Vitamin D is made by skin cells in response to sunlight. Just 10-15 minutes of daily exposure to the sun is sufficient to make enough vitamin D for maintaining good health. It is also found naturally in fatty fish, fish liver oils, egg yolks as well as in fortified grains and dairy products.

Along with helping to build strong bones by maintaining proper calcium and phosphorus levels in the body, emerging research shows that vitamin D may help to protect against many other health problems. For instance, having adequate levels of 25-hydoxyvitamin D (the biologically active form of vitamin D) can lower the risk of a first heart attack and peripheral vascular disease, along with reducing risk for prostate, colon and breast cancer. Observational data further suggests that low levels of 25-hydoxyvitamin D are associated with increased likelihood of type 2 diabetes, hypertension and cognitive impairment. Increasing vitamin D intake to about 800 international units (IU) per day has been reported to reduce the risk of developing type 2 diabetes by as much as a third.

A recent study investigated a possible link between vitamin D and cognition. It used 15-year residential history along with satellite and ground monitor data to determine solar radiation and air temperature exposure for over 19,000 cognitively normal participants aged 45 years and older. Solar radiation was seen to interact with temperature, age, and gender - while lower levels of solar radiation were clearly associated with increased odds of cognitive impairment.

Associations have previously also been noted between low levels of 25-hydroxyvitamin D and Alzheimer's disease and dementia in both Europe and the US. In fact the risk of cognitive impairment was nearly four times greater in elderly people who were severely deficient in 25-hydoxyvitamin D when compared to elderly individuals with normal levels.

And now, two large prospective studies indicate that low vitamin D concentrations may actively increase the risk of cognitive decline specifically in elderly women. In the first study, researchers examined whether dietary intake of vitamin D was an independent predictor for the onset of dementia among women aged 75 years and older. A total of 498 community-dwelling women of average age 80 years were placed into three groups according to the onset of dementia within 7 years - no dementia, Alzheimer's disease, or other dementias. Baseline vitamin D dietary intakes were estimated from a self-administered food frequency questionnaire. The study authors found that women who developed Alzheimer's disease had the lowest average vitamin D intake of 50.3 mg/week, whereas those who developed other forms of dementia had an average of 63.6 mg/week and those who didn't develop dementia at all averaged 59 mg/week. This study clearly shows that higher vitamin D dietary intake means a lower risk of developing Alzheimer's disease among older women, although there seems to be no association between vitamin D levels and onset of other dementias.

Another cross-sectional study tested whether lower 25-hydroxyvitamin D levels were associated with a greater likelihood of cognitive impairment and risk of cognitive decline in over 6,000 community-dwelling elderly women who were followed for 4 years. Global cognitive function was measured by the Modified Mini-Mental State Examination (MMSE) and executive function was measured by Trail Making Test Part B (Trails B). Here too, the study authors found that women with very low 25-hydoxyvitamin D levels had greater odds of global cognitive impairment at baseline relative to those with normal levels. Low 25-hydoxyvitamin D levels among older women were associated with greater odds of global cognitive impairment and a higher risk of global cognitive decline.

Both these studies highlight the important role played by vitamin D levels in the development of Alzheimer's disease - a severe form of dementia associated with aging that affects memory, thinking, and behavior. Late-onset Alzheimer's is by far the most common form, typically seen in people aged 60 and older.

All forms of dementia first appear as forgetfulness. Eventually they lead to difficulties with emotional behavior, language, memory, perception and cognitive skills. Early Alzheimer's symptoms include difficulty performing tasks that used to come easily, such as balancing a checkbook, playing complex games and learning new information or routines. Getting lost on familiar routes, trouble remembering names of familiar objects and losing interest in formerly pleasurable activities are other typical symptoms. Alzheimer's sufferers may also exhibit personality changes and lose interest in social interactions.

Later, changes in sleep patterns, delusions, depression, and agitation begin to emerge. Patients may forget details about current events and even their own life history. Hallucinations, striking out and violent behavior become commonplace - till finally, Alzheimer's patients may withdraw from social contact altogether.

In the final stages, Alzheimer's patients no longer understand language, do not recognize their own family members, even their spouses and become unable to perform simple tasks such as eating, dressing themselves or bathing.

Given the healthcare costs and the severe physical, emotional and social consequences for Alzheimer's patients and their immediate families, large, well designed randomized controlled trials are now needed to determine whether vitamin D supplementation is effective at preventing or treating Alzheimer's disease and other dementias in elderly men and women.