When vitamin D was first discovered, scientists didn't quite know how special it was. But more modern research has significantly changed our understanding of the "D."

Since the early 1900s, it has been known that vitamin D protects against the development of soft brittle bones—a condition known as rickets in children and osteomalacia in adults—by promoting the absorption of calcium. For similar reasons, vitamin D is also recommended as a safeguard against osteoporosis—particularly women—later in life.

Research over the past fifteen years has brought about a completely new understanding about this undervalued fat-soluble vitamin. After an ongoing debate, vitamin D is now categorized as a steroid hormone, rather than a nutrient. Ergo, like other steroid hormones, vitamin D also helps control metabolism, immune function, and inflammation. Tests have led scientists to believe that almost every cell in the body contains vitamin D receptors, and that vitamin D regulates bone and heart health, immune system response, the body's control of insulin and blood sugar, as well as regulation of calcium and phosphorus metabolism.[1]  That's quite a lot of work for a simple little "vitamin."

Sources of Vitamin D

The body naturally produces vitamin D when exposed to UVB sunlight. Fifteen to thirty minutes of sun during the warmer months (late spring through early fall), can help produce enough daily vitamin D. In the winter, if you live north of Atlanta, the sun never gets high enough in the sky for its UVB rays to penetrate the atmosphere. However, sunscreen containing SPF 8 or higher can decrease results by 95 percent.

Unfortunately, vitamin D isn't found in many foods. It is therefore often added to foods like milk, cereal, and orange juice. Chicken feed is sometimes fortified with D, to produce egg yolks that contain this valuable vitamin. Several types of fish naturally contain vitamin D, such as salmon, tuna, mackerel, cod, and sardines.

Here's a fun fungi fact for you: if you expose mushrooms to sunlight, they'll absorb the vitamin D, which transfers to you if you eat it!

Factors that affect vitamin D status include:

• Age– vitamin D status declines with age, around age 70 for men and 50 for women
• Skin Pigmentation– melanin not only determines skin color but also blocks UVB; meaning the darker your skin color, the more at risk you are for a vitamin D deficiency
• Sun Exposure– people who spend a lot of times indoors during the day or cover their skin with clothes or sunscreen are more at risk for deficiency
• Season and Latitude– People who live in the Northeast US don't get adequate sun exposure for vitamin D production during the winter months
• Breastfed Infants- If you or your infant aren't taking a supplement, there is an increased risk for deficiency
• Adiposity- People who are overweight or obese also have an increased risk
• Low dietary intake– Insufficient attention to eating vitamin D-rich foods
• Malabsorption– conditions that affect the absorption of nutrients such as celiac disease, gastric bypass or gastric sleeve, irritable bowl syndrome, Crohn's disease, chronic kidney or liver disease and pancreatic insufficiency can reduce vitamin D levels.
  
  

How much?

Recommended Dietary Allowances (RDA) for Vitamin D

One international unit (IU) of vitamin D is equivalent to .025 mcg.
* Quantity listed here represents an adequate intake  .

Tolerable Upper Intake Levels (ULs) for Vitamin D

Deficiency and Insufficiency

The number of Americans with a vitamin D deficiency or insufficiency depends on which parameters are used. The main two are the Institute of Medicine (IOM) and the Endocrine Society's Clinical Practice Guidelines. Vitamin D insufficiency and deficiency, combined, affects approximately one-third of Americans by the IOM standards. Supplementation may be required for those at risk or with insufficiency.

Getting Tested

Serum levels of 25-hydroxyvitamin D are a reliable indicator of vitamin D status. If you're interested in learning about your vitamin D status, discuss with your doctor about prescribing the simple blood test at your next visit.

Interaction with Medications

Vitamin D supplements have the potential to interact with different medications as well as medications affecting the metabolism of vitamin D. People who take the medications below should talk to their healthcare providers about their vitamin D intake and status.

• Corticosteriod medications, such as prednisone, can reduce calcium absorption and impair vitamin D metabolism. With long-term use, the effects can lead to the loss of bone mass and development of osteoporosis.
• The common cholesterol lowering drug cholestramine (brand names include Questran ®, LoCholest ® and Prevalite ®) and the weight-loss drug orlistat (brand names Xenical ® and Alliâ„¢) can reduce the absorption of vitamin D and other fat soluble vitamins (A, E, and K).
• Phenobarbital and phenytoin (brand name Dilantin ®), used to prevent and control epileptic seizures, increase the liver metabolism of vitamin D to inactive compounds and reduce calcium absorption.[2]

Vitamin D performs a plethora of vital roles in your body. For optimal health, know your vitamin D levels, eat plenty of vitamin D-rich foods, take the recommended supplements if necessary, and make sure to get adequate sun exposure during the warmer months.

References

[1]  See, for instance:  http://www.ncbi.nlm.nih.gov/pubmed/19601865  and  
http://www.endocrineweb.com/professional/meetings/can-vitamin-d-help-control-weight-blood-glucose

[2]  Gough H, Goggin T, Bissessar A, Baker M, Crowley M, Callaghan N. A comparative study of the relative influence of different anticonvulsant drugs, UV exposure and diet on vitamin D and calcium metabolism in outpatiets with epilepsy. Q J Med 1986;59:569-77.