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Vitamin D is a nutrient your body needs for building and maintaining healthy bones. That's because your body can only absorb calcium, the primary component of bone, when vitamin D is present. Vitamin D also regulates many other cellular functions in your body. Its anti-inflammatory, antioxidant and neuroprotective properties support immune health, muscle function and brain cell activity.
Vitamin D isn't naturally found in many foods, but you can get it from fortified milk, fortified cereal, and fatty fish such as salmon, mackerel and sardines. Your body also makes vitamin D when direct sunlight converts a chemical in your skin into an active form of the vitamin (calciferol).
The amount of vitamin D your skin makes depends on many factors, including the time of day, season, latitude and your skin pigmentation. Depending on where you live and your lifestyle, vitamin D production might decrease or be completely absent during the winter months. Sunscreen, while important to prevent skin cancer, also can decrease vitamin D production.
Many older adults don't get regular exposure to sunlight and have trouble absorbing vitamin D. If your doctor suspects you're not getting enough vitamin D, a simple blood test can check the levels of this vitamin in your blood.
Taking a multivitamin with vitamin D may help improve bone health. The recommended daily amount of vitamin D is 400 international units (IU) for children up to age 12 months, 600 IU for people ages 1 to 70 years, and 800 IU for people over 70 years.
From the Mayo clinic