A new study has found that half of older adults who develop depression or bipolar disorder may already have abnormal brain proteins linked to dementia. Researchers say this could mean that some late-life mood disorders are actually early signs of the memory-stealing disease.
Scientists from Japan’s National Institutes for Quantum Science and Technology (QST) conducted brain scans on 52 people who developed depression or bipolar disorder after age 40. They found that 26 of them had high levels of a protein called tau, and 15 of those also had amyloid beta—two substances known to play a key role in dementia.
By comparison, scans from 47 healthy adults showed tau buildup in only seven individuals and amyloid beta in just one.
The findings suggest that certain psychiatric symptoms—like depression, delusions, and hallucinations—could be triggered by the same brain changes that later lead to dementia.
Study Suggests New Way to View Late-Life Mood Disorders
The research team used PET scan technology to view the brains of patients with late-life mood disorders (LLMD). These included severe depression and bipolar disorder that began in middle or old age. Patients were recruited mainly from university hospitals.
Lead author Dr. Keisuke Takahata explained that the scans revealed tau protein deposits in regions of the brain responsible for emotion and decision-making—areas often affected in both dementia and mood disorders.
The study supports a growing theory: that in some cases, depression and similar conditions in older adults may not be mental health problems alone, but early signs of neurological decline.
Protein Deposits May Predict Dementia Diagnosis
According to data from Japan’s national brain bank, people diagnosed with depression or bipolar disorder later in life often develop dementia within seven years. This timeline supports the idea that mood changes could be part of the early disease process.
“Hallucinations and delusions in these patients may be linked to higher tau buildup in the frontal lobes,” said the researchers. These brain regions are deeply involved in mood regulation and are known to deteriorate early in dementia.
New Avenues for Diagnosis and Treatment
The findings could help reshape how doctors approach depression in older patients. Instead of treating mood disorders in isolation, doctors may begin to view some cases as potential early dementia and intervene earlier.
Dr. Shin Kurose, another researcher on the project, said the team now plans to use PET scanning to tailor treatment strategies based on protein levels. The technology could also help doctors identify dementia before memory loss begins.
“This opens the door to developing medications and therapies that slow the disease at a much earlier stage,” Kurose said.
Promising New Drugs Show Potential
New dementia treatments, such as lecanemab, aim to block the buildup of amyloid beta in the brain. If caught early, such drugs may slow the disease before memory loss and confusion appear.
The researchers hope that as PET scans become more common in mental health care, they’ll help detect early dementia in patients who might otherwise be misdiagnosed with depression alone.
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