More good news about Irisin:
Irisin: An Exercise Hormone Tracks With Bigger Memory Hubs in the Aging Brain
Australian researchers measured weekly exercise, fasting blood irisin, and high-resolution hippocampal subfield volumes in 74 healthy older adults, and report the first human evidence that the muscle-derived hormone irisin sits on the path between physical activity and hippocampal structure. More exercise went with more irisin, and more irisin went with larger hippocampi, most strongly on the right side and concentrated in the CA3 and CA4 subfields that house the dentate gyrus and are central to memory formation and Alzheimer’s pathology.
Exercise protects the ageing brain. That has been settled for more than a decade. People who move more tend to have larger hippocampi, the seahorse-shaped structures that build new memories and that shrink early in Alzheimer’s disease. What nobody has shown in humans is the courier. How does a contracting thigh muscle send a message that ends up as preserved tissue inside the skull?
A team at the Thompson Institute in Queensland has put forward the first direct human evidence for one candidate courier: irisin, a hormone released from skeletal muscle when it works hard. In mice, irisin looks close to essential for the brain benefits of exercise. Delete the gene that produces it and the gains vanish. Give it back and the gains return. Whether any of that carries over to people has been an open question, partly because measuring irisin in human blood is notoriously difficult and partly because the two or three small imaging studies that tried gave contradictory answers.
The new study took 74 healthy Australians aged 50 to 84, asked how much they exercised in a typical week, drew fasting blood, and scanned their brains at high resolution, splitting the hippocampus into eleven compartments per side rather than treating it as a single lump.
Three results emerged. People who exercised more carried more irisin in their blood. People with more irisin had larger hippocampi, especially on the right. And the irisin signal was not spread evenly. It concentrated in CA3 and CA4, the inner compartments that handle pattern separation and that contain the dentate gyrus, the one region of the adult human brain where new neurons are thought to be born. A visual cortex region used as a control showed nothing at all, which argues against the finding being a generic marker of bigger brains or better health.
The awkward result is what they did not find. In this sample, exercise showed no relationship whatsoever to hippocampal size. The statistical method they used can still report a pathway under those conditions, and it did, for all nine hippocampal compartments tested. But the honest reading is narrower than the headline. Exercise tracks with irisin. Irisin tracks with hippocampal volume. Whether exercise causes irisin to cause brain preservation is not something this design can settle, because everything was measured on a single occasion with no follow-up.
That distinction governs how the result should be used. If irisin really is the courier, the target for intervention shifts from the brain to the muscle, and from generic activity to whatever specifically drives muscle to release this hormone. It would also open a route for people who cannot exercise at all, since irisin can in principle be administered as a drug. If instead irisin is a passenger, a proxy for muscle mass or metabolic health that happens to travel alongside brain size, then chasing the hormone directly is a distraction and the muscle itself remains the intervention.
This study does not separate those two possibilities. It does establish that the question now justifies the cost of a proper longitudinal trial.
Actionable Insights
What the numbers actually say. Exercise explained an extra 11.8 percent of the variation in blood irisin. In plain terms, someone one standard deviation more active than average, roughly two to three additional vigorous sessions per week on the questionnaire used, carried about 1.1 nanograms per millilitre more irisin, against a group average near 5.9.
On the brain side, irisin explained an extra 12.4 percent of right hippocampal volume and 7.5 percent of left. Translated into tissue, one standard deviation more irisin corresponded to roughly 145 cubic millimetres more right hippocampus, close to 4.8 percent of that structure. Comparing the top quarter of irisin levels with the bottom quarter gives a gap near 12 percent. At the roughly 1.5 percent annual hippocampal loss typical after age 60, that gap is equivalent to about eight years of ageing.
Treat those figures as an upper bound. They come from 74 people measured once, and correlations of this kind reliably shrink when tested properly. The defensible take-home is that resistance and vigorous aerobic training, which build the muscle that secretes irisin, remain the intervention with the best mechanistic case behind it.
Context and Source
- Open Access Paper: The Myokine Irisin Represents an Indirect Pathway Linking Exercise to Hippocampal Subfields Relevant to Alzheimer’s Disease and Neurogenesis. published: 24 April 2026.
- Institution: Thompson Institute, University of the Sunshine Coast, Birtinya, Queensland
- Country: Australia
- Journal: Aging Cell (Wiley, on behalf of the Anatomical Society)
- Impact evaluation: The impact score of this journal is 7.7 (2025 Journal Impact Factor; CiteScore 13.8), evaluated against a typical high-end range of 0 to 60+ for top general science, therefore this is a Medium impact journal. Qualifier: within the narrower field of biogerontology, Aging Cell is a leading specialty venue and ranks Q1 in the Aging category, sitting below Nature Aging but above most ageing-focused titles