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Most people have heard two things about sources of vitamin D. One is that human skin can use sunlight to make this "sunshine vitamin." The other is that vitamin D is essential to fighting infection, reducing the risk of cancer, and building healthy bones. But do we need supplemental vitamin D, and if we do, what are the best sources?
Scientists used to assume that anyone who got 20 to 40 minutes of sunlight every day made enough vitamin D for all their body's needs. (The skin only makes vitamin D when the UV index is 4 or higher.) It turns out this is not so. Many more people than scientists originally thought need additional vitamin D:
Researchers discovered that 51 per cent of visibly tanned surfers and skate boarders in Hawaii, which enjoys bright sunshine all year round, had low vitamin D levels. On the other hand, other researchers found that women in Wisconsin in the winter had normal levels of vitamin D, if they took vitamin D supplements. Despite the fact that our bodies can make much of the vitamin D we need, a little "vitamin D insurance" is usually a good idea. Who needs extra vitamin D?
But it is not especially easy to get enough vitamin D even from the best vitamin D-rich foods.
There are several foods that are relatively rich in vitamin D. These include butter and cream from cow's milk and whole cow's milk, even if vitamin D is not added during processing, fish liver, beef liver, and catfish, eel, mackerel, salmon, and sardines. The exact amount of vitamin D in these foods varies tremendously:
A tablespoon of cod liver oil packs nearly 1500 IU of D, which is more than your body needs for maintenance for an entire day (which is as often as most people want to take it). If you are vitamin D-deficient, then taking cod liver oil three times a day (are you thinking "Yum?") is enough. But consider how much of other foods you need to make up a vitamin D deficiency with 5,000 IU a day:
Obviously, except for cod liver oil, there are no foods that are sufficiently rich in vitamin D to make up a vitamin D deficiency. That's why supplements are so useful.
Getting vitamin D from supplements is blissfully easy—and inexpensive. Just take 5,000 IU a day to treat deficiency, or 1,000 to 2,000 IU a day for maintenance, especially in the winter. At these levels, you won't overdose, and you will spend very little money for a very important nutritional protection.
Institute of Medicine (IOM). Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride (1997) Access date: 01-16-2011.