To ward off cognitive decline and impaired memory as we age, it's important to get a sufficient dose of vitamin D everyday, according to a new study from the University of California at Davis and Rutgers University in the US.

"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," says Joshua Miller of Rutgers. "Even if doing so proves to not be effective, there's still very low health risk to doing it."

Individuals who don't get enough experience cognitive decline three times faster than those who do, according to the study, which suggests that vitamin D has a substantial effect on the aging brain.

Miller says that vitamin D insufficiency was associated with significantly speedier declines in both episodic memory (one's ability to remember moments) and executive function (the brain's ability to organize and make decisions) regardless of individuals' cognitive abilities from the get-go.

Working with just under 400 men and women, who were at a mean age of 76 and who were either in good cognitive health, had mild cognitive impairment or dementia, the research team began by measuring their serum vitamin D status as a first step.

Both deficiency and insufficiency -- the milder of the two -- were rampant among participants, with 26 per cent demonstrating deficiency and 35 per cent classified as insufficient.

Over the course of a five-year follow-up, participants who were vitamin D deficient saw cognitive declines that were two to three times more rapid than those whose serum levels measured adequate at baseline, according to the study.

"We expected to see declines in individuals with low vitamin D status," says Charles DeCarli, director of the Alzheimer's Disease Center at UC Davis. "What was unexpected was how profoundly and rapidly [low vitamin D] impacts cognition."

Sunlight is an important source of vitamin D and those with darker skin don't absorb it as quickly due to the higher concentration of melanin that gives them their pigment.

Vitamin D can also be obtained from dietary sources, particularly dairy products.

"This is a vitamin deficiency that could easily be treated and that has other health consequences," says DeCarli. "We need to start talking about it."

The research is published online in JAMA Neurology, a JAMA Network journal.