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Objectives: To examine the effect of perceived loneliness on the development of dementia (all-cause), Alzheimer's disease (AD), and vascular dementia (VaD).

Method: The study comprised 1,905 nondemented participants at baseline, drawn from the longitudinal Betula study in Sweden, with a follow-up time of up to 20 years (mean 11.1 years). Loneliness was measured with a single question: "Do you often feel lonely?".

Results: During the follow-up, 428 developed dementia; 221 had AD, 157 had VaD, and 50 had dementia of other subtypes. The entire dementia group is denoted "all-cause dementia". Cox regression models, adjusted for age, gender, and a baseline report of perceived loneliness, showed increased risk of all-cause dementia (hazard ratio [HR] = 1.46, 95% confidence interval [CI] 1.14–1.89), and AD (HR = 1.69, 95% CI 1.20–2.37), but not VaD (HR = 1.34, 95% CI 0.87–2.08). After adjusting for a range of potential confounders, and excluding participants with dementia onset within the first 5 years of baseline (to consider the possibility of reverse causality), the increased risk for the development of all-cause dementia and AD still remained significant (HR = 1.51, 95% CI 1.01–2.25 for all-cause dementia; HR = 2.50, 95% CI 1.44–4.36 for AD).

Discussion: The results suggest that perceived loneliness is an important risk factor for all-cause dementia and especially for AD, but not for VaD. These results underscore the importance of paying attention to subjective reports of loneliness among the elderly adults and identifying potential intervention strategies that can reduce loneliness.