mitigate the negative health impacts of obesogenic urban environments [10]. Likewise, the relevant role of neighborhood environments in promoting children´s physical activity and reducing obesity has recently been acknowledged [11]. In contrast, we investigate the role of neighborhood aesthetics on neighborhood-home-based physical activity and the neuroendocrine response to chronic stress and obesity in a sample of school children. To achieve this, in the following lines, we will start with the typical studies on the beneficial effects of physical activity and its concomitant psycho-physiological effects on obesity via stress-reducing effects. Then, we add a psycho-environmental approach to document the stress recovery influences of nature-built environments on cortisol recovery and chronic stress. We finally expose some tentative antecedents about the restorative influences of nature on eating behavior.
In children, physical activity (light, moderate to vigorous intensity activity, or LPA/MVPA, respectively) is essential for disease prevention and health promotion [12]. Physical activity is significantly associated with several indicators of body composition (Body Mass Index, % body fat), fat mass index, physical fitness, and cardiometabolic biomarkers [12, 13]. The potential benefits of MVPA for obesity prevention have been systematically documented in several studies. However, even the practice of LPA (defined as any activity with intensity between sedentary behavior and moderate intensity or activity performed at under 3 METs) has been found to protect against childhood obesity [14], resulting in an alternative health promotion strategy against obesity [14]. In the same way, several studies have shown negative associations of LPA and MVPA with anxiety and depression in both children and adults [15–19]. The effects of physical activity on stress have been documented through subjective (self-reports) [20] and cortisol biomarkers, which are frequently used in the analysis of exercise responses.
Hair cortisol concentration (HCC) is one of the most promising approaches for evaluating the endocrine chronic stress-related [21]. HCC measures systemic cortisol concentrations related to the hypothalamic-pituitary-adrenal (HPA) axis activity [22]. HCC measures are useful in investigating the links between real-life stressors and long-term cortisol responses [23]. Also, HCC within the physiological range has been linked to cardiovascular disease and obesity [24]. In athletes, regular MVPA resulted in significant salivary and plasma cortisol concentrations, with retrospective hair cortisol concentrations (HCC) increasing independently of any source of psychosocial stress [25]. Conversely, a study by Teychenne et al. [26] finds no association between women's sedentary behavior (TV viewing, computer use) and HCC. In children and adolescents, the links between the cortisol response and levels of physical activity are less established [27], with some recent antecedents suggesting that LPA and MVPA can support healthy cortisol decreases in children [28, 29]. Other studies have suggested that physical activity has been associated with lower cortisol secretion levels during stress [30]. Chronic elevated cortisol secretion in response to stress represents a risk factor for developing obesity [31]. A substantial amount of studies have documented a significant relationship among physiological biomarkers of chronic stress (cortisol levels), BMI, waist circumference, and waist-hip ratio among adult samples [32–36], confirming, thus, the role of stress-related cortisol concentrations in adipocyte biology and weight gain [37]. Relevant to obesity, obese adults used to report higher perceived stress [38] and higher HCC than average weight and overweight groups [39]. Like adults, recent research has thoroughly shown the relationships between BMI, waist circumference, salivary cortisol, and HCC in obese children [40–44].
Ecological models of healthy environments and behaviors [45] suggests that children´s physical activity is concatenated by an interaction of a multiple set of scenarios, contexts (individual, home-family), and behaviors, ranging from personal levels (e.g., age, sex) to social, neighborhood, and policy levels [46]. Individuals' behavioral choices influence physical activity [47] and sociocultural and environmental factors [48]. From an environmental point of view, higher levels of physical activity in children are linked to a set of environmental attributes related to the neighborhood-built environment, where specific locations correlate with higher children´s physical activity [49]. Neighborhood walkability [50], neighborhood aesthetics (e.g., attractive buildings and green areas [49] and recreational amenities [49, 51, 52] are neighborhoods´ attributes related to a lower prevalence of obesity in several studies [53–56]. In other studies, a higher risk of being overweight or obese (due to reduced children's physical activity) is linked to traffic (or fewer pedestrian-friendly streets), community physical disorder [57, 58], and homicide rates [59]. In line with these studies, a lack of aesthetics in neighborhood may hinder neighborhood walkability and increase the likelihood of childhood obesity [60, 61]. In this context, neighborhood aesthetics is referred to as the perception of “whether there are many interesting things to look at while walking in my neighborhood” ([62], p. 282). In this paper, the environmental attributes of a neighborhood’s aesthetics are referred to as the perceived attractiveness of natural contents (e.g., trees and urban natural sights) and buildings/homes in an urban environment. In addition to the neighborhood qualities, some homes' socio-physical features of the family environment may contribute to sedentary behavior, children's informal physical activity, and weight status [63, 64]. Specifically, physical factors at home (e.g., sports equipment, basketball hoops, playsets) are practical tools for supporting youth physical activity [65]. The nexus between green areas and obesity in adults and children shows mixed results. In some cases, higher community greenness and tree density are associated with lower odds of adiposity [59, 66, 67], and decreased access to green areas is related to higher BMI scores in boys and girls [68]; in others, playground areas have no effect on overweight in low- income preschoolers [69]. Assuming that the benefits of access to nature on obesity can be explained in terms of the increase in physical activity [70–72], it must be considered that in some cases, green spaces may also encourage sedentary rather than MVPA [73], and experiences that can promote movements skills rather overall physical activity [74].
In addition to the well-documented literature on natural and built environment-based physical activity, research in environmental psychology suggests other environmental influences that can positively affect obesity outcomes [75]. Spending time in natural environments may have significant behavioral, cognitive, emotional, and physiological benefits for children. Time spent in green outdoors promotes a feeling of being away from daily routines, increasing relaxation [76], improves children’s mood and self-regulation [76, 77], the ability to focus [78], pro-environmental behaviors [79], increases intergenerational social interactions and children´s sociability [80]. Likewise, vegetation and natural areas can help to promote the resilience of children and others with stress adversities [81], resulting in a better stress response [82]. These beneficial effects have been explained by two well-established theories concerning the promotive (instorative effects), protective (through mitigation or buffer impacts), and restorative effects of nature encounters [82, 83]: the attention restoration theory [84] and the stress recovery theory-SRT [85]. The ART proposes the positive effects of contact with natural environments on attention by encouraging effortless attention (e.g., fascination) to non-damaging features of the natural environment, thus directing attention resources [84]. The SRT proposes that natural environments are settings for stress-relieving and recovery from negative affect and physiological stress. Ulrich SRT postulates that exposure to natural surroundings positively impacts affective states, affecting brain function and neuroendocrine system activity (such as HPA axis activity) for better stress recovery [86, 87]. Compared with adults, studies on restorative environments (the environment of psychological restoration) in children and adolescents are less documented [88]. Restorative experiences are not limited to pristine landscapes but also include a variety of urban-built settings, ranging from various types of green areas (indoor and outdoor) to residential streetscapes, blue spaces, and architectural and cultural attractions [89]. Some restorative features are linked to the degree of naturalness, vegetation diversity, tree cover, and aesthetics features [90–93]. In such traits, the experience of urban aesthetics and place-based attractiveness contribute to positive experiences that support environmental preferences and perceived restoration in urban built settings [94–96]. According to these, in this paper, it is considered that both the public perception of the attractiveness of natural sights and built elements in the neighborhood are contents that contribute to the neighborhood aesthetics and restorative qualities [89] with potential effects on stress recovery outcomes [85].
Children’s restorative experiences occur where they spend most of their free time: residential areas and school playgrounds [97]. In such settings, some psychological effects related to physical activity are enhanced through multisensory experiences of pleasantness and distance from daily hassles [75, 98]. Synergies between the contact of restorative environments and physical activity revealed that, compared with indoor physical activity, outdoor green exercise (physical activity carried out in greener environments, including green walking [99], is more positively associated with emotional well-being than physical activity indoors [100]. The same pattern has been documented in the effects of physical activity in outdoor green natural environments vs. outdoor urban environments [101]. Akers et al. [102] found that levels of perceived exertion in natural areas are typically lower than those in indoor, artificial environments lacking natural amenities. In the choice of environments for physical activity, the literature shows some positive characteristics of neighborhood environments that motivate people to practice physical activity [103]. Predictors of exercise frequency in outdoor environments suggest that people prefer to build environments with natural aesthetic features such as trees and vegetation [46, 104]. As a result, given differing restoration needs and environmental preferences (i.e., aesthetics), people are motivated to walk in a potentially restorative environment [105]. In line with this, aesthetically pleasing environments may promote physical activity because they can boost the psychological benefits of exercise, including enhancing emotional states and reducing stress and anxiety [104, 106]. According to this, physical activity is not only a consequence of being in a particular setting but also reflects psychological experiences linked with stress reduction, making physical activity an essential mediator between an affordable restorative setting and mental and physical health outcomes. Under this scenario, reducing body weight and physical fitness are children’s primary physical health outcomes [107, 108]. The above studies provide the background to explain the positive link between a restorative urban space and its conjoint benefits to children's physical activity.
The positive influences of restorative environments on children have been considered through several mechanisms, including those related to accessibility (opportunity-based) and exposure types. Levels of accessibility can range from geographical proximity to green spaces to vegetation density within an area [109]. Exposure to the restorative environment includes (a) mediated (indirect) expositions (i.e., pictures of plants; [110], (b) in situ (direct) engagement experiences through active or passive encounters with nature and built amenities in neighborhoods and school greenspaces [111, 112], (c) active experiences (i.e., regular walking in green areas; [99], and (d) passive experiences (trough viewing nature, [113]. Most of these studies adopted cross-sectional designs to examine the acute effects of exposure to nature. To date, long-term biomarkers involved with accessibility and exposure to nature in children are unavailable. In such measures, antagonist processes to restoration, such as chronic stress, are critical variables in the chronic restoration-related pathways, or how environmental settings and experiences “can over time become positively associated with indices of health through pathways that involve the cumulative effects of repeated episodes of adequate restoration” ([114], p. 185).
Antecedent research on neuroendocrine restoration [114, 115] provides some promissory evidence on the alternative pathways of the links between the neighborhood aesthetics restorative qualities and its stress-recovery influence on chronic stress. Field experiments show that walking or staying in green spaces improves mood and reduces salivary cortisol levels [116, 117]. In this line, Todorova et al. [118] showed that salivary cortisol levels were significantly lower in a forest group intervention compared with one urban group. Other experimental studies show that nature sounds (birds and water sounds) [115] and gardening [114] are more beneficial for cortisol recovery. Some preliminary research suggests that living in greener neighborhoods may be associated with lower chronic stress, as analyzed by HCC in adults [86, 119]. To date, no antecedents of the environmental influences of nature-restorative environments on biomarkers of accumulative stress (HCC concentrations) in children are documented. Given the above background, the following question arises: Can the neuro-endocrine response related to exposure to neighborhood aesthetics and restorative qualities and children's physical activity be reflected in the development of obesity in children? In addition to the effects of restorative environments on stress recovery [85], positive experiences from restorative environments might also be applied to new health outcomes involved with the dietary domain and eating behavior [115]. Chronic stress has been identified as a potential underlying mechanism associated with obesity because it may lead to hyperphagia [42]. Assuming the phenomenon of “stress-eating” [120] (i.e., related to dietary decisions or influenced by emotional eating) [121], nature exposure is viewed as an environmental source whose restorative effects (stress reduction and cognitive control) interfere positively with emotional eating [122], food craving and impulsive eating [9] and healthy choice of foods [123]. According to such, mental restorativeness can promote less eating out of desire, considering both emotional stimuli and food cues [124]. Those above were examined in cross-sectional data with adult samples, with no data available on children. To the best of our knowledge, no evidence of longitudinal studies on mental restorativeness, eating behavior, and overweight are explored yet in the literature.
1.1 Study overview
Existing research on the impact of physical activity on body composition suggests that physical activity may protect against childhood obesity and mental health disorders. Likewise, significant associations between stress-related biomarkers and adiposity measures have been widely documented. Incipient studies on the effects of physical activity on the neuroendocrine response to stress in children suggest that physical activity can promote healthy cortisol decreases in children. In such studies, the directionality of the causal mechanisms is not clear, nor are the relative contributions of the environmental influences of children´s physical activity on the expression of chronic stress and its effects on children´s obesity and overweight. Socioenvironmental factors influence children´s physical activity. Among environmental factors, neighborhood environmental attributes such as walkability and aesthetics are some environmental qualities related to a lower prevalence of childhood obesity because of the affordability of the spaces to promote physical activity. The perceived attractiveness of natural and built contents contributes to positive experiences that support restorative experiences. Such experiences can enhance the psychological benefits of being active in specific locations. Stress relieving, recovering from negative affect, and reducing physiological stress are potential benefits of active (including LPA to MVPA) and passive experiences from the accessibility and exposure to a restorative environment (e.g., a highly pleasurable and aesthetic settings) [85]. Studies of neuroendocrine restoration show that active or passive encounters with restorative environments benefit cortisol recovery and lower levels of chronic stress as measured with HCC. In line with the studies above, the empirical background suggests the links between neighborhood aesthetics and restorative qualities, physical activity, and its effects on biomarkers of accumulative stress. Our study considers the extension of such effects on the development of obesity in healthy children. We examined such effects on a generalized index of obesity, the BMI index. Therefore, this study documents the predictive associations of neighborhood aesthetics on children´s obesity through home and neighborhood-based physical activity and long-term HPA axis activity in elementary school children. Overall, it was expected that some physiological effects of children´s physical activity on HCC and BMI could be modeled through the neighborhood aesthetics. Our study analyzes the indirect effects of HCC and physical activity as those that indirectly influence the relationship between neighborhood aesthetics and children´s obesity.
Thus, a set of questions emerges:
Q1. Does the neighborhood aesthetics influence children´s obesity?
Q2: Is children´s obesity influenced by the expression of children´s physical activity at home and neighborhood, given the neighborhood aesthetics?
Q3. Is children´s obesity influenced by the expression of children´s HCC, given the neighborhood aesthetics?
Q4. Is children´s obesity influenced by the expression of children´s HCC and physical activity, given the neighborhood aesthetics?
For Q1-Q4, we have proposed a conceptual model of the influences of neighborhood aesthetics on obesity through children´s physical activity and chronic stress (Fig. 1).
Our model departs from a socioecological approach to health promotion [45], which considers the phenomenon of obesity as a multifactorial disease. It pretends to promote a better comprehension of the links between the environment and behavioral transactions and their attendant effects on the physiology and expression of obesity [33]. The hypothesized relationships between the model variables are now briefly discussed. Path A examines the direct impact of neighborhood aesthetics on children's obesity, in congruence with previous studies [49, 60, 61]. Path B evaluates the environmental effects of exposure to neighborhood aesthetics on children’s obesity through active experiences with children’s physical activity [53, 125]. Path C considers the restorative (passive exposure) influences of neighborhood aesthetics (stress-reducing effects on HCC as a biomarker of chronic stress) on children’s obesity [115] (Path C). As stated before, the model considers the SRT [85] and the empirical research on typologies and characteristics of urban restorative environments [84, 89–93] to explain the promotive, protective, and restorative effects of neighborhood aesthetics [82] on the children´s physical neighborhood-home-based physical activity [64, 75, 98, 126] and chronic stress. Thus, according to the model, the more favorable appreciation of the parent´s neighborhood aesthetics, the greater the frequency of physical activity in the home and neighborhood and the lower the levels of chronic stress in children [29, 86, 119, 127]. Such levels are expected to influence children´s obesity BMI outcomes [40, 43, 44] (main path D).