Although vitamin D is found in foods like oily fish, most people cannot get enough of the vitamin through diet alone, and those living in higher latitudes are at greater risk.
"Some people may have had melanoma or fear getting it," added Prof Miller. "Or, they may live in climates where the sun isn't powerful enough, or do work that keeps them out of the sun. That's where supplements come in."
The study looked at 400 people with a mean age of 76, who were either cognitively normal, had mild cognitive impairment or were suffering from dementia.
Over five years of follow-up, vitamin D deficient individuals experienced cognitive declines that were two-to-three times faster than those with adequate serum vitamin D levels. In other words it took only two years for the deficient individuals to decline as much as their counterparts with adequate Vitamin D declined during five years.
"We expected to see declines in individuals with low vitamin D status," said Professor Charles DeCarli, director of the Alzheimer's Disease Center at the University of California.
"What was unexpected was how profoundly and rapidly low vitamin D impacts cognition.
"I don't know if replacement therapy would affect these cognitive trajectories. That needs to be researched and we are planning on doing that.