The researchers suggest that low vitamin D levels should now be considered a risk factor for dementia, particularly among people with darker skin tones.
“On average, people with low vitamin D declined two to three times as fast as those with adequate vitamin D,” said Professor Joshua Miller, of the Department of Nutritional Sciences at Rutgers University.
"This work, and that of others, suggests that there is enough evidence to recommend that people in their 60s and older discuss taking a daily vitamin D supplement with their physicians.
"Even if doing so proves to not be effective, there's still very low health risk to doing it," he said.
A recent survey suggested that around 50 per cent of all adults have some degree of vitamin d deficiency, which has been blamed on too much time spent indoors and increased use of sunblock.