How to Go About Measuring Health in Aging is its Own Sizable Debate
Health is like art; one knows it if one sees it. But that isn't good enough for the development of therapies. Health must be pinned down and actually measured. Numerous bodies have offered guidelines to this end, and those guidelines have been interpreted in countless different ways by the scientists who conduct studies. Everyone has their own definition at the detail level, which makes comparison between studies a sizable challenge. This is the usual picture for any area of human endeavor in which standards would be useful. Coordination is not a solved problem, and about the best one can hope for is a few competing standards that are fairly rigorously adhered to. In the matter of assessing and treating degenerative aging, even that remains an aspirational goal.
For the first time in human history, we may be able to measure health directly, rather than infer it from the presence or absence of disease. Doing so could enable earlier detection of age-related declines and catalyze new interventions to extend healthy longevity. But this shift will require a clear and operationalizable definition of "health" and rigorous methods to assess it. The World Health Organization's (WHO) healthy ageing framework introduced intrinsic capacity (IC) - "the composite of all the physical and mental capacities of an individual."
As a measure of individual level functioning, intrinsic capacity could become a transformative outcome for geroscience research, clinical practice, and population health. From the outset, however, intrinsic capacity has been interpreted inconsistently. While ambiguity was acceptable in a policy context, it has become problematic as IC is increasingly applied in clinical care and biological research.
In 2017, WHO-affiliated investigators identified five subdomains - locomotor, cognitive, sensory, psychological, and vitality - as "pivotal" to intrinsic capacity. While the first four comprise overt measures of functioning consistent with traditional approaches to capacity, vitality was conceptualized as describing the biological attributes required to maintain homeostasis. Vitality emerged within these biological analyses with a biologically grounded profile, including DHEAS, IGF-1, hemoglobin, FEV1, and grip strength. This 5 subdomain pattern (with variation in the specific measures used) has since been replicated in cohorts across continents and found to predict subsequent mortality and incidence of specific diseases.
Yet despite broad acceptance of the five-domain structure, methodological inconsistency remains high. Studies differ in scoring approaches, data sources, and measurement tools, particularly for vitality. As IC increasingly becomes a candidate outcome for clinical trials and aging biology studies, lack of standardization poses serious risks - misinterpretation, non-comparability, and potential loss of credibility.