Features: The vitamin-D debate

August 19, 2014

Vitamin D: How much is enough? By @RajMukhop #IOM recommends 600 IU (21-29 ng/ml in blood) but some want >4k, IOM max
http://www.asbmb.org/asbmbtoday/201406/Features/VitaminD

By Rajendrani Mukhopadhyay

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Other experts disagree. They say that there is solid evidence to connect vitamin D only to bone health, on which the IOM based its recommendation in 2011. “There’s this view that vitamin D is a miracle hormone that does literally everything,” says J. Wesley Pike at the University of Wisconsin-Madison. “The fact of the matter is that’s just simply not true.”
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cure all v just bone health?

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There are two types of vitamin D: vitamin D2 and vitamin D3. Vitamin D2 is generally found in fortified foods, such as milk and some other dairy products, cereals and orange juice. The few natural sources of vitamin D include shitake mushrooms and fatty fish, such as salmon, sardines and mackerel. Vitamin D3 is the product of sunshine. Ultraviolet light from the sun converts 7-dehydrocholesterol in the skin into previtamin D3. The previtamin D3 gets turned into vitamin D3.

But both forms of vitamin D need to be activated to work. Vitamin D2 and D3 get hydroxylated by a P450 enzyme in the liver to be turned into 25-hydroxyvitamin D. This prohormone binds to a protein in the blood called the serum vitamin D-binding protein. Clasped to its binding partner, the prohormone then travels through the blood circulation system for about two weeks. When the bound prohormone gets to the kidneys, an enzyme called CYP27B1 hydroxylase in the proximal tubular epithelial cells turns it into the active hormone
1α,25-dihydroxyvitamin D.

This active form of vitamin D has a fleeting existence in plasma. “}}

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The IOM report in 2011 stated that a level between 21 ng/mL and 29 ng/mL of 25-hydroxyvitamin D was considered to be sufficient for maximum bone health. This is the range being disputed between vitamin-D experts. Some believe we need a blood level of more than 30 ng/mL of 25-hydroxyvitamin D; others think the IOM committee behind the report got it right.

Ross says the only clinical outcome that the committee could see where vitamin D had a direct effect was bone health. The literature on that was more extensive and conclusive, so the committee used it to base its recommendation of a daily vitamin D intake of 600 IU.

Critics of the IOM report had other bones to pick. One was the question of sun exposure. Sun-driven production of vitamin D is the most effective source of the nutrient. In the summer, a white person can make 10,000 IU after spending 30 minutes in the early-afternoon sun without sunscreen…..She says the 600 IU recommended for vitamin D was made to maintain an adequate serum level of the vitamin even in the absence of sun “but knowing full well that, for many people, sun adds to that. Chances are their actual total exposure, considering both diet and sun, is actually higher.”

Meanwhile, experts point out that black people have different requirements for vitamin D and calcium; melanin absorbs UV light and interferes with its transmission into the skin, reducing the production of vitamin D. Also, black Americans tend to consume less dairy, which is usually fortified with vitamin D. As John Adams at the University of California, Los Angeles, explains, the first values for serum levels of vitamin D were established by studying white people. He says that, if we look at the U.S. population today, about 60 percent of the white population has a serum level of 25-hydroxyvitamin D of less than 30 ng/mL. “If you look at African-Americans who have a (25-hydroxyvitamin D) level less than 30 ng/mL,” says Adams, “it’s 95 percent of the population.”

The IOM report set the upper limit for taking vitamin D at 4,000 IU for adults and between 2,500 and 3,000 IU for children, based on their ages. Critics say the upper limit could have been higher. They point to a review that showedthat doses of vitamin D up to 10,000 IU were not toxic.

In the meantime, everyone is waiting to see the results from a large-scale randomized trial now happening under the purview of the National Institutes of Health. The agency is supporting a long-term study of daily intake of vitamin D3 pills of 2,000 IU. Spearheaded by JoAnn Manson and Julie Buring at Harvard University-affiliated Brigham and Women’s Hospital, the trial is called the vitamin D and omega-3 trial, from which various letters have been plucked to give the trial the name VITAL.
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Related to this:
http://www.medscape.com/viewarticle/589256_8

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If the vitamin D blood test was 30 ng/ml (75 nmol/L) and a 40 ng/ml (100 nmol/L) level was desired, 1,000 IU (25 mcg) of vitamin D per day over several months should be taken to achieve a normal blood level or 40 ng/ml (100 nmol/L). Upon reaching the goal, most individuals need to supplement with 800 to 1,000 IU per day to maintain this level. “}}