Rapamycin and calorie restriction have attracted serious scientific attention as longevity interventions, with studies in animals consistently showing extended average lifespans. A new reanalysis of that data complicates the picture considerably.
The reanalysis builds on a 2025 study that examined 167 research papers spanning eight non-human species — including fish, mice, rats, and rhesus monkeys. That earlier work found animals lived longer on average when given rapamycin or subjected to calorie restriction, leading the researchers to conclude the same likely applied to humans. The new phase of the research shifts focus from group averages to individual responses, and the spread is wide.
“Some individuals will be much longer lived, some will be a little longer lived and some might not live any longer than they would have anyway,” says Tahlia Fulton at the University of Sydney. According to the announcement, either intervention is “likely beneficial, but you don’t know how beneficial” at the individual level — a meaningful distinction that average-outcome data obscures entirely.
The Survival Curve Problem
Fulton frames the benchmark for a genuinely effective longevity intervention as “squaring the curve” — a concept from population health where the graph of survival over time shifts from a gradual slope to a near-vertical drop at an advanced age. In practical terms, it means the majority of a population lives a full lifespan and declines sharply at its end, rather than dying in a long, dispersed tail. The research finds that neither rapamycin nor dietary restriction achieves this. Both interventions extend average lifespan without reliably bringing more individuals to that upper bound.
Matt Kaeberlein at the University of Washington in Seattle adds a further dimension: squaring the curve does not automatically improve healthy years of life. He identifies “healthspan inequality” — whether the gap in healthy life years between individuals widens or narrows with a given intervention — as the more meaningful question, one the current research does not resolve.
What Rapamycin Actually Does
Originally developed as an immunosuppressant for organ transplant patients, rapamycin works by blocking the mTOR protein, which regulates cell growth and division. At low doses, the drug has demonstrated lifespan extension in flies and mice, with DNA damage protection identified as a possible mechanism. Its repositioning as a potential anti-ageing compound rests on that animal data — data now shown to mask substantial individual variability.
Fulton’s position is that expectations should be moderated until research can identify which individuals benefit most, with genetic profiles and life history as likely factors. “Hopefully we can address individual genetic codes and life experiences and be able to say to them, ‘Alright, cool, this is exactly what you need in order to live your longest possible life,'” she says. The findings were published in Biology Letters.
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