If you work indoors, odds are that you're deficient in Vitamin D -- even more so in the Winter, if you're dark-skinned, and the further away from the equator you live. This supplement is an excellent way to help correct your levels. Dr John Cannell at the Vitamin D Council has researched dose extensively, and found 10,000 IU per day to be safe over the long-term; the skin produces about that much Vit D in response to 20 to 30 minutes of summer sun exposure. However, correcting low blood levels often requires a higher dose, which is where this product comes in. You should only use it daily for one to three months, hopefully with a blood test every month or so to check your progress. Research shows that ideal levels are between roughly 60 ng/mL and 120 ng/mL (150 to 300 nmol/L). After your levels correct, to maintain them you can switch to taking either one 50,000 IU capsule per week, or 5,000 to 10,000 IU per day on days that you don't get plenty of sun. Low levels of Vit D have been associated with higher rates of cancer (including breast, ovarian, prostate, colon, kidney, endometrial and lymphoma), heart disease, diabetes, Multiple sclerosis, and even the flu -- as well as lower rates of injury from falls and fractures. There is good evidence to suggest that adequate levels of Vit D can protect you from H1N1 as well or better than immunization. In fact, the Winter decline in Vit D has been implicated as one of the underlying causes of flu season. Another tidbit about getting Vit D from the sun: while sun exposure does increase the risk of certain skin cancers, those cancers are also the most curable forms. The flip-side is that limiting sun exposure reduces your Vit D level, which puts you at much higher risk for getting the really nasty, incurable cancers. Is the trade-off really worth it? Our Paleolithic ancestors certainly spent plenty of time in the sun