Measuring functional ability in Healthy Ageing: testing its validity using Japanese nationwide longitudinal data

Background The United Nations Decade of Healthy Ageing 2021-2030 suggests nations should monitor functional ability as an indicator of healthy ageing progress. Functional ability is the attribute of people to do something they value and consists of five domains. We examined its validity in terms of a construct, cross-validation across multiple waves' data, and predictivity for subsequent well-being.Methods Using panel data from 35,093 community-dwelling adults aged >= 65 years from the Japan Gerontological Evaluation Study, we performed factor analyses to explore the construct of functional ability domains in both 2013 and 2016. A modified Poisson regression analysis was employed to test their associations with well-being (subjective health and happiness) in 2019.Results The mean age (standard deviation) of participants was 72.1 (5.0) years, and 52.0% were women. A total of 85.0% reported good subjective health, and 50.6% reported high happiness levels. Factor analyses with 31 logically checked candidate items from 2016 data suggested a three-factor model comprising 24 items, which were compatible with the 2013 data results. Based on the World Health Organization's original domains, we named domains as domain #1: ability to build and maintain relationships; domain #2: ability to meet basic needs + ability to move around and domain #3: ability to learn, grow and make decisions + ability to contribute. All three domains predicted both subjective health and happiness in 2019.Conclusions Empirical data from Japan supports the functional ability concept among older individuals. Validating this concept with data from other nations is warranted.