Social isolation and loneliness are significant public health concerns, exacerbated by the COVID-19 pandemic. These conditions have been linked to increased mortality, impaired affective well-being, and increased risk of mood disorders. The pandemic highlighted the urgent need for effective remedial strategies. Physical activity is a known protective factor for mental health, with potential neural mechanisms linked to emotion regulation. However, its role in mitigating the negative effects of social isolation in daily life remained largely unknown. This study hypothesized that physical activity could compensate for the negative affective consequences of lacking social contact, particularly in individuals at higher risk for depression.

Existing research demonstrates a strong correlation between social isolation and negative mental health outcomes. Studies have shown that lack of social contact is associated with increased mortality, reduced affective well-being, and altered brain structure and function in emotion regulatory networks. The COVID-19 pandemic dramatically increased social isolation, emphasizing the need for interventions. While physical activity has been established as beneficial for mental health, its direct impact on mitigating the effects of social isolation in real-world settings was not fully understood. This study aimed to bridge this gap by examining the dynamic interplay between social contact and physical activity on affective well-being.

This study employed a multimodal approach combining accelerometry, electronic diaries (ecological momentary assessment – EMA), and neuroimaging (fMRI). Study 1 included 317 healthy young adults (aged 18-28) who wore accelerometers for seven days to measure physical activity and used smartphone-based e-diaries to report real-life social contact and affective valence multiple times daily. A subset (175) also underwent resting-state fMRI scans to assess default mode network (DMN) connectivity. Study 2, a replication study during the COVID-19 pandemic, included 30 participants who wore accelerometers and used e-diaries for six months. Multilevel models were used to analyze the interaction effects of social contact and physical activity on affective valence, controlling for covariates such as time of day, location, sex, age, and BMI. DMN connectivity was included as a moderator in Study 1. Statistical analyses were performed using SAS and CONN toolbox in MATLAB.

Study 1 found a significant interaction effect between social contact and physical activity on affective valence. Higher physical activity significantly reduced the negative impact of lacking social contact on affective well-being. Approximately 349 milli-g of physical activity over an hour (e.g., walking at 3 mph) was estimated to fully compensate for the affective deficit of social isolation. Individuals with higher DMN connectivity, a neural risk factor for depression, showed the most pronounced compensatory effect of physical activity. Individuals with smaller social networks and higher habitual physical activity reported lower trait loneliness. Study 2 replicated the main finding during the COVID-19 pandemic, demonstrating the robustness of the effect even under restrictive conditions. The effect of physical activity remained significant even when considering only light physical activity and home-based activity.

The findings demonstrate a clear compensatory mechanism between physical activity and social isolation on affective well-being. This suggests that even moderate physical activity can effectively buffer against the negative emotional consequences of social isolation. The stronger effect observed in individuals with higher DMN connectivity highlights a potential personalized approach to interventions, targeting at-risk individuals. The replication during the pandemic underscores the real-world applicability and importance of physical activity as a protective factor. The study provides a strong basis for recommending physical activity as an accessible and effective strategy to mitigate the negative mental health impacts of social isolation.

This multimodal study provides robust evidence supporting the role of physical activity as a compensatory mechanism for the negative affective consequences of social isolation, particularly among those at higher neural and psychological risk for affective disorders. The findings highlight the potential of physical activity as a readily accessible and effective public health strategy. Future research could focus on investigating the underlying neural mechanisms further, exploring the effects of different types and intensities of physical activity, and developing targeted interventions incorporating just-in-time adaptive interventions.

The correlational nature of the study design prevents causal inferences. While the use of real-life data enhances ecological validity, it also limits the control over potential confounding variables. Future studies employing experimental designs could help establish causality. The focus on affective valence as a single outcome measure limits the scope of findings; investigating specific emotions could provide a more nuanced understanding. The generalizability of findings might be limited by the relatively young age of participants in the main study.