Research suggests that people with very low levels of vitamin D in their blood, a condition known as vitamin D deficiency, are more likely to develop Alzheimer's disease and other forms of dementia. For example, a large study published in Neurology showed that people with extremely low blood levels of vitamin D were more than twice as likely as those with normal vitamin D levels to develop Alzheimer's disease or other types of dementia. Other studies have shown no association between vitamin D levels and dementia. At this point, the association between vitamin D deficiency and dementia risk is only observational. More research is needed to show cause and effect. Vitamin D is vital to bone metabolism, calcium absorption and other metabolic processes in the body. Its role in brain function, cognition and the aging process is still unclear. Some studies suggest that vitamin D may be involved in a variety of processes related to cognition, but more research is needed to better understand this relationship. Most vitamin D is produced within the body in response to sunlight exposure. Vitamin D occurs naturally in only a few foods, including fatty fish and fish-liver oils. The most common dietary sources of vitamin D are fortified foods, such as milk, breakfast cereals and orange juice. Vitamin D supplements also are widely available. Vitamin D deficiency is common among older adults, partly because the skin's ability to synthesize vitamin D from the sun decreases with age. It's too early to recommend increasing your daily dose of vitamin D in hopes of preventing dementia or Alzheimer's disease. But maintaining healthy vitamin D levels can't hurt and may pay off in other ways, such as reducing the risk of osteoporosis. According to the National Institutes of Health, adults age 70 and younger need 600 international units (IU) of vitamin D daily, and adults over age 70 need 800 IU daily. More studies are needed to determine if vitamin D deficiency is indeed a risk factor for Alzheimer's disease and dementia and if the use of vitamin D supplements or sun exposure can prevent or treat these conditions.