For this study, researchers looked at mentally healthy men and women who participated in the U.S. Cardiovascular Health Study between 1992-93 and 1999. Their blood samples were collected at the start, and their mental status was assessed roughly six years later.
Participants' vitamin D blood levels reflected vitamin D from food, supplements and sun exposure. Dietary sources include fatty fish such as salmon, tuna or mackerel, as well as milk, eggs and cheese.
During the follow-up, those with low levels of vitamin D were about 1.7 times more likely to develop dementia than those with normal levels. Those with severely low levels were about 2.2 times more likely than those with normal levels to develop dementia, the study found.
The results echo some findings from other, smaller studies, Fargo said. "What's important about this study is the large number of participants," he said.
Experts disagree about the best blood level of vitamin D. In this study, risk for dementia and Alzheimer's significantly increased below a threshold of 50 nanomoles per liter (nmol/L), the researchers said.
Exactly how low vitamin D may be linked with dementia isn't known. Experts speculate that the vitamin may clear plaques in the brain linked with dementia. This has been shown in the lab, Fargo said. Vitamin D deficiency has been linked with brain atrophy as well, according to background information in the study.
Until more research is in, Fargo recommended that people ''try to eat a brain healthy diet," which is the same as a heart-healthy diet. That includes foods low in fat and cholesterol.
Getting regular physical activity and keeping blood pressure under control are other good measures, Fargo said.
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