While other studies have shown people with more extensive social networks were at reduced risk of cognitive impairment, the study by Dr. David A. Bennett, and his colleagues from the Rush Alzheimer's Disease Center, is the first to examine the relations between social networks and Alzheimer's disease pathology. Researchers studied elderly people without known dementia who are participating in the Rush Memory and Aging Project, an epidemiological and clinicopathological study of aging and Alzheimer's disease that involves over 1,100 volunteers across northeastern Illinois. Brain autopsy was done at the time of death and post mortem data was available for analysis from the first 89 people.
"Many elderly people who have the tangles and plaques associated with Alzheimer's disease don't clinically experience cognitive impairment or dementia," said Bennett. "Our findings suggest that social networks are related to something that offers a 'protective reserve' capacity that spares them the clinical manifestations of Alzheimer's disease."
Participants in the study underwent clinical evaluations and 21 cognitive performance tests each year. To determine social network, participants were asked about the number of children they have and see monthly. They were asked about the number of relatives, excluding spouse and children, and friends to whom they feel close and with whom they felt at ease and could talk to about private matters and could call upon for help. They were asked to specify how many of these people they see monthly. Their social network was the number of these individuals seen at least once per month.
The relationship between the amount of Alzheimer's disease pathology and cognitive performance changed with the size of the social network. As the size of the social network increased, the same amount of pathology had less effect on cognitive test scores. In other words, for persons without much pathology, social network size had little effect on cognition. However, as the amount of pathology increased, the apparent protective effect on cognition also increased. Thus, social network size appears to have offered a protective reserve capacity despite the fact that their brains had the tangles and plaques indicative of Alzheimer's disease.
The effect was evident across different kinds of cognitive abilities, but was most evident for semantic memory, which is the repository of knowledge about the world and is fundamentally involved in unique human cognitive processes such as language. The results were unchanged after controlling for cognitive, physical, and social activities, depressive symptoms, or number of chronic diseases.
"Identifying factors associated with the ability to tolerate the pathology of Alzheimer's disease has important implications for disease prevention," said Bennett. "Previous studies suggest one factor is education. Now we know that healthy and frequent interactions with friends and family have a positive impact as well."
The researchers are extremely grateful for the remarkable dedication and altruism of the volunteers participating in the Rush Memory and Aging Project. The research was supported by grants from the National Institutes on Aging, which leads the Federal effort to support and conduct basic, clinical, and social and behavioral studies on aging and on Alzheimer's disease.
The Lancet Neurology