Vitamin D is known as the "sunshine vitamin" for good reason. When exposed to sunlight, humans manufacture copious amounts of the nutrient. But shorter days combined with winter clothes that have most people bundled from head to toe leave little chance for the sun to work its magic.

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Two new studies in the Journals of Gerontology reinforce why older adults should get enough vitamin D. The first study linked low levels of vitamin D to cognitive impairment in older women. The second study found that men and women with higher levels of vitamin D were less likely to develop Alzheimer's.

The government recommends adults 70 and younger take 600 international units of vitamin D a day. Those 71 and older should get at least 800 IU. Without sun exposure, it may be hard to get the daily requirement without taking a supplement. For example, an 8-ounce glass of milk has only 100 IU. For a government fact sheet, go to