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Frequency of social contact was tied to brain volume in cognitively normal older adults, a cross-sectional study in Japan showed.

Total brain volume was smaller in people who had the lowest frequency of social contact versus those with the highest frequency (67.3% vs 67.8%), reported Toshiharu Ninomiya, MD, of Kyushu University in Fukuoka, and co-authors in .

Less social contact was linked with 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.

Relationships between social contact and brain volumes appeared to be partly mediated by depressive symptoms, "which could be a potential mechanism underlying the link between social isolation and cognitive function or dementia," the researchers said.

The findings are in line with recent data from the Framingham Study, which found that dementia incidence tripled in lonely older adults who otherwise were expected to have relatively low risk. A longitudinal analysis of U.K. Biobank participants also linked social isolation with lower brain volume and a higher risk of subsequent dementia.

Loneliness and social isolation are serious public health risks, especially in older adults, noted Alexa Walter, PhD, and Danielle Sandsmark, MD, PhD, both of the University of Pennsylvania in Philadelphia, in an .

"A National Academies of Sciences, Engineering, and Medicine found that more than one-fourth of adults [over] 65 years are socially isolated," Walter and Sandsmark wrote. "Social isolation has been associated with a variety of negative health outcomes including premature mortality, increased risk of coronary heart disease and stroke, increased reporting of depressive symptoms, as well as increased dementia risk and cognitive decline."

Ninomiya and co-authors evaluated MRI and survey data from 8,896 dementia-free participants in the ongoing Japan Prospective Studies Collaboration for Aging and Dementia (). Women comprised 57% of the study group, which had a mean age of 73.

Participants rated their frequency of social contact as every day, several times a week, several times a month, or seldom. They answered questions about depression, educational level, marital status, living situation, and occupation. The survey also asked about regular medications including antihypertensive or antidiabetic drugs, medical history, smoking, and exercise.

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; scores between 25 and 30 are considered normal). Depressive symptoms were lower in the daily-contact group compared with the seldom-contact group (P<0.001).

Less social contact also was associated with lower education, unemployment, hypertension, smoking, alcohol use, and a history of cardiovascular disease.

Mediation analysis suggested that 15% to 29% of the associations between frequency of social contact and each brain volume were mediated by depressive symptoms. The direct effects of social contact frequency on brain volumes ranged from 71% to 85%, the model hypothesized.

Several mechanisms may explain the link between low social contact and brain atrophy, Ninomiya and colleagues noted. "[S]ocially isolated individuals were more prone to have diabetes mellitus, hypertension, and unhealthy lifestyles, including smoking and physical inactivity," they observed.

Decreased cognitive stimulation due to less social contact may cause brain atrophy, the researchers suggested. Fewer social interactions also could be an early symptom of structural brain changes like atrophy or white matter lesions, they pointed out.

"Since the present analysis had a cross-sectional design, causal interpretations of the association between low frequency of social contact and brain atrophy should be avoided," Ninomiya and co-authors cautioned.

"Rather, it is expected that the relationship between low frequency of social contact and brain atrophy may be bidirectional, with a vicious cycle existing between social isolation, brain atrophy, and cognitive function decline," they noted.

The analysis had other limitations, the researchers acknowledged. The survey did not collect information about loneliness, and findings may not apply to younger populations.

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