From exercise dose to social dose: a narrative review of the components, mechanisms, and design principles of group-based physical activity interventions to reduce loneliness

Loneliness is a growing public health concern associated with adverse mental, physical, and social outcomes. Group-based physical activity (GBPA) has emerged as a potentially scalable approach for reducing loneliness, but intervention effects are heterogeneous and are not adequately explained by exercise dose alone. In this narrative review, we propose the concept of social dose to complement conventional exercise dose. Whereas exercise dose refers to the frequency, intensity, and duration of activity, social dose refers to the structured exposure to opportunities for social connection embedded within group-based activity. We conducted a theory-guided narrative synthesis of literature from social psychology, exercise science, and public health, with emphasis on systematic reviews, intervention studies, qualitative studies, and foundational theoretical work relevant to loneliness, GBPA, belongingness, social identity, and peer support. We organize the evidence around three psychosocial pathways—belongingness, social identity, and peer support—and identify actionable intervention components including stable cohorts, repeated co-presence, predictable rituals, psychologically safe group climates, shared goals and symbols, buddy systems, peer mentoring, and inclusive leadership behaviors. We also discuss cross-cutting moderators at the individual, group, contextual, and activity-type levels, and outline implications for measurement and mechanism-sensitive trial design. We argue that GBPA should be understood not only as an exercise intervention, but also as a socially structured exposure intervention. However, the current evidence base remains heterogeneous, GBPA-specific causal evidence is uneven across pathways, and many studies underreport social-interaction components. Treating social dose as a measurable and designable construct may help make GBPA interventions more precise, equitable, and scalable for reducing loneliness.