Association of Aging Trajectories in the Japan Science and Technology Agency Index of Competence With Instrumental Activities of Daily Living Among Community‐Dwelling Older Japanese Adults: The Otassha Study

Aim The Japan Science and Technology Agency Index of Competence (JST‐IC) was developed to measure competence at much higher levels than the Tokyo Metropolitan Institute of Gerontology Index of Competence (TMIG‐IC), which evaluates higher‐level functional capacity necessary for independent living. This study explored the JST‐IC aging trajectories and their associations with instrumental activities of daily living (IADL) impairment in community‐dwelling older Japanese individuals. Methods Participants were 665 adults who responded to the 2014 baseline survey and at least one follow‐up survey (2015–2019) of the Otassha study. The JST‐IC and TMIG‐IC aging trajectories were identified using group‐based trajectory modeling. The association with nine‐year IADL impairment onset was investigated using Cox proportional hazards models. Results Three JST‐IC trajectory groups were identified: high (29.5%), medium (53.1%), and low (17.3%), with scores decreasing by one to two points with increasing age over the follow‐up period. High‐stable (50.6%), late‐onset decrease (41.1%), and early‐onset decrease (8.3%) TMIG‐IC trajectory groups were identified with a one‐ to three‐point decrease in late‐ and early‐onset groups. The low JST‐IC trajectory group had an adjusted hazard ratio of 2.90 (95% confidence interval, 1.60–5.24) for IADL impairment (reference: high group). Low technology usage, information practices, and social engagement trajectories were significantly associated with IADL impairment. Conclusions The JST‐IC scores gradually declined in all groups, even among those who maintained their TMIG‐IC scores. The low JST‐IC trajectory group—particularly low technology usage, information practices, and social engagement—was associated with the onset of IADL impairment.