Severe vitamin D deficiency, and the associated increased risk for dementia, was rare in the study, however. Only four percent of the older people included were “severely deficient” based on their blood samples.
“It is too early to tell whether improving vitamin D levels helps to delay or prevent dementia - clinical trials are now urgently needed,” said senior author David J. Llewellyn of the University of Exeter Medical School in the U.K.
Researchers used data from an existing study of heart disease risk among 1,658 elderly adults. When the study began in 1993, none of the participants had dementia, heart disease or stroke, and all gave blood samples for analysis, which were then stored in a lab at the University of Vermont.
In 2008, researchers at the University of Washington retested the samples for circulating vitamin D levels.
Most people in the study did have sufficient vitamin D levels in their blood samples, defined as at least 50 nanomoles of the vitamin per liter of blood (nmol/L).
But about 30 percent of people had less than that: 419 people were deficient, with more than 25 nmol/L but less than 50, and 70 people were ‘severely deficient,’ with less than 25 nmol/L.
By 1999, 171 people in the study did develop dementia, including 102 cases of Alzheimer’s disease.
People who had been severely deficient in vitamin D at the start of the study were more than twice as likely to develop dementia in the coming years than people with sufficient levels, according to the results published in Neurology.
The researchers did not test whether taking vitamin D supplements or changing diet plans would have affected dementia risk.
“The study does not provide any specific advice for older people in relation to vitamin D status,” said Cynthia Balion of the Canadian Longitudinal Study on Aging and a clinical biochemist at Hamilton Regional Laboratory Medicine Program in Ontario.
“It only shows there is data to support its importance in cognitive function, but therapy trials are needed to determine its clinical significance,” said Balion, who was not part of the new study.
Llewellyn agreed. Previous studies have suggested that vitamin D supplements may help protect some older adults from fractures or falls, he told Reuters Health by email.
Aside from supplements, eating a balanced diet including oily fish and regularly venturing outdoors as part of an active lifestyle can help boost vitamin D levels, he said.
The absolute change in risk in this study is somewhere between five and 12 percent, Balion said by email. Only 10 percent of the group as whole developed dementia, which increased to 15 and 22 percent of vitamin D deficient and severely deficient people, respectively.
“We do not yet know with certainty how to reduce the risk of dementia,” Llewellyn said. “However, our current knowledge suggests that eating a balanced Mediterranean style diet rich in oily fish, moderate intensity exercise and the careful management of diabetes, high blood pressure and depression may help.”
Many other observational studies have tied low vitamin D levels to increased risk of any type of disease, said Dr. Philippe Autier, but that by no means implies that low vitamin D causes those diseases.
Autier coauthored a systemic review of vitamin D and ill health published in The Lancet in 2013 and studies the question at the International Prevention Research Institute in Lyon, France.
“Our paper in the Lancet showed that low vitamin D status would rather be the consequence of systemic inflammatory processes that are common in many chronic conditions, infectious diseases, ageing, dementia and even depression,” Autier said. “Inflammatory processes are strongly involved in Alzheimer’s disease, and these processes are likely to be present well before Alzheimer’s disease is clinically detectable.”
The one randomized trial that has investigated vitamin D levels and dementia did not find a causal connection, he noted.