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In the 1930's, vitamin D3 was the recognized as a nearly miraculous tool for preventing the then-common childhood disease known as rickets.
Children who had to go to work at an early age in poorly lit factories, and who lived in cities where the sky was permanently dark with coal ash, developed weak and wobbly bones. Giving these children as little as 200 IU of vitamin D3 a day (a single gram of vitamin D3 is 40,000,000 IU) was enough to prevent rickets, and this dosage of D3 was considered an appropriate amount for general daily supplementation for everyone under the age of 50.
Cutting edge research has revealed, however, that much higher doses of D3 can be beneficial. Vitamin D3 dosage for adults, in particular, can be much higher than the 200 IU of D3 daily recommended for adults under the age of 50 and the 400 IU of D3 daily recommended for adults 50 and older. In certain situations, up to 5,000 IU of D3 each and every day is appropriate for correcting vitamin D3 deficiency caused by lack of sunlight or poor nutrition.
Much of the latest research about vitamin D3 comes from Finland, which lies in near or above the Arctic circle and where summer's midnight sun is followed by winter's noontime darkness. Finns who do not eat large quantities of foods that are rich in D3, such as butter and oily fish, tend to have winter-time D3 deficiencies.
Correcting a deficiency in healthy adults is not hard to do. Clinical studies have shown that taking just 400 IU of D3 a day—about twice the recommended dosage for D3—is enough to boost the immune system so colds and flu are less frequent.
Bone health in children, teens, and adults under 50 may also require just 400 IU of vitamin D a day, along with several servings of leafy green vegetables every week to provide vitamin K. Vitamin K is an important, and frequently overlooked, co-factor for vitamin D.
If you don't get your leafy greens, then you need to take a vitamin K supplement. If you happen to take a "blood thinning" drug for cardiovascular disease or recovery from frostbite, your doctor will advise you when it is safe to take vitamin K, which can cancel out the effects of anti-clotting medications.
If you are 50 and over, you need more vitamin D3. Part of the reason for this is that your parathyroid glands are usually less efficient at converting the storage form of vitamin D, D3, into the active form of vitamin D, D2.
Keeping enough vitamin D2 in your bloodstream requires having more D3 stored in skin and fat. Taking a supplement of up to 1,000 IU of vitamin D a day acts as "insurance" that your tissues will always have the activated vitamin D they need.
How much vitamin D3 should I take if I am deficient? Children and adults at any age can become deficient in vitamin D3, even if they get lots of sun. One study found that 1 in 3 professional surfers on the north shore of Maui, where the sun shines nearly every day, had a vitamin D3 deficiency.
Since toxicity symptoms are unknown below daily dosages of 40,000 IU per day, the US Institute of Medicine recommends taking up to 4,000 IU per day as an unquestionably safe dosage. The idea behind setting the limit low is making sure that even if a vitamin manufacturer accidentally made the product 10 times too strong or even if the consumer accidentally took more than one dose per day, there still would be no injurious side effects. Actually, you can take 5,000 to 10,000 IU per day will no ill effects.
Since there are two kinds of vitamin D, you need to have a blood test for both to know what dosage of vitamin D3 is appropriate for you. If you have low levels of vitamin D3, the storage form of the vitamin, then you definitely need to take more vitamin D3, up to 5,000 IU per day for up to two weeks, and then 1,000 to 2,000 IU per day as a maintenance dose thereafter. This is what is needed if the problem is not getting enough sun.
If you body has a problem converting vitamin D3 to D2, however, and your levels of D3 are high but your levels of D2 are low, then you need to speak with a nutritionally oriented doctor or a nutrition specialist about the full range of supplements that will help your body use its daily dosage of vitamin D3 effectively in making the active form of the vitamin, D2. Be sure you have both tests to make sure you are getting all the vitamin D you really need.
Laaksi I, Ruohola JP, Mattila V, Auvinen A, Ylikomi T, Pihlajamäki H. Vitamin D supplementation for the prevention of acute respiratory tract infection: a randomized, double-blinded trial among young Finnish men. J. Infect Dis. 2010 Sep 1;202(5):809-14.