Healthspan

Healthspan is the part of a life spent in good health, as distinct from lifespan, which counts every year lived whether healthy or not. Two people can die at the same age having had very different healthspans: one independent until the final months, the other dependent for a decade.

The distinction matters because it separates two goals that are easy to confuse. Adding years to the end of a life is not the same as moving the point at which health begins to fail, and a treatment can do one without doing the other. Asked which they would prefer, most people choose the second.

The difficulty is measurement. Lifespan has an unambiguous endpoint. Healthspan does not: it has to be assembled out of proxies — the age at which a chronic disease is diagnosed, the age at which everyday tasks become difficult, a score on a frailty index — and different research groups assemble it differently. A study reporting an effect on healthspan is reporting an effect on whichever proxy it chose, which is why that choice is worth finding before the result is believed.

In animal research the problem is sharper, because a mouse cannot report how it feels. Healthspan in mice is inferred from grip strength, activity, coat condition and similar measures, none of which maps cleanly onto what the word means for a person.