Dynamical modelling of the frailty index indicates that health reaches a tipping point near age 75

Abstract

The frailty index (FI) serves as a useful quantitative summary of age-related health. We quantitatively modelled FI trajectories with age. We fit directly to longitudinal transitions in health attributes from normal to deficit and vice-versa. We used data from two large longitudinal studies: the Health and Retirement Study and the English Longitudinal Study of Ageing. The studies included 47592 individuals with 254357 total visits. Using damage (deficit emergence) and repair (deficit recovery) transitions we estimated changes to robustness and resilience, respectively. We find that both robustness and resilience decrease continuously with both increasing age and FI. Remarkably, these declines caused a tipping point in health near age 75, when damage and repair rates are equal. Beyond this tipping point, the ongoing loss of both robustness and resilience leads to a sharp increase in the FI and a commensurate increase in risk of mortality. This tipping point was observed in both sexes, noting that males showed higher initial robustness and resilience, and commensurately steeper decline, consistent with the sex-frailty paradox. We infer that robustness and resilience mitigate environmental stressors only up to an age of 75, beyond which health deficits will increasingly accumulate leading to death.