How human and animal frailty index studies can inform each other: a brief narrative review

Frailty is common in older people, who are much more often studied than older animals. This makes it less surprising that research in frailty largely began with insights from humans before being translated into frailty in animals. Back translation from animal models to humans followed. Here, we review this interplay. We highlight how properties of the deficit accumulation approach to frailty (our focus) have been reproduced in animals. These include the distribution of frailty scores, the regularity of their change, and the presence of a submaximal limit that appears to reflect the mortality risk associated with significant health deficit accumulation. We offer examples of how animal models have helped us to understand age-related health changes. These changes reflect an increase in health deficits but also a reduction in the ability to withstand lesser potentially damaging stress (robustness) or to repair such damage when it occurs (resilience). Investigations into how age and sex affect the dynamics of deficit accumulation, in both human and animal models, are underway. Studying frailty in preclinical models will benefit our understanding of health dynamics and health, both for preclinical models and for us.