| Sumario: | Background Evidence on how intrinsic capacity and environment interact to sustain social participation remains limited. We examined how intrinsic capacity, neighbourhood environments and their interaction relate to social participation among urban adults aged ≥50 years in Brazil. Methods This cross-sectional study comprised 5068 urban residents aged ≥50 years from the nationally representative 2015–16 Brazilian Longitudinal Study of Ageing. Intrinsic capacity was assessed as a five-domain composite (cognitive, psychological, sensory, locomotor, vitality). Environment was measured using a 15-item neighbourhood perception scale and a derived three-item neighbourhood environment indicator capturing mobility barriers and access to food outlets. Social participation, assessed with a 14-item scale covering relational, leisure and productive roles, was classified as high for participants in the highest quartile. Associations of intrinsic capacity, neighbourhood measures and their interaction with social participation were estimated using Poisson regression adjusted for sociodemographic, clinical and modifiable factors. Results Intrinsic capacity was associated with high social participation (adjusted prevalence ratio per 1-SD increase = 1.31; 95%CI = 1.22–1.40), but both neighbourhood measures modified this relationship (P -value for interactions≤.001). The modifying role of neighbourhood measures was most evident among adults with low intrinsic capacity, for whom more supportive neighbourhoods were associated with higher probabilities of high social participation. With the three-item neighbourhood indicator, predicted probabilities ranged from 12% in the least supportive settings to 30% in the most favourable ones, approaching levels observed in adults with higher intrinsic capacity. Conclusions Supportive neighbourhoods may help offset reduced intrinsic capacity and sustain high social participation in later life, offering scalable strategies to promote healthy ageing in under-resourced settings.
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