Loss of Smell Could Be a Warning Sign of Alzheimer’s, Study Finds

New Delhi: A decline in the sense of smell could signal the early stages of Alzheimer’s disease, even before memory loss or other cognitive issues appear, according to new research from Germany.

The study, conducted by scientists at DZNE and Ludwig-Maximilians-Universitat Munchen (LMU), suggests that the brain’s own immune cells — called microglia — may play a role in damaging nerve fibres that are vital for processing smells. Their findings were published in the journal Nature Communications.

Researchers found that these immune cells mistakenly attack connections between two important brain regions: the olfactory bulb, which processes smells, and the locus coeruleus, a region involved in regulating blood flow, sleep-wake cycles, sensory processing, and other vital functions.

“Our study suggests that in the early stages of Alzheimer’s, nerve fibres linking the locus coeruleus to the olfactory bulb undergo changes,” explained Dr. Lars Paeger, a scientist at DZNE and LMU. “These changes send signals to microglia, which then mistakenly break down the fibres.”

The team discovered that in affected nerve fibres, a fatty substance called phosphatidylserine — normally found inside cell membranes — had shifted to the outside. This change acts as an “eat-me” signal for microglia, prompting them to clear away these fibres. Such processes are usually part of normal brain maintenance, called synaptic pruning, but in this case, they appear to harm healthy connections needed for the sense of smell.

The findings are based on studies in both mice and humans, including brain tissue analyses and advanced PET scans. According to the researchers, this mechanism could help explain why many Alzheimer’s patients notice a fading sense of smell long before memory loss begins.

Scientists believe the discovery could pave the way for earlier diagnosis of Alzheimer’s. If smell loss is recognised as a reliable warning sign, doctors may be able to screen patients sooner and begin treatments at an earlier stage — potentially slowing down the disease’s progression.