Vitamin D deficiency is associated with a substantially increased risk of dementia and Alzheimer’s disease in older people, according to a study published in Neurology.

An international team led by Dr. David Llewellyn at the University of Exeter Medical School found that study participants who were severely vitamin D deficient were more than twice as likely to develop dementia and Alzheimer’s disease.

The team studied elderly Americans who took part in the Cardiovascular Health Study. They discovered that adults in the study who were moderately deficient in vitamin D had a 53% increased risk of developing dementia of any kind, and the risk increased to 125% in those who were severely deficient.

Similar results were recorded for Alzheimer’s disease, with the moderately deficient group 69% more likely to develop this type of dementia, jumping to a 122% increased risk for those severely deficient.

The study was funded in part by the Alzheimer’s Association, and was published August 6 online in Neurology, the medical journal of the American Academy of Neurology. It looked at 1,658 adults aged 65 and over who were able to walk unaided and were free from dementia, cardiovascular disease and stroke at the start of the study. The participants were then followed for six years to investigate who went on to develop Alzheimer’s disease and other forms of dementia.

“We expected to find an association between low vitamin D levels and the risk of dementia and Alzheimer’s disease, but the results were surprising—we actually found that the association was twice as strong as we anticipated,” Dr. Llewellyn said. “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. We need to be cautious at this early stage and our latest results do not demonstrate that low vitamin D levels cause dementia. That said, our findings are very encouraging, 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.”

Research collaborators included experts from Angers University Hospital, Florida International University, Columbia University, the University of Washington, the University of Pittsburgh and the University of Michigan. The study was supported by the Alzheimer’s Association, the Mary Kinross Charitable Trust, the James Tudor Foundation, the Halpin Trust, the Age Related Diseases and Health Trust, the Norman Family Charitable Trust, and the National Institute for Health Research Collaboration for Leadership in Applied Research and Care South West Peninsula (NIHR PenCLAHRC).

Dementia is one of the greatest challenges of our time, with 44 million cases worldwide—a number expected to triple by 2050 as a result of rapid population aging. A billion people worldwide are thought to have low vitamin D levels and many older adults may experience poorer health as a result.

The study also found evidence that there is a threshold level of vitamin D circulating in the bloodstream below which the risk of developing dementia and Alzheimer’s disease increases. The team had previously hypothesized this might lie in the region of 25-50 nmol/L, and their new findings confirmed that vitamin D levels above 50 nmol/L are most strongly associated with good brain health.

Dr. Doug Brown, director of research and development at Alzheimer’s Society, commented on the study: “Shedding light on risk factors for dementia is one of the most important tasks facing today’s health researchers. While earlier studies have suggested that a lack of the sunshine vitamin is linked to an increased risk of Alzheimer’s disease, this study found that people with very low vitamin D levels were more than twice as likely to develop any kind of dementia. Large scale clinical trials are needed to determine whether increasing vitamin D levels in those with deficiencies can help prevent the dementia from developing.”