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You'd think it barely gets a passing grade, but vitamin D has managed to assume stratospheric stardom on the nutritional report card. Only loosely defined as a vitamin, vitamin D is actually more like a hormone, produced partly from cholesterol in the skin when we're exposed to the sun.

As we learn more about vitamin D, myths are being busted, and surprising and disturbing trends are emerging. Researchers are discovering receptors for vitamin D in organs, tissues and an array of glands throughout the body.

Meanwhile, testing of vitamin-D levels indicates widespread deficiency among people everywhere.

Best known for supporting bone health and mood, vitamin D appears to also play roles in regulating immune function and having distinctive cancer-prevention abilities.

Vitamin D helps maintain bone in concert with other nutrients and may boost calcium absorption by one-third. Paired with a nutritionally targeted diet, exercise and, in many cases, supplementation, vitamin-D levels can be restored. Nevertheless, big hurdles exist.

All kinds of people are at risk for vitamin-D deficiency. Old folks in assisted living who don't get outside and those at higher latitudes, both north and south, receive less sun exposure. And people of all ages may be at risk because of widespread use of sunscreen.

In our effort to protect ourselves from the sun, we may actually be halting vitamin-D production. The light we block, including ultraviolet B (UVB), would normally lead to production of the most active form of vitamin D: D-3, aka cholecalciferol. A recent study showed that sun-protection factor (SPF) 15 lowers absorption of vitamin D by 99 percent.

Americans, on average, get fewer than 250 international units of vitamin D per day, far less than what's needed for a 21st-century immune system.

Talk to a health care provider about testing, food sources, supplementation and personal risk for vitamin-D deficiency.

Michael Altman is a nutritionist at Ventana Wellness and the Centre for Natural Healing. He teaches at Southern Oregon University and College of the Siskiyous. E-mail him at michael@ventanawellness.com.