For a long time, vitamin deficiency diseases, like scurvy and rickets, were common. Then when nutritious food became more plentiful, research shifted to investigating whether large quantities of vitamins might be able to fend off certain diseases. But just because someone gets a definitive disease due to a serious lack of a particular vitamin doesn't mean that consuming more than the Recommended Daily Allowance can prevent a range of diseases. Remember when megadoses of Vitamin C were thought to be a cure-all for everything from colds to cancer? Then Vitamin E was in the spotlight because of a possible link to Alzheimer's and heart disease. Even the B's had their moment of fame. And now it's D. But follow-up studies haven't shown any conclusive preventive benefit to megadosing. And in some cases, taking too much can be harmful.