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A new study published in JAMA looked at longitudinal associations of mental disorders with dementia, a 30-year analysis of 1.7 million New Zealand citizens.

“This study is about links between mental health and dementia,” study author Barry J. Milne told us. “We wanted to investigate whether there are associations between a range of mental health disorders and subsequent dementia, and whether associations are apparent for both early- and late-onset dementia.”

Researchers hypothesized that mood disorders such as depression would be associated with subsequent dementia, but they were unsure whether associations would extend to other mental health conditions.

“Dementia peaks late in life and is not currently curable, whereas mental health disorders peak in adolescence and young adulthood and are treatable,” Milne told us. “If the same people who have mental disorders when young tend to then develop dementia years later, that would mean that mental health treatments might be an opportunity to prevent or delay dementia.”

Previous studies investigating the link between mental health disorders and dementia have tended to assess mental health from mid-life only (past the young-adult period of peak prevalence) and have not often explored a range of mental health conditions.

Researchers studied New Zealanders born between 1928 and 1967 and followed them for 30 years, from 1988-2018 (so they were aged 21-60 at the start of the period and 51-90 at the end of the period).

“We assessed mental health hospitalisations and dementia (defined through hospitalisation, pharmaceutical and death records) over the 30-year period (1988-2018),” Milne told us. “Analyses were conducted using data in the Integrated Data Infrastructure (IDI), a de-identified nationwide database of New Zealanders’ interactions with government services.”

Analyses involved assessing the association between mental health hospitalisations and subsequent dementia, controlling for gender, birth year, prior physical health hospitalisations, and neighbourhood deprivation (as a proxy for people’s socioeconomic status).

Analyses were conducted for all mental health disorders and for six mental health disorder types (substance use disorder, psychotic disorder, mood disorder, anxiety disorder, self-harm, and other disorders), and for all dementias as well as Alzheimer’s and other dementias separately.

“Those who had experienced a mental health hospitalisation had 3.5 times greater risk of later developing dementia than those who did not,” Milne told us. “Results held for men and women, for both early and late onset dementia, for both Alzheimer’s and non-Alzheimer’s dementia, and across all mental health disorder types.”

Results held after accounting for pre-existing physical illness, and socioeconomic deprivation.
Mental health predicted dementia more strongly than physical health predicted dementia.

“We were surprised that mental health was a stronger predictor of dementia than physical health, and a little surprised how consistent the associations were across mental health disorders,” Milne told us. “If the association is causal, preventing mental disorders early in life may help prevent or delay later dementia.”

Even if the association is not causal, mental disorders are important early warning signs for later dementia. People with mental disorders could, for instance, be encouraged to increase their physical activity – physical inactivity is a modifiable risk factor for dementia, and is elevated among those with mental health disorders.

“Even though risk of dementia is 3.5 times greater in those who have experienced mental health problems, most people with mental health problems will not develop dementia,” Milne told us. “Mental health problems are not a ‘life sentence’ that always result in dementia.”