Although estimates vary, there is good evidence from population statistics that those whose functioning is significantly impaired by mental illnesses such as schizophrenia, major depression, and bipolar disorder have shorter lifespans, on average, compared with those who don’t have these disorders. According to the World Health Organization, average lifespans in such individuals may be shortened from 10 to 25 years.

This major, if infrequently discussed, impact of serious mental illness is related to the separate question of how such illnesses affect specific aspects of physical health over the course of the lifespan.

A team that included two BBRF prize winners set out to determine whether having a psychiatric disorder was associated with the onset of dementia. To determine this, they employed an extraordinary sample: data on all individuals born in New Zealand between 1928 and 1967, and who lived in that country at any time in the period between 1988 and 2018. The New Zealand Integrated Data Infrastructure, maintained by the government, thus provided anonymous, searchable health records for over 1.7 million people, 50.6% male, and aged 21 to 60 at the start of the study. Since the study covered a 30-year period, this enabled the team to look at the health trajectories of people from the beginning of adulthood through age 90.

The study was led by Leah S. Richmond-Rakerd, Ph.D., of the University of Michigan. Avshalom Caspi, Ph.D., and Terrie E. Moffitt, Ph.D., were members of the team. They are, respectively, the 2010 and 2011 winners of BBRF’s Ruane Prize and are affiliated with Duke University.

The team noted in its paper appearing in JAMA Psychiatry that a commission of dementia experts recently identified depression among 12 preventable risk factors that are most robustly associated with dementia. While various studies have implicated other psychiatric disorders including anxiety, schizophrenia, and bipolar disorder, depression is the only psychiatric diagnosis represented in most dementia risk assessments. Most such studies have observed individuals from midlife or later, and thus by design are not able to capture psychiatric disorders earlier in the lifespan, including the years of peak onset. Nor have most studies distinguished between early-onset and late-onset dementia.

The study led by Dr. Richmond-Rakerd and colleagues addresses these issues, evaluating a full range of psychiatric conditions, from early in life, and noting dementia onset throughout the lifespan.

Analysis of the data revealed that individuals diagnosed with a psychiatric disorder were more likely to develop dementia than those without a psychiatric disorder.

During the 30-year period, 64,857 (3.8%) of the 1.7 million people in the records were identified as having a psychiatric disorder; 34,029 (2%) were identified as having dementia. Yet 6.1% of those with a psychiatric disorder were also identified as having dementia, compared with only 1.8% of those without a psychiatric disorder. Dementia was more prevalent among those with psychiatric disorders in men, women, and those in every age group. The mean time to a dementia diagnosis following a first diagnosis of psychiatric disorder was, in this population sample, 8.6 years.

Those with psychiatric disorders were also at elevated risk for younger onset of dementia, and the linkage was present between psychiatric disorders and dementia across the full range of psychiatric diagnoses. It was also present when considering those who developed early and later-onset dementia, Alzheimer’s disease, and other dementias. All of these relationships remained intact after the team accounted for preexisting physical illnesses and socioeconomic deprivation.

“These results have several implications,” the team noted. The most important: “If associations [between psychiatric disorders and dementia] are causal, then ameliorating psychiatric disorders early in life might mitigate neurodegenerative disease in later life,” including dementia.

“Mental disorders are a particularly salient antecedent of dementia among younger individuals,” the team also noted. While the reasons are not clear, their analysis “does reveal that poor mental health is associated with poor cognitive health among younger individuals, not just older individuals.”

The team noted that different mechanisms might connect different psychiatric disorders with dementia. “Depression may prompt neuroinflammation, excessive alcohol use can lead to brain damage, and psychosis may precipitate accelerated cognitive and functional decline,” they said, offering three examples. Future research should try to specify different mechanisms, if indeed the connection is causal.

The team also noted that psychiatric illness may be an indicator—a marker—of risk rather than a cause of dementia. Even if this is so, however, psychiatric disorders, they say, “are a very early warning sign of subsequent cognitive decline.” Thus, their findings “support embedding dementia prevention into mental disorder treatment across the life course.” An example would be trying to address social isolation or disconnection in a person diagnosed with depression or schizophrenia.