Background
Serum lipid traits are associated with cardiovascular disease, but uncertainty remains regarding their associations with dementia.

Methods
From 2006 to 2010, 254,575 women and 214,891 men were included from the UK Biobank. Cox regression estimated overall and sex-specific hazard ratios (HRs) for apolipoprotein A (ApoA), apolipoprotein B (ApoB), HDL, LDL, total cholesterol, triglycerides, lipoprotein A, and various lipid ratios, by quarters and standard deviation (SD) higher, associated with all-cause dementia, Alzheimer's disease (AD) and vascular dementia (VaD). Subgroup analyses by age and social deprivation were conducted.

Findings
Over 11·8 years (median), 3734 all-cause dementia (1,716 women), 1231 AD and 929 VaD were recorded. Compared to respective lowest quarters, highest quarter of ApoA was associated with lower dementia risk (HR, [95% confidence interval (95% CI)]: 0·77 [0·69, 0·86]) while the highest quarter of ApoB was associated with greater risk (HR, 1·12 [1·01, 1·24]). Higher HDL/ApoA and ApoB/ApoA, were associated with greater risk of dementia (HR, 1·12 [1·00, 1·25], per standard deviation (SD), 1.23 [1·11, 1·37], per SD, respectively), LDL/ApoB was inversely associated (HR, 0·85 [0·76, 0·94], per SD. Higher triglycerides was associated with higher dementia risk in <60 years, but the inverse was observed for ≥60 years. Similar associations were observed for VaD and AD.

Interpretation
Apolipoproteins, and their ratios, were associated with the risk of dementia. It may be prudent to consider apolipoproteins, along with circulating cholesterol, when assessing dementia risk.

Funding
University of New South Wales, UK Medical Research Council, and the Australian National Health and Medical Research Council.