Vitamin D has long provided significant support for healthy bone density. However, scientists have also validated the critical role that vitamin D plays in regulating healthy cell division and differentiation, and its profound effects on human immunity. These findings link a deficiency of vitamin D to a host of common age-related problems. 
 
Vitamin D3 can be synthesized by humans in the skin upon exposure to Ultraviolet-B (UVB) radiation from sunlight. But, due to the winter season, weather conditions, and sun block, the body's ability to produce optimal vitamin D levels may be inhibited. In fact, it has been proposed that annual fluctuations in vitamin D levels explain the seasonality of influenza.  All of these factors point to the value of taking a daily vitamin D supplement.