08/10/2025
🧠Scientists have identified a new sign of Alzheimer's — smell.
If you lose your sense of smell, there is a chance it could be an early sign of Alzheimer’s disease, according to new research.
Scientists have long suspected that a fading sense of smell might be linked to the early stages of Alzheimer’s, but the exact cause wasn’t clear until now. In a new study, researchers used brain scans, human tissue samples, and experiments with mice to uncover what’s going on.
They found that the brain’s own immune cells, called microglia, may be breaking down important nerve connections between two key brain areas: the olfactory bulb, which processes smells, and the locus coeruleus, which helps regulate sensory input like smell. These microglia are meant to clean up damaged or unneeded connections, but in this case, they may be attacking nerve fibers too early, possibly because the affected neurons are firing abnormally, a known symptom in the early phases of Alzheimer’s. A fatty molecule called phosphatidylserine, which usually stays hidden inside nerve cell membranes, starts showing up on the outside of these cells, acting like a distress signal.
When microglia detect this signal, they interpret it as a sign the neuron is damaged and destroy the connection, leading to a reduced sense of smell. This process is normally part of brain maintenance, but in Alzheimer’s, it seems to kick in too soon and in the wrong places.
The damage to smell pathways seems to happen before major memory loss or other cognitive issues, meaning it could serve as an early warning sign. If doctors can catch Alzheimer’s this early, before memory problems begin, they may be able to start treatment sooner, potentially improving the outcome.
source
Meyer, C., Niedermeier, T., Feyen, P.L.C. et al. Early Locus Coeruleus noradrenergic axon loss drives olfactory dysfunction in Alzheimer’s disease. Nat Commun 16, 7338 (2025).
