Dr. Michele Couri can pinpoint precisely when the notion of Vitamin D deficiency crossed her radar - at an integrative medicine conference in fall 2009.

"There wasn't one lecture that didn't bring up Vitamin D," she said. "I thought, 'I think I must be missing something.' "

She came home, did some research and started offering to screen patients for Vitamin D deficiency. She was surprised to find a third of her patients' levels were low.

But she was more shocked to learn just how important Vitamin D is.

"It has so many functions in the body," she said. "It helps with calcium absorption, bone mineralization. It helps the pancreas process insulin. It helps our immune systems."

Fatigue and depression are commonly reported in patients with low levels of Vitamin D. But being deficient also can put people more at risk for certain kinds of cancer, osteoporosis, autoimmune disorders, diabetes, possibly even multiple sclerosis.

"We don't want to sell Vitamin D as the cure for everything," she said. "But it does play a very valid role in optimal health."

Last year, Couri's office manager, Tina Collins of Hanna City, complained she was tired every day. She had dark circles under her eyes, and her primary care physician had ruled out some likely culprits. Couri offered to check her Vitamin D level and found she was low. The physician set up a supplement plan for her staff member.

"Now my dark circles are gone, and I have a ton of energy," Collins said.

In 2010, the Institute of Medicine upped its guidelines of how much Vitamin D people need daily. For people up to age 70, it's 600 international units a day. For age 71 and older, it's 800 units. That change and a growing body of research are squelching some of the debate over the importance of Vitamin D.

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Fatigued? It might be time to check your Vitamin D levels