Further, the association between social isolation and reduced brain volume appears to be at least partly mediated by depressive symptoms.
“We believe that efforts should be made to reduce social isolation among the elderly as much as possible,” investigator Toshiharu Ninomiya, MD, PhD, professor of epidemiology and public health at Kyushu University in Fukuoka, Japan, said in an interview.
The study was published online in Neurology.
A dementia prevention strategy
Dr. Ninomiya noted there have been several studies suggesting that social interaction is beneficial in preventing cognitive decline and the onset of dementia.
In addition, recent epidemiological studies have shown social isolation is associated with a risk for cognitive decline and dementia.
Although the investigators note that very little is known about the link between the two, some studies have shown that social isolation is linked with depressive symptoms in older adults, and late-life depression has been associated with brain atrophy.
To explore the potential link between social isolation and brain atrophy, as well as the role of depression as a potential mediator, the investigators studied nearly 9,000 citizens aged 65 and older as part of the Japan Prospective Studies Collaboration for Aging and Dementia (JPSC-AD), an ongoing, community-based nationwide cohort study of dementia in Japan.
Participants were recruited from eight research sites across Japan, and each had a baseline MRI scan between 2016 and 2018. The investigators excluded those with a dementia diagnosis at baseline. Self-reported frequency of social contact was categorized as every day, several times a week, several times a month, or seldom.
Participants also answered questions about medical history and treatment, antihypertensive or antidiabetic medications, exercise, current alcohol intake, and smoking habits. Depressive symptoms were assessed with the Geriatric Depression Scale. Of the participants, 57% were women, and the mean age was 73 years.
Lower brain volume
Total brain volume was lower in those with the lowest frequency of social contact vs. those with the highest frequency (67.3% vs. 67.8%). Less social contact was also linked to smaller temporal lobe, occipital lobe, cingulum, hippocampus, and amygdala volumes.
White matter lesion volume increased with fewer social interactions, from 0.26% in the most social group to 0.30% in the least.
Cognitive function was higher in participants who had daily social contact, compared with those who had the least contact (28 vs. 27 on the Mini-Mental State Examination; P < .001). Scores between 25 and 30 are considered normal.
Depressive symptoms were lower in the daily contact group, compared with the seldom-contact group (P < .001).
The team also found that lower frequency of social contact was significantly associated with the smaller superior, middle, or inferior temporal gyrus; and a smaller fusiform gyrus, transverse temporal gyrus, temporal pole, and entorhinal cortex, among other subregions.
Mediation analyses indicated that depressive symptoms accounted for only 15%-29% of the associations of lower frequency of social contact with each regional volume.
Worse physical health
The results also showed that socially isolated participants were more likely to have diabetes, to have hypertension, to smoke, and to be physically inactive.
“Cardiovascular risk factors have been reported to cause endothelial dysfunction in the brain, which could in turn lead to problems in maintaining microcirculation and blood-brain barrier function,” the investigators write.
Some epidemiological studies have associated cardiovascular risk factors with brain atrophy, they noted, which could have been one of the underlying mechanisms.
Another possibility is that reduced cognitive stimulation due to social isolation may cause brain atrophy, they add.
“Ultimately,” Dr. Ninomiya said, “the detailed mechanism of the relationship between social isolation and brain volume is not yet clear.”
He also said more research is needed to know whether the findings would apply to people in other countries.
In an accompanying editorial, Alexa Walter, PhD, and Danielle Sandsmark, MD, PhD, from the University of Pennsylvania, Philadelphia, note that isolation has been associated with many adverse health outcomes, including increased risk of heart disease, stroke, and premature death.
“Given these findings, future work considering social health factors in the context of neurological disease is an important area of research to consider. Additionally, leveraging other existing longitudinal studies could provide us with an opportunity to better understand these relationships within populations and inform public policy to address these issues,” Dr. Walter and Dr. Sandsmark write.
The study was funded by the Japan Agency for Medical Research and Development and Suntory Holdings Limited. Dr. Ninomiya reports receiving grants from Suntory Holdings Limited.
A version of this article first appeared on Medscape.com.