Susceptibility to stress and nature exposure: Unveiling differential susceptibility to physical environments; a randomized controlled trial

BACKGROUND

Emerging epidemiological evidence indicates nature exposure could be associated with greater health benefits among groups in lower versus higher socioeconomic positions. One possible mechanism underpinning this evidence is described by our framework: (susceptibility) adults in low socioeconomic positions face higher exposure to persistent psychosocial stressors in early life, inducing a pro-inflammatory phenotype as a lifelong susceptibility to stress; (differential susceptibility) susceptible adults are more sensitive to the health risks of adverse (stress-promoting) environments, but also to the health benefits of protective (stress-buffering) environments.

OBJECTIVE

Experimental investigation of a pro-inflammatory phenotype as a mechanism facilitating greater stress recovery from nature exposure.

METHODS

We determined differences in stress recovery (via heart rate variability) caused by exposure to a nature or office virtual reality environment (10 min) after an acute stressor among 64 healthy college-age males with varying levels of susceptibility (socioeconomic status, early life stress, and a pro-inflammatory state [inflammatory reactivity and glucocorticoid resistance to an in vitro bacterial challenge]).

RESULTS

Findings for inflammatory reactivity and glucocorticoid resistance were modest but consistently trended towards better recovery in the nature condition. Differences in recovery were not observed for socioeconomic status or early life stress.

DISCUSSION

Among healthy college-age males, we observed expected trends according to their differential susceptibility when assessed as inflammatory reactivity and glucocorticoid resistance, suggesting these biological correlates of susceptibility could be more proximal indicators than self-reported assessments of socioeconomic status and early life stress. If future research in more diverse populations aligns with these trends, this could support an alternative conceptualization of susceptibility as increased environmental sensitivity, reflecting heightened responses to adverse, but also protective environments. With this knowledge, future investigators could examine how individual differences in environmental sensitivity could provide an opportunity for those who are the most susceptible to experience the greatest health benefits from nature exposure.

Neural and peripheral markers of reward during positive social evaluation are associated with less clinician-rated depression symptom severity in adolescence

Although blunted sensitivity to reward is thought to play a key role in promoting risk for depression, most research on this topic has utilized monetary reward paradigms and focused on currently depressed adults. To address this issue, we analyzed neural reward and β-endorphin data from the Psychobiology of Stress and Adolescent Depression (PSY SAD) Study, which recruited a well-characterized sample of adolescent girls at high vs. low risk for major depressive disorder (MDD) (N = 52, M_age = 14.90, SD = 1.35) based on their mothers’ lifetime history of MDD. As hypothesized, greater striatal activity while receiving positive (vs. neutral) social evaluation was associated with lower depression symptom severity as independently assessed by the Kiddie Schedule for Affective Disorders and Schizophrenia (K-SADS). This association was present for girls at high but not low risk for MDD, suggesting that this neural response may represent a pre-clinical marker of risk for depression. Consistent with these results, higher post-social evaluation levels of a peripheral marker of reward sensitivity, β-endorphin, were related to lower clinician-rated depression symptom severity. Together, these results indicate that neural and peripheral markers of responsivity to social reward are both related to depression severity, which may have implications for understanding the pathophysiology of depression.

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The neurobiological reward processes underlying adolescent depression risk are unclear.

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To address this issue, we recruited adolescent girls at high vs. low risk for MDD.

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We assessed their striatal activity, β-endorphin levels, & depression symptom severity.

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Striatal activity & β-endorphin levels during social evaluation predicted depression severity.

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These effects differed for high-vs. low-risk youth, suggesting pre-clinical risk biomarkers.

Psychobiology of Stress and Adolescent Depression (PSY SAD) Study: Protocol overview for an fMRI-based multi-method investigation

Depression is a common, often recurrent disorder that causes substantial disease burden worldwide, and this is especially true for women following the pubertal transition. According to the Social Signal Transduction Theory of Depression, stressors involving social stress and rejection, which frequently precipitate major depressive episodes, induce depressive symptoms in vulnerable individuals in part by altering the activity and connectivity of stress-related neural pathways, and by upregulating components of the immune system involved in inflammation. To test this theory, we recruited adolescent females at high and low risk for depression and assessed their psychological, neural, inflammatory, and genomic responses to a brief (10 minute) social stress task, in addition to trait psychological and microbial factors affecting these responses. We then followed these adolescents longitudinally to investigate how their multi-level stress responses at baseline were related to their biological aging at baseline, and psychosocial and clinical functioning over one year. In this protocol paper, we describe the theoretical motivations for conducting this study as well as the sample, study design, procedures, and measures. Ultimately, our aim is to elucidate how social adversity influences the brain and immune system to cause depression, one of the most common and costly of all disorders.

Associations of Residential Brownness and Greenness with Fasting Glucose in Young Healthy Adults Living in the Desert

Evolutionary psychology theories propose that contact with green, natural environments may benefit physical health, but little comparable evidence exists for brown, natural environments, such as the desert. In this study, we examined the association between "brownness" and "greenness" with fasting glucose among young residents of El Paso, Texas. We defined brownness as the surface not covered by vegetation or impervious land within Euclidian buffers around participants' homes. Fasting glucose along with demographic and behavioral data were obtained from the Nurse Engagement and Wellness Study (n = 517). We found that residential proximity to brownness was not associated with fasting glucose when modeled independently. In contrast, we found that residential greenness was associated with decreased levels of fasting glucose, despite the relatively low levels of greenness within the predominantly desert environment of El Paso. A difference between the top and bottom greenness exposure quartiles within a 250 m buffer was associated with a 3.5 mg/dL decrease in fasting glucose levels (95% confidence interval: -6.2, -0.8). Our results suggest that within the understudied context of the desert, green vegetation may be health promoting to a degree that is similar to other, non-desert locations in the world that have higher baselines levels of green.