A functional approach to model intrinsic capacity in ageing trajectories.

The ageing process is a multifaceted phenomenon that affects individuals' physical, cognitive, and psychological well-being over time. To promote healthy ageing, in the mid-2010s, the World Health Organisation introduced the concept of intrinsic capacity (IC) as the set of physical and mental capacities of an individual. These capacities, categorised into five domains, namely cognition, vitality, locomotion, psychological well-being, and sensory, are assessed by healthcare practitioners using the ICOPE guidelines, a laborious and time-consuming task. This study leverages existing data from longitudinal studies, such as the well-known Health and Retirement Study, to align their dimensions with the ICOPE framework and understand how existing ageing trajectories fit within the new IC paradigm. Moreover, a novel IC scoring system has been validated statistically using linear mixed models, and a simple yet comprehensive data visualisation strategy has been designed to illustrate IC trajectories. Results show the significant negative impact of time on all IC domains, except psychological well-being, showing a gradual IC decline over the years. Additionally, significant associations between lifestyle factors, such as physical activity, alcohol and tobacco consumption, and body mass index, with IC trajectories have been identified. This work suggests the feasibility of using existing longitudinal studies to quantitatively model IC ageing trajectories following the ICOPE framework.