Hardly a day goes by when vitamin D isn't in the health care news, with deficiency linked to everything from vertebral and hip fractures, lower extremity dysfunction (poor muscle tone), and periodontal disease, to colorectal and breast cancers, autoimmune disorders such as multiple sclerosis, and cardiovascular conditions. Should these reports be considered red flags? Or do they signal a need for action?

Until fairly recently, our understanding of vitamin D importance in human health was limited to its role in the prevention of rickets and contributions to bone health. Milk fortification began in the 1930s to prevent rickets in children, a national public health problem at the time.

Researchers only recently discovered that most cells contain vitamin D receptors, and mounting evidence supports the assertions that current dietary recommendations are insufficient to prevent falls and fractures; cardiovascular, autoimmune, and infectious diseases; some cancers; and type 1 and type 2 diabetes, and that they're also inadequate for preserving muscle strength and optimal health throughout one's lifespan, especially in the absence of adequate sun exposure.

As a dietitian, I find it difficult to accept that a single nutrient could prevent a half dozen or more chronic diseases, and that we can't meet all of our nutrient needs from diet alone.

However, says Catherine Peterson, PhD, a nutrition scientist at the University of Missouri, Columbia, who is studying vitamin D's role, because vitamin D functions as a prohormone, "it may have the potential to have much more profound effects than a true nutrient (for which health effects are more subtle)."

Erin Michos, MD, a researcher at Johns Hopkins who is studying vitamin D and mortality rates, says, "It's very difficult to get all of the vitamin D you need from diet alone." Sunlight is the body's primary source of vitamin D, through synthesis in the skin. Whereas 10 to 15 minutes of exposure to full-body ultraviolet B provides 10,000 to 20,000 international units (IU) of vitamin D, high-calcium foods provide far less; a cup of fortified milk, for instance, provides only 100 IU, and even cod liver oil provides only about 1,360 IU per tablespoon.

WHAT THE RESEARCH IS SAYING

Karen Rafferty, RD, senior research dietitian for the Osteoporosis Research Center at Creighton University, says of the recent boom in vitamin D press, "It was only in 2003 that the first papers were published relating specific serum 25-hydroxyvitamin D levels with specific health outcomes. Most of what we've learned about vitamin D has come through scientific research conducted and published in the past five years." And what we're learning suggests that we're not getting enough vitamin D.

It is well known that the risks of a number of conditions change in relation to one's proximity to the equator. The prevalence of multiple sclerosis, for instance, rises as the distance from the equator increases (although even in very hot climates, the risk does not decrease if the people wear clothing that covers their bodies well, as they do in the Middle East).

Similarly, Mohr and colleagues, reporting in the May-June 2008 issue of the Breast Journal, found an inverse relationship between exposure to sunlight (higher levels of ultraviolet B radiation and therefore vitamin D) and the risk of breast cancer.

The cardiovascular protection we attribute to the fatty acid content in fish may also be related to its vitamin D content. And in a review published in the December 9, 2008, issue of the Journal of the American College of Cardiology, Lee and colleagues suggest that the secondary hyperparathyroidism related to insufficient levels of 25-hydroxyvitamin D likely contributes to hypertension, insulin resistance, inflammatory response, and the associated increase in cardiovascular risk.

In November 2008 the American Academy of Pediatrics doubled the amount of vitamin D it recommends children and adolescents receive to 400 IU daily, because of concerns over widespread deficiency and new clinical evidence supporting the vitamin's possible role in maintaining children's innate immunity and preventing diseases. Obesity, sedentary and indoor lifestyles, routine sunscreen use, and increasing age all decrease cutaneous vitamin D synthesis. Rafferty says that it takes much longer for an older than a younger person to synthesize vitamin D. "From age 20 to 80, we experience a 50% decline" in our ability to synthesize it. And obesity increases risk because vitamin D is sequestered in body fat.

WHO NEEDS TO WORRY MOST?

Populations at greatest risk for deficiency include people with malabsorptive disorders, the institutionalized or homebound elderly, prisoners, those who keep most of their bodies clothed, and dark-skinned populations of all ages living at latitudes far north and south of the equator. According to Michael Holick in the July 19, 2007, issue of the New England Journal of Medicine, dark pigment (or a high level of melanin) reduces "vitamin D₃ synthesis by as much as 99%."

People living more than 37[degrees] south or north of the equator (roughly, above Washington, DC, or below Buenos Aires, Argentina) cannot make adequate vitamin D from sun exposure during winter months, regardless of skin pigmentation or time outdoors. Certain medications, such as corticosteroids and anticonvulsants, increase the risk of deficiency as well. And there are populations in whom vitamin D supplementation could even cause harm. Michos warns that individuals with "granulomatous diseases such as sarcoidosis may be prone to vitamin D intoxication."

WHAT TO TELL PATIENTS

* Get screened. A serum 25-hydroxyvitamin D test is the best tool for assessment. Says Holick in his 2007 article, "Although there is no consensus on optimal levels of 25-hydroxyvitamin D as measured in serum, vitamin D deficiency is defined by most experts as a 25-hydroxyvitamin D level of less than 20 ng per milliliter (50 nmol per liter)." (There is no consensus to date for vitamin D insufficiency in infants and children.)

* Practice sensible sunlight exposure. Rafferty and Michos both suggest using sunscreen on your face, and then getting 15 to 20 minutes of sun exposure on your arms and legs two or three times a week (after which sunscreen should be applied). You cannot get an overdose of vitamin D from the sun, but you can increase your risk of skin cancer. Beware of tanning beds, too. Some lamps give off high levels of ultraviolet A radiation, which is most associated with melanoma.

* Choose fortified foods to help meet your needs. All milk is fortified, regardless of fat content, as are the majority of ready-to-eat cereals and some juices; most cheeses and yogurts are not.

* Supplement daily with 1,000 to 2,000 IU vitamin D₃ year-round, unless you're a sunbather. The upper limit is 10,000 IU per day.

* Work with your physician or child's pediatrician to assess risk and adjust supplements and sun exposure accordingly.

So, is vitamin D worth all the hype? Yes. It does a lot of what its proponents claim. But Peterson doesn't believe vitamin D is a cure-all and adds that "one cannot deny the data-the prevalence of vitamin D deficiency and insufficiency is at epidemic proportions in North America and Europe."

Melinda Hemmelgarn, MS, RD, LD