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Introduction: Observational studies have reported a higher risk of dementia in individuals with type 2 diabetes and high glucose concentrations have been associated with dementia independent of type 2 diabetes. However, is not known whether glucose per se has a causal effect on risk of dementia. Hypothesis: High plasma glucose due to genetic variation is causally associated with risk of Alzheimer's dementia, vascular dementia, and unspecified dementia in the general population. Methods: We examined observational associations of plasma glucose concentration with risk of disease; genetic variants with plasma glucose concentrations; and genetic variants with risk of disease as an indication of a causal effect of high plasma glucose on disease, using a Mendelian randomization design largely free of confounding and reverse causation in 125,875 individuals from the Copenhagen General Population Study and the Copenhagen City Heart Study. Results: In observational multifactorially adjusted analyses hazard ratio for Alzheimer's dementia was 1.18 (95% confidence interval: 0.89-1.56; p=0.25), for vascular dementia 1.74 (1.02-2.96; p=0.043), and 1.50 (1.15-1.95; p=0.003) for unspecified dementia in individuals with a glucose above 9 mmol/L versus between 5-6 mmol/L. Corresponding hazard ratios were 1.13 (0.96-1.32; p=0.14), 1.66 (1.25-2.21; p<0.001) and 1.39 (1.21-1.60; p<0.001) in individuals with type 2 diabetes versus without. A weighted allele score of glucose increasing alleles was associated with a 4.1% higher plasma glucose. In genetic, causal analyses a 1 mmol/L higher plasma glucose had risk ratios of 0.85 (0.70-1.04; p=0.11) for Alzheimer's dementia, of 1.03 (0.74-1.89; p=0.87) for vascular dementia, and of 1.17 (0.64-2.13; p=0.61) for unspecified dementia. Summary level data from the International Genomics of Alzheimer's Project using Egger Mendelian randomisation analysis gave a risk ratio for Alzheimer's dementia of 1.09 (0.60-1.96; p=0.79). Conclusions: Genetically high plasma glucose had no causal effect on risk of Alzheimer's dementia, vascular dementia, or unspecified dementia, suggesting that the increased observational risk of vascular and unspecified dementia is likely due to confounding and/or reverse causation. [ABSTRACT FROM AUTHOR]