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Vitamin D

Dr. Weil in Garden

What is vitamin D?
Vitamin D, often referred to as the "sunshine vitamin," is actually a fat-soluble hormone that the body can synthesize naturally. There are several forms, including two that are important to humans: D2 and D3. Vitamin D2 (ergocalciferol) is synthesized by plants, and vitamin D3 (cholecalciferol) is synthesized by humans when skin is exposed to ultraviolet-B (UVB) rays from sunlight. The active form of the vitamin is calcitriol, synthesized from either D2 or D3 in the kidneys. Vitamin D helps to maintain normal blood levels of calcium and phosphorus.

Why is vitamin D necessary?
Vitamin D assists in the absorption of calcium and promotes bone mineralization, which may prevent or slow the progression of osteoporosis. It also helps to strengthen the immune system and protect against a number of serious diseases, including rickets and osteomalacia. Research suggests vitamin D may also provide protection from hypertension, psoriasis, several autoimmune diseases (including multiple sclerosis and rheumatoid arthritis), and reduce the incidence of fractured bones. In addition, growing evidence has demonstrated its important role in defending against cancer (studies link a deficiency of vitamin D to as many as 18 different cancers).

What are the signs of a deficiency?
Deficiencies of vitamin D are common, especially in industrialized countries in northern latitudes, where sun exposure is typically infrequent. Low levels of vitamin D may be indicated by porous bones, weak muscles and easy fracturing.

How much, and what kind, does an adult need?
According to the National Institutes of Health (NIH), the daily Adequate Intake (AI) for adults is 5 mcg (200 IU) daily for males, female, and pregnant/lactating women under the age of 50. People 50 to 70 years old should get 10 mcg daily (400 IU) daily, and those over 70 should get 15 mcg daily (600 IU). Based on recent research, Dr. Weil recommends 2,000 IU of vitamin D per day. Look for supplements that provide D3 (cholecalciferol) rather than D2 (ergocalciferol). Anyone with vitamin D deficiencies should discuss intake levels with his or her physician.

How much does a child need?
According to the NIH, AI for children from birth until 5 years of age should take 5 mcg per day (200 IU).

How do you get enough vitamin D from foods?
It isn't easy to get enough vitamin D from your diet. While fortified foods such as milk and cereals are available, most provide vitamin D2, a form which is much less well utilized by the body than D3. Good dietary sources include fortified foods, eggs, salmon, tuna, mackerel and sardines. Since sunlight causes our bodies to make vitamin D, daily exposure is helpful. A helpful rule: estimate the amount of time it would take for your skin to turn pink - or slightly, but noticeably darker - in the sun. Then reduce that time by 50 percent if you have fair skin, 25 percent for darker skin, and get that amount of exposure between 11 a.m. and 3 p.m. two to three times per week.

Are there any risks associated with too much vitamin D?
No adverse effects have been seen with supplemental vitamin D intakes up to 10,000 IU daily. Exposing the face and hands to roughly 10 minutes of direct sunlight daily is also quite safe and a good way to boost vitamin D; for some tips on general sun safety, read the DrWeil.com Q&A: "No Fun in the Sun."


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Information on this web site is provided for informational purposes only and is not intended as a substitute for the advice provided by your physician or other healthcare professional. You should not use the information on this web site for diagnosing or treating a health problem or disease, or prescribing any medication or other treatment.

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