Seasonal vitamin D and incident dementia
What is it about?
There are significant seasonal variations in vitamin D levels. However, little is known regarding the association of incident dementia with vitamin D insufficiency (25-49.9 nmol/L) and deficiency (≥50 nmol/L) across seasons. We assessed the association between vitamin D insufficiency and deficiency using both non-seasonal (traditional method) and seasonal (penalized spline smoothing method) specific cut-offs.
Why is it important?
There was significant variation across seasons, with mean vitamin D levels for spring, summer, autumn, and winter at 45.4 nmol/L, 58.6 nmol/L, 54.9 nmol/L, and 43.5 nmol/L, respectively. Compared with the non-seasonal specific cut-off for vitamin D deficiency of <25 nmol/L, we defined cut-off points for vitamin D deficiency as <17.8 nmol/L for spring, <28.0 nmol/L for summer, <24.0 nmol/L for autumn, and <16.9 nmol/L for winter, all of which were significantly associated with an increased risk of dementia. Seasonal-specific vitamin D deficiency was associated with a 1.51-fold, 2.16-fold, 1.99-fold, and 1.70-fold increased risk of dementia in spring, summer, autumn, and winter, respectively.