Does Alzheimer's have a smell? A new study links smell to the early stages of the memory-affecting disease.

A team of German scientists has found a possible key to early detection of Alzheimer's disease: the sense of smell.

In research recently published in the journal Nature Communications, researchers led by Jochen Herms of the University of Munich identified a mechanism in the brain that could explain why the loss of smell often precedes other, more well-known symptoms of this neurodegenerative disease.

The study suggests that the immune system, upon detecting certain chemical changes in specific neurons, triggers an attack that damages the connections between the olfactory bulb and the locus ceruleus , the brain area involved in the perception of smells.

This finding raises the possibility that an alteration in the sense of smell could serve as an early warning sign for Alzheimer's , even before obvious cognitive symptoms appear.

Scientists explain that this phenomenon originates when the membranes of certain neurons begin to display phosphatidylserine on their outer surface , a substance that acts as a "food signal" for microglia, the brain's immune cells. In response, microglia attack and eliminate these nerve connections, compromising the pathway that allows them to process odors.

The research is based on a multidisciplinary approach that includes three sources of evidence: tests in mice, which showed an early loss of olfactory connections; PET imaging performed on living patients, which showed alterations in brain regions related to smell; and post-mortem analysis of brain tissue from people diagnosed with Alzheimer's, which confirmed the biochemical changes responsible for this immune response.

"Our findings could pave the way for the early identification of patients at risk of developing Alzheimer's... This allows them to undergo comprehensive testing to confirm the diagnosis before cognitive problems arise," Herms said, highlighting the value of these invisible signs as an early diagnostic tool.

Interest in the relationship between smell and neurodegenerative diseases is not new. Previous studies have already suggested that an unexplained loss of olfactory ability can be an early warning sign . A longitudinal study in the United States, which analyzed nearly 3,000 older adults between the ages of 57 and 85, concluded that people with a diminished sense of smell were twice as likely to develop dementia within five years, compared to those with a normal sense of smell.

These data, according to the University of Chicago, show that olfactory loss can anticipate classic symptoms such as memory decline . This is supported by another study, published in Springer Nature, with a 12-year follow-up, which supports the idea that olfactory impairment, whether alone or combined with cognitive decline, is a strong early indicator of dementia risk.

These findings are especially important given the current landscape of Alzheimer's diagnoses. An estimated 944,000 people are living with dementia in the United Kingdom, and projections indicate that this number will exceed one million by the end of the decade.

However, healthcare systems struggle to detect the disease in its early stages. A recent survey by the Alzheimer's Society, conducted by Walnut Unlimited among nearly 3,500 people (including patients, family members, and unpaid caregivers), revealed that only a third had a satisfactory experience with diagnosis.

Fifty-two percent of respondents said the process took too long, and 41 percent said they had consulted several professionals before receiving a proper assessment. "Nearly one million people are living with dementia, but its scale and everyday realities often remain hidden... An early and accurate diagnosis is vital to ensuring people with dementia can access the care, support, and treatments they so desperately need," stressed Professor Fiona Carragher, the organization's director of policy and research.

The greatest contribution of the study led by Herms is the possibility of transforming an everyday experience (such as smell) into a clinical tool for detecting Alzheimer's. Early identification of biological signals could significantly improve the effectiveness of existing treatments.

Recent medications, such as beta-amyloid antibodies, have demonstrated some efficacy, but only when applied in the early stages of the disease. Therefore, detecting the disease before the onset of cognitive decline could be crucial in the patient's outcome . In the researcher's words: "This would allow for earlier intervention with beta-amyloid antibodies, increasing the likelihood of a positive response."

*This content was rewritten with the assistance of artificial intelligence, based on information from O Globo, and reviewed by a journalist and an editor.