Studies of vitamin D have been on the rise in recent years, and with good reason—a 2009 estimate suggests that nearly three quarters of teens and adults in the U.S. are deficient in this vital nutrient. Vitamin D deficiency not only causes rickets, a skeletal disorder in which the bones are soft and weak, but has also been associated with a rapidly increasing range of chronic conditions like cancer, heart disease, and type 2 diabetes. Now, two new studies suggest a link between vitamin D and neurological disorder: Older people with insufficient vitamin D levels may be more likely to develop Parkinson's disease and experience cognitive decline.

The first, led by Paul Knekt and colleagues at the National Institute for Health and Welfare, Finland, examined levels of vitamin D in the blood of 3,173 Finnish men and women aged 50 to 79 determined to be free of Parkinson's disease at the start of the study. The researchers then examined the incidence of Parkinson's disease in these participants over a 29-year follow-up period. They found that participants with the highest levels of vitamin D (more than 50 nmol/L) had a 65 percent lower risk of developing Parkinson's disease than those with the lowest vitamin D levels (less than 25 nmol/L). The researchers accounted for potentially confounding variables such as age, sex, marital status, education, alcohol consumption, smoking, physical activity and month of blood draw.

Parkinson's disease is a progressive neurodegenerative disorder that leads to impaired movement and speech, and is thought to result from insufficient dopamine levels in the brain. How vitamin D may protect against Parkinson's is not understood, although there is limited evidence from cell-based and animal models that vitamin D may prevent the loss of dopaminergic neurons (cells that produce dopamine).

One important limitation to the study is that the average vitamin D concentration of all the study participants (approximately 40 nmol/L) falls well below what is considered to be optimal (more than 75 nmol/L). Therefore, whether supplementation with vitamin D would further lower the risk for Parkinson's remains unknown. Nevertheless, the study suggests that not having enough vitamin D may predispose individuals to Parkinson's, and provides a starting point for further investigation. The results were published online July 12 in the Archives of Neurology.

In the second study, David Llewellyn of the University of Exeter and colleagues examined vitamin D levels among 858 Italian men and women age 65 and older. They found that more than half of the participants with dementia were vitamin D deficient (less than 50 nmol/L). What's more, cognitive tests revealed that severely deficient individuals (less than 25 nmol/L) were 60 percent more likely to undergo cognitive decline over the six-year follow-up period. This study appears online July 12 in the Archives of Internal Medicine.

Humans can obtain vitamin D by eating oily fish or fortified foods, and it is also photosynthesized in the skin upon exposure to adequate amounts of ultraviolet B (UVB) rays in sunlight. Major factors that influence vitamin D status in humans include season, latitude, age, skin tone, diet and supplement use. The U.S. Institute of Medicine currently recommends that adult men and women aim for a daily intake of 200 to 600 International Units (IU) of vitamin D.

New guidelines for vitamin D intake were published online July 12 in the Canadian Medical Association Journal by scientists from Osteoporosis Canada, a nonprofit organization. Because vitamin D influences calcium absorption and may protect against osteoporosis, the authors advise an increased daily intake of 400 to 1000 IU for healthy Canadians under age 50, and up to 2000 IU for those older than 50. The researchers state the changes are necessary because winter sunlight north of the 35th parallel (which coincides with the southern border of Tennessee) provides insufficient UVB for people living in that region to adequately make vitamin D.

The studies by Knekt and Llewellyn are not the first to link vitamin D deficiency with neurological problems, however. A role for vitamin D has previously been suggested in multiple sclerosis, autism and schizophrenia.

Some experts advise interpreting the results of these and other observational studies of vitamin D with caution. The above studies relied on participants from specific geographic areas, so more study is needed to determine whether the findings apply to other regions. Furthermore, "low vitamin D levels may simply be a marker for lower health status rather than a cause of it," Andrew Grey, professor of medicine at the University of Auckland, wrote in an editorial in the Archives of Internal Medicine. This is because vitamin D levels are directly related to sunlight exposure and physical activity; less healthy individuals are therefore likely to be less active and more sunlight-deprived, and have lower levels of vitamin D.

"It is now time to test the various hypotheses generated by observational studies of vitamin D…in adequately designed and conducted randomized controlled trials," Grey concluded. "We should invest in trials that provide the best possible evidence on the benefits and risks of vitamin D before we invest in costly, difficult, and potentially unrewarding interventional strategies."

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