Vitamin D is important for the body's immune function, growth and repair of bones, and normal calcium and phosphorus absorption. It can be obtained from fish, milk, eggs and cheese.
Dementia is a collective term used to describe the problems that people with various underlying brain disorders can have with their memory, language and thinking. Alzheimer's disease is the best known and most common disorder under the umbrella of dementia.
Alzheimer's disease is the sixth leading cause of death in the US and is believed to currently affect 5.3 million Americans, according to the Centers for Disease Control and Prevention (CDC). It is most common in people aged over 65, in which a tenth of the population has the condition.
The authors of the study, published in Neurology, state that low concentrations of vitamin D are associated with the development of Alzheimer's disease. Worryingly, there are high rates of vitamin D deficiency in older adults - the group most at risk from developing dementia.
The CDC report that one third of the US population do not get sufficient amounts of vitamin D, with 8% of the population at risk of vitamin D deficiency. Vitamin D is obtained from sun exposure and foods such as milk, eggs, cheese and fatty fish.
Vitamin D and dementia: a strong association
For the study, the researchers tested 1,658 dementia-free people aged over 65 who had participated in the US population-based Cardiovascular Health Study. The vitamin D levels in their blood were tested, and they were followed up for an average of 5.6 years.
During this follow-up period, 171 of the participants developed dementia and 102 participants developed Alzheimer's disease. The researchers found the participants with low levels of vitamin D were 53% more likely to develop dementia, and those who were severely deficient were 125% more likely, when compared with participants with regular levels of vitamin D.
Similarly, participants with low levels of vitamin D saw a 70% increased risk of developing Alzheimer's disease, and those with severe deficiency had an increased risk of 120%, again when compared with participants with normal levels of the vitamin.
Study author David J. Llewellyn, of the University of Exeter Medical School in the UK, was surprised by the extent of their results, saying, "we actually found that the association was twice as strong as we anticipated."
The results of the study remained the same even after adjusting for other variables - such as alcohol consumption, smoking and education - that could affect the risk of developing dementia.
Llewellyn urges caution following the findings of the study, stating that the results do not demonstrate that low vitamin D levels cause dementia. He suggests the direction that future research needs to take:
"Clinical trials are now needed to establish whether eating foods such as oily fish or taking vitamin D supplements can delay or even prevent the onset of Alzheimer's disease and dementia."
The study was unable to account for all forms of dementia, as by excluding participants with cardiovascular disease and stroke at the beginning of the study, the researchers encountered few cases of vascular dementia. The authors acknowledge that further research will be required to incorporate this area of the population.
Despite this, the study could provide a good starting point for this area of research. "Our findings are very encouraging," says Llewellyn, "and even if a small number of people could benefit, this would have enormous public health implications given the devastating and costly nature of dementia."
Medical News Today also reported on the benefits of oily fish earlier in the week, with a study suggesting that eating baked or broiled fish every week is good for the brain.