The study, published in the Journal of Neurology, Neurosurgery and Psychiatry, included more than 3,100 men 40 and older in eight European cities: Florence, Italy; Leuven, Belgium, Lodz, Poland; Malmo, Sweden; Manchester, England.; Santiago de Compostela, Spain; Szeged, Hungary, and Tartu, Estonia.
The men, who were about 60 years old, on average, provided blood samples so the researchers could check their vitamin D levels. Their average vitamin D level was in the adequate range.
The men also took three tests of their visual memory, visual scanning, and speed at processing visual information.
Men in their 60s and 70s with low levels of vitamin D were the most likely participants to have low scores on the visual scanning and processing test.
Those findings held when the researchers considered various factors, including the men's years of education, smoking, alcohol consumption, physical activity, BMI (body mass index), depression symptoms, the season of the year, and the location where the men were tested.
Those last two factors -- the season and the location -- were considered because the body makes vitamin D best when exposed to bright sunlight. For instance, during winter in a northern city, people may not be able to make vitamin D as well as they could during summer in a southern city.
The researchers, who included David Lee, PhD, MPH, of England's University of Manchester, don't rule out the possibility that other factors, besides vitamin D, may have affected the results.
The findings were a snapshot in time; Lee's team didn't follow the men over time to see if their vitamin D levels or test scores changed.
In the journal, Lee and colleagues call for further studies to test whether vitamin D supplements might help minimize age-related declines in specific mental skills.