Geographical variation in dementia mortality in Italy, New Zealand, and Chile: the impact of latitude, vitamin D, and air pollution

Background: Dementia risk is reported as being higher in the north compared to the south, which may be related to vitamin D deficiency. If this were the case, an opposite gradient of risk would be observed in the southern hemisphere, but this has not been investigated previously. Methods: We calculated standardised mortality ratios (SMRs) for deaths in 2012 where dementia (Alzheimer's disease, vascular or unspecified dementia) was recorded as the underlying cause for 20 regions in Italy, 20 District Health Board areas in New Zealand and 29 Health Service areas in Chile. Results: Dementia SMRs were higher in northern than central or southern Italy. The inverse pattern was seen in women in New Zealand, with rates higher on South Island than North Island. However, dementia risk was raised in eight regions in the north and centre of Chile in both men and women. Conclusions: Geographical variation plays a key role in dementia risk, but patterns vary in men and women. In the northern hemisphere, dementia mortality is higher in the north, but the pattern in the southern hemisphere is more complex.