People living in the north have a greater risk of developing dementia than their southern counterparts, according to recent research.
Scientists at the University of Edinburgh say this could be due to northerners' lower exposure to sunlight, resulting in decreased amounts of vitamin D.
The researchers conducted a study on twins in Sweden and discovered that those living in the north were two to three times more likely to develop dementia compared with those in the south, when factors such as age, gender and genes were taken into account.
Data from another study of children born in 1921 in Scotland corroborated these findings. Although there was no link to where people lived as children, by the time men and women reached middle-age there was once again a higher risk for those who lived in areas further north.
Tom Russ, clinical lecturer in old age psychiatry at Edinburgh University's Alzheimer Scotland Dementia Research Centre, said: "The north-south divide does make you think about latitude and it may be it is something to do with sunlight exposure and vitamin D - that is a possibility and it has certainly been linked to healthy brain function and dementia."
He added that the Swedish study enabled the researchers to eliminate genetic factors that could explain the north-south divide.
Research carried out last year revealed evidence of a link between levels of vitamin D and Alzheimer's disease.
Those lacking the vitamin were twice as likely to develop dementia and Alzheimer's disease compared with people with healthy levels.
Although the study demonstrated a link between low levels of vitamin D and an increased risk of developing the condition, it did not show that the former causes the latter.
Most of the body's vitamin D is made in the body using sunlight, although small amounts can be derived from foods such as oily fish.
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