ReviewSocial relationships and risk of dementia: A systematic review and meta-analysis of longitudinal cohort studies
Introduction
Dementia has emerged as major public health issue because of high prevalence rates, high burden to patient, carer and society, and high health care costs (Langa et al., 2001, World Health Organization, 2012). In 2010, more than 35 million people worldwide were affected by dementia and this is expected to increase to 115 million people in 2050 (Ferri et al., 2006, Prince et al., 2013). Increased longevity and the aging of the baby boom generation largely contribute to the increased prevalence of dementia (Middleton and Yaffe, 2009, World Health Organization, 2012). The prevalence of dementia approximately doubles with every 5-year increase in age after the age of 60, from 1% among people aged 60 to 25% among people aged ≥85 years from Western Europe in 2001 (Ferri et al., 2006). An average delay of two years in onset of Alzheimer’s disease (AD) could decrease the worldwide prevalence of AD by 22.8 million cases by the year 2050 which would subsequently lower the number of cases that need high level care and associated costs (Brookmeyer et al., 2007). Currently, there is no effective treatment available to cure dementia. Therefore, identification of modifiable risk factors is of utmost importance in order to delay or prevent the onset of dementia (Middleton and Yaffe, 2009). One potentially important modifiable risk factor for incident dementia is the absence of good social relationships. Good social relationships were found to protect against multiple adverse outcomes, including depression (Santini et al., 2014), coronary heart disease (Hemingway and Marmot, 1999), functional decline (Avlund et al., 2004), and mortality (Holt-Lunstad et al., 2010). The definition and operationalization of social relationships differs across studies. Social relationships can for example contain structural features such as the number of one’s social ties, but also qualitative aspects such as levels of social support (Seeman et al., 2001). It is important to distinguish between these different aspects of social relationships, because they may influence health through different mechanisms (Cohen, 2004). For example, social integration may have a beneficial effect on health through influencing health behaviors. Whereas social support may benefit health through stress reduction, by providing psychological and material resources needed to cope with stress (Cohen, 2004).
Previous reviews have investigated the influence of social relationships (i.e., socially integrated lifestyle, social engagement, social activities) on incident dementia, but conclusions were contradictory (Di Marco et al., 2014, Fratiglioni et al., 2004, Pillai and Verghese, 2009, Wang et al., 2012, Williams et al., 2010). The reviews did not perform a meta-analysis, did not distinguish between various social relationship factors, or did not take into account the methodological quality of the included studies.
Therefore, our aim is to investigate the relation between social relationships and incident dementia in the general population by conducting a systematic review, including a meta-analysis of longitudinal cohort studies, in which we will take into account different aspects of social relationships (e.g., social network size, social participation, loneliness).
Section snippets
Methods
This systematic review was conducted according to the methods of the Cochrane Collaboration (Higgins and Green, 2008) and in addition, we followed PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines (Moher et al., 2009) for the reporting of this systematic review and meta-analysis.
Results
Reviewing 8527 titles and abstracts and 133 full articles, resulted in inclusion of 15 articles (Amieva et al., 2010, Andrew and Rockwood, 2010, Boyle et al., 2010, Chen et al., 2011, Crooks et al., 2008, Fabrigoule et al., 1995, Fratiglioni et al., 2000, Gureje et al., 2011, He et al., 2000, James et al., 2011, Lobo et al., 2008, Saczynski et al., 2006, Valenzuela et al., 2011, Wang et al., 2002, Wilson et al., 2007) (see Fig. 1). Screening the references of all articles selected for full-text
Discussion
This meta-analysis shows that people with less social participation, less frequent social contact and more feelings of loneliness have an increased risk to develop dementia. Low satisfaction with social network seems to be associated with incident dementia, but results were not statistically significant. There was insufficient evidence to draw firm conclusions about the association between small social network and incident dementia due to large heterogeneity between the studies with regard to
Conflict of interest
The authors have no potential conflict of interest to report.
Acknowledgements
This work was supported by the multidisciplinary research programme Healthy Ageing, Population & Society (HAPS). HAPS is supported by the University of Groningen. The funding source had no role in study design, data collection, data analysis, data interpretation, writing of the report, or in the decision to submit the paper for publication. We would like to thank Truus van Ittersum of the Research Institute SHARE of the University of Groningen, University Medical Center Groningen, for her
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