Tau protein in the brain is one of Alzheimer’s biomarkers, the team from the University of Galway said.
It does not prove that vitamin D levels lower the level of tau in the brain or the risk of dementia – but indicates an association.
The study was led by Professor Emer McGrath and Dr Martin Mulligan, from the College of Medicine, Nursing and Health Sciences at University of Galway and Galway University Hospital.
It was carried out in collaboration with Boston University, the Framingham Heart Study and University of Texas San Antonia.
Prof McGrath, Associate Professor in Medicine, University of Galway and Consultant Neurologist, Galway University Hospital, said: “This study demonstrates an association between higher vitamin D in midlife and a lower burden of abnormal tau protein in the brain, a marker of Alzheimer’s disease.
“Vitamin D in midlife could be an important factor to protect future brain health. While previous research has linked low vitamin D in adults over 70 with an increased risk of dementia, this study is among the first to look at younger adults at midlife, around the average age of 39.
“Low vitamin D in midlife may be an important target to reduce the risk of early signs of preclinical dementia in the brain.
“However, while these findings are very interesting, they only demonstrate an association between vitamin D and early signs of dementia in the brain.
“Further studies, for example a clinical trial, will be required to determine if vitamin D supplements could prevent dementia.”
Dr Mulligan said there are many theories as to how vitamin D influences brain health.
“Our research did not look at this specific question, however, previous research has suggested that vitamin D may work by reducing inflammation, or by improving antioxidant defences and cell signalling, which may keep the tau protein from accumulating,” he said.
“Addressing vitamin D deficiencies in midlife is particularly important because it targets the disease at a much earlier stage, providing a greater window of opportunity to intervene and potentially modify the disease before any noticeable memory loss or dementia symptoms begin.”
The study involved nearly 800 people with an average age of 39 who did not have dementia.
All participants had the level of vitamin D in their blood measured at the start of the study.
They had brain scans an average of 16 years later that measured levels of tau and amyloid beta proteins in the brain, which are both biomarkers for Alzheimer’s disease.
A high level of vitamin D was defined as greater than 30 nanograms per millilitre (ng/mL) and a low level was less than that number.
A total of 34pc of participants had low levels of vitamin D and 5pc were taking vitamin D supplements. The results included factors that could affect tau levels, such as age, sex and symptoms of depression.
A limitation of the study is that the blood level of vitamin D was measured only once.
The findings have been published today in Neurology Open Access, an official journal of the American Academy of Neurology.
The Framingham Heart Study is the longest running longitudinal cohort study worldwide. It started in 1948 with the recruitment of 5,200 adults from the town of Framingham in Massachusetts, with around 15pc of the participants of Irish ancestry.
