Cognitive decline and depression often occur simultaneously in older adults. These conditions share some common symptoms such as difficulty concentrating, sleep disturbances and changes in appetite.

Previous studies have shown a two-way association between cognitive function and depressive symptoms, but the direction of this connection has been inconsistent. Now a team of British researchers has found depressive symptoms may lead to subsequent cognitive decline and cognitive impairment may result in depression later on.

“Our study shows that the relationship between depression and poor memory cuts both ways, with symptoms of depression preceding memory decline and memory decline linked to subsequent depressive symptoms,” senior author, Dorina Cadar, a senior lecturer in neuroepidemiology and dementia at Brighton and Sussex Medical School, said in a statement.

Frustration and feelings of incompetence often accompany the debilities of aging and are common triggers of depression. Memory impairment can trigger depression, too.

The findings suggest that treating depressive symptoms in older adults may slow cognitive decline. It is also important to monitor memory changes in older adults with depressive symptoms to identify memory loss in the early stages and prevent or slow worsening depression, the study's lead author, Jiamin Lin, a graduate student at the University of Rochester, explained.

The researchers followed almost 8,300 adults enrolled in the English Longitudinal Study on Aging for up to 16 years. The nationally representative study population answered questions every two years. The participants' average age was 64.

Depressive symptoms at the beginning of the study and an increase in symptoms during the study period were associated with a faster rate of cognitive decline over time. An increasing rate of cognitive decline was in turn associated with a rapid increase in depressive symptoms.

Depression-related changes in the brain may affect memory, the researchers said. These changes include a decrease in the neurotransmitters serotonin and dopamine and structural changes in the memory-processing regions of the brain such as the hippocampus. Chronic stress and high levels of the hormone cortisol can damage neurons in these regions.

Memory impairment may also be the result of psychological factors such as rumination or repetitive thinking focusing on negative feelings. People experiencing memory lapses or who have difficulty retaining new information may become frustrated and lose confidence in themselves, said the researchers. Frustration and feelings of incompetence often accompany the debilities of aging and are common triggers of depression. Memory impairment may also disrupt daily functioning and social interactions, which can trigger depression, too.

The researchers noted certain limitations of their study. As an observational study, its findings cannot prove cognitive decline causes depression or vice versa. In addition, participants were not asked about feelings of loneliness, which can affect cognitive functioning and depression. People in the study were also older, more likely to be men, have lower levels of education and poor overall health and cognition at baseline, so the findings may not be applicable to a wider population.

The study is published in JAMA Network Open.