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Vitamin D: Not just for bones

(article, Catherine Bennett Dunster)

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p(blue). Editor's note: Catherine Bennett Dunster wrote the Health+Food column from June 2007 to April 2008.

"I recently learned that I'm low on vitamin D, and my doctor prescribed supplements. She also indicated that many people suffer from low levels of vitamin D. Can you shed some light on this?"

You're in good company — I’m often asked this question. In fact, I'm considering taking vitamin D supplements myself. Let me explain.

For centuries, we’ve known that a vitamin D deficiency causes rickets — the childhood disease characterized by softening, weakening, and bending of bones — which often leads to fractures. 

[%image "reference-image" float=right width=400 caption="Your body needs winter sunshine."] 

For decades, we’ve known that vitamin D increases calcium absorption to maintain a constant serum calcium level; without enough vitamin D, calcium is leached from bones, leading to osteomalacia or osteoporosis.  

But while vitamin D’s role in bone health has long been examined, more recent research explores its other roles in maintaining good health.

Last summer, a comprehensive review appeared in the New England Journal of Medicine addressing the role of vitamin D in health and disease. As the review reported, scientists have linked insufficient levels of vitamin D with certain cancers (colon, breast, and prostate among them), autoimmune diseases (multiple sclerosis, rheumatoid arthritis, and Type 1 diabetes), infections, autism, and cardiovascular disease. 

h1. The sunlight connection


When sunlight (specifically UVB radiation) hits skin, previtamin D3 is formed and then turned to vitamin D3 by the heat. Next, the liver converts vitamin D3 to the major circulating form of vitamin D in the body — serum 25(OH)D — which is metabolically inactive. Active vitamin D — 1,25(OH)2D — is formed in the kidney. New research tells us that other tissues in the body (like the colon, prostate, breast, and immune cells) can also convert inactive D to the active form.  

While the sun is an excellent source of vitamin D, several factors determine the amount produced by the sun for a given individual. Skin color (darker skin makes less vitamin D), seasonality, latitude (people above 35 degrees north or below 35 degrees south have difficulty producing adequate vitamin D), sunscreen use (an SPF of 15 or more decreases vitamin D production by 99 percent), and age (older people typically get less exposure to sunlight) are just some of these factors.  


The implications of these studies have moved out of the laboratory and into the doctor’s office, as health practitioners have begun to embrace and appreciate vitamin D’s impact on more than just bone health (though plenty of research continues).  

Currently, there’s no consensus on optimal levels. Meanwhile, the question of why so many are suffering effects from insufficient vitamin D levels is met with ample debate.  

Some experts suspect that the Dietary Reference Intake (DRI) for vitamin D, set in 1997, is much too low. (DRIs are different from Recommended Daily Allowances (RDAs), which were set according to the amount of a specific nutrient needed to prevent a deficiency disease, such as rickets. The newer DRIs reflect a shift in emphasis to the nutrient amounts necessary to decrease the risk of chronic disease.) They believe that while recommended amounts may be sufficient to prevent bone disease in most people, they may not be high enough to promote cellular health and prevent non-bone diseases. 

Others point at the hysteria associated with sun exposure, our single most prolific source of vitamin D (see sidebar). Since the late 1980s, we’ve been warned to avoid sunlight without protection from a thick layer of sunscreen. Jon Hanifin, a professor of dermatology at Oregon Health and Science University, warned against sun exposure in a recent Oregonian article:

"It’s dangerous to encourage fair-skinned people to go out and get sun exposure, especially in midsummer. I take 1,000 IU \[of vitamin D\] a day myself."

John Cannell, a psychiatrist who founded the nonprofit Vitamin D Council, disagrees with Hanifin in the same article:

"If you put on your bathing suit in the summer in Oregon, your body makes 200 times more vitamin D than the government says you need every day. Whether it’s nature or God or whatever you believe in, why would there be a system in our skin that makes so much vitamin D so quickly? Probably nature set this up for a good reason. What did our mothers, and their mothers before them, say? Drink your milk, and go play outside."

Other experts side with each of these scientists, although most policy makers and scientists agree that the suggested amounts of recommended vitamin D need to be reassessed. But until that happens, what should you do? 

First, if you’re in one of the high-risk categories (living in extreme northern or southern latitudes, are darker-skinned, of advanced age, or have limited unprotected sun exposure) or suspect your vitamin D level might be low, consult your health-care provider to have your serum vitamin D level checked.  

Second, enjoy a few minutes of sunshine (but avoid burning) when the sun is out. Third, do your best to include vitamin-D rich foods in your daily diet:

|!Food||!Vitamin D|
|Cod-liver oil, 1 Tbsp.||1,360|
|Salmon, 3.5 oz.||360|
|Tuna, 3 oz.||200|
|Sardines, 1 oz.||250|
|Fortified milk, 1 cup||100|
|Egg, 1 whole||20|
|Fortified juice, 1 cup||100|
|Fortified cereal, 1 cup||40-60|
|Beef liver, 3.5 oz.||15|

h1. How much vitamin D?


The adequate intake of vitamin D recommended for infants, children, pregnant women, and adults aged 19 to 50 is 200 IU per day; for adults aged 51 to 70, 400 IU per day; and for adults aged 71 and up, 600 IU daily. Currently, the upper limit recommended for vitamin D is 2000 IU per day. 


I’m a proud member of the Get Your Nutrients From Your Food Club, but I think these new implications of vitamin D make a compelling case for considering supplementation. While it's possible to meet the DRIs with careful consumption of dietary vitamin D sources, the research tells us that on average, a mere 10 percent of our vitamin D comes from food. That, coupled with the fact that experts believe the current DRIs are too low, may mean supplementing is a good idea.

If you, too, are contemplating vitamin D supplements, seek the advice of your health-care provider first.

p(bio). Catherine Bennett Dunster is a registered dietitian and a former instructor at Oregon Health and Science University. She lives with her husband and two children in Portland, Oregon. 

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reference-image, l