Sunshine could ward off dementia and strokes after scientists have shown a direct link between vitamin D and the conditions in a world-first study.

A new study based on British people said that cases of dementia could drop by nearly a fifth if people who were deficient in the vitamin took supplements to bring them up to healthy levels,

It is known as the sunshine vitamin because the skin makes it when exposed to light.

The team from the University of South Australia looked at nearly 300,000 people from the UK Biobank examining the impact of low levels of vitamin D and the risk of dementia and stroke.

They found that low levels of vitamin D were associated with lower brain volumes and an increased risk of dementia and stroke.

Further genetic analyses supported a causal effect of vitamin D deficiency and dementia.

They said that in some populations as much as 17 percent of dementia cases might be prevented by increasing everyone to normal levels of vitamin D.

Dementia is one of the major causes of disability and dependency among older people worldwide, affecting thinking and behaviours as you age.

Globally, more than 55 million people have dementia with 10 million new cases diagnosed every year. With no cure in sight, there is an increasing focus on preventative behaviors.

Alzheimer's and other forms of dementia affect over 920,000 people in the UK—a figure that will rise to two million in the next three decades.

Study author Professor Elina Hyppönen, Senior investigator and Director of UniSA's Australian Centre for Precision Health, said the findings are important for the prevention of dementia and appreciating the need to abolish vitamin D deficiency.

"In this UK population we observed that up to 17 per cent of dementia cases might have been avoided by boosting vitamin D levels to be within a normal range," she said.

"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.

"Vitamin D is a hormone precursor that is increasingly recognized 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.

"In some contexts, where vitamin D deficiency is relatively common, our findings have important implications for dementia risks.

"Dementia is a progressive and debilitating disease that can devastate individuals and families alike.

"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.

"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."

The genetic study analyzed data from 294,514 participants from the UK Biobank, examining the impact of low levels of vitamin D (25 nmol/L) and the risk of dementia and stroke.

Nonlinear Mendelian randomization (MR)—a method of using measured variation in genes to examine the causal effect of a modifiable exposure on disease – were used to test for underlying causality for neuroimaging outcomes, dementia, and stroke.

The study was published in the American Journal of Clinical Nutrition.

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