"Experts recommend somewhere between 1500 to 2600 IU daily," Dr. Binkley said. "It's considered a very safe vitamin. One would need daily doses of 40,000 IU or higher before seeing negative side effects."

Vitamin D is essential for bone health, but Binkley discussed its role in improving muscle function.

"One of the primary killers among older adults is falls," Binkley said. A sufficient amount of Vitamin D not only enhances bone strength but also improves muscle function thereby reducing the risk of fractures."

Vitamin D receptors have been discovered in many tissues throughout the body. "There's a lot that is unknown about vitamin D right now," Dr. Binkley said. "But there is one certainty - its importance is widespread."

The ubiquitous effects of the "sunshine vitamin" are demonstrated by research studies associating lower circulating vitamin D levels with cancer, type 1 diabetes and cardiovascular disease.

Although young people are perfectly capable of producing vitamin D in their skin after brief exposure to sunlight (10-15 minutes of UVB ray exposure per day), Americans often avoid sun exposure or utilize sunscreen. Additionally, older adults have less ability to produce vitamin D in the skin and generally require vitamin D supplementation.

"Nearly 40% of our endocrinology clinic patients over the age of 50 have inadequate vitamin D levels," R. Mack Harrell, MD with Broward Health in Ft. Lauderdale, Florida, said. "We used to think that sunscreen utilization was the main cause, but the problem is clearly more complicated than that."

As Vitamin D's importance comes into focus, questions and confusion remain about different versions of the vitamin.

There are two types of Vitamin D: D2 (ergocalciferol) and D3 (cholecalciferol). D2 comes from plant life, while D3 is derived from animals. Binkley recommends checking the labels when buying Vitamin D, because existing data indicates that D3 may be more effective.