There is a direct link between vitamin D deficiency and the risk of dementia, according to a new study.
Alzheimer's disease is a form of dementia, which refers to a group of symptoms affecting memory, thinking and language for which there is no cure.
Researchers at the University of South Australia have found that vitamin D deficiency can lead to dementia.
The study found that low levels of the vitamin was associated with increased risks of dementia and stroke risk and lower brain volumes.
The team also claims that almost a fifth of dementia cases in the UK could have perhaps been prevented.
"In the UK population we observed that up to 17 percent of dementia cases might have been avoided by boosting vitamin D levels to be within a normal range," said senior investigator Professor Elina Hyppönen.
Sunlight and a small number of foods are sources of vitamin D, such as oily fish like salmon and sardines.
Researchers analysed information about 294,514 participants from the UK Biobank, a large database and research resource with in-depth genetic and health information.
The study, which was published in The American Journal of Clinical Nutrition, could be significant given the high prevalence of dementia around the world.
The NHS says there are more than 850,000 people in the UK who have dementia.
Prof Hyppönen says the findings are important for the prevention of dementia and increasing people's levels of the essential nutrient.
She said: "Vitamin D is a hormone precursor that is increasingly recognised for widespread effects, including on brain health, but until now it has been very difficult to examine what would happen if we were able to prevent vitamin D deficiency."
"Our study is the first to examine the effect of very low levels of vitamin D on the risks of dementia and stroke, using robust genetic analyses among a large population.
"In some contexts, where vitamin D deficiency is relatively common, our findings have important implications for dementia risks. Indeed, in this UK population we observed that up to 17 percent of dementia cases might have been avoided by boosting vitamin D levels to be within a normal range."
Early diagnosis means the condition can be slowed in some cases, but there is no cure for dementia.
"Dementia is a progressive and debilitating disease that can devastate individuals and families alike," continued Prof Hyppönen.
"If we're able to change this reality through ensuring that none of us is severely vitamin D deficient, it would also have further benefits and we could change the health and wellbeing for thousands."
The researcher said some people could need vitamin supplements.
She added: "Most of us are likely to be ok, but for anyone who for whatever reason may not receive enough vitamin D from the sun, modifications to diet may not be enough, and supplementation may well be needed."
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