Learn more about some of the vitamins with newly recognized or suspected roles in health and disease:

Vitamin A does much more than help you see in the dark. It stimulates the production and activity of white blood cells, takes part in remodeling bone, helps maintain the health of endothelial cells (those lining the body's interior surfaces), and regulates cell growth and division. This latter role had researchers exploring for years the relationship between vitamin A and cancer. Specifically, researchers looked at whether people could reduce their cancer risk by taking supplements of

beta-carotene,one of several precursor compounds that the body can transform into vitamin A, or by taking the active form of vitamin A (also called retinol or preformed vitamin A). Several studies and randomized trials have dashed this hypothesis. Although it's possible to get too little vitamin A, it's easy to get too much preformed vitamin A (retinol) from supplements. Intake of up to 3,000 micrograms of preformed vitamin A, more than three times the current recommended daily level, is thought to be safe. However, there is some evidence that this much preformed vitamin A might increase the risk of hip fracture or some birth defects. Another reason to avoid too much preformed vitamin A is that it may interfere with the beneficial actions of vitamin D.

In contrast to preformed vitamin A, beta-carotene is not toxic even at high levels of intake. The body can form vitamin A from beta-carotene as needed, and there is no need to monitor intake levels, as there is with preformed vitamin A. Therefore, it is preferable to choose a multivitamin supplement that has all or the vast majority of its vitamin A in the form of beta-carotene; many multivitamin manufacturers have already reduced the amount of preformed vitamin A in their products. Smokers should avoid high-dose single supplements of beta-carotene, since some randomized trials in smokers have linked high dose supplementation with increased lung cancer risk. There is no strong reason for anyone to take separate beta-carotene supplements.

There's no question that vitamin C plays a role in controlling infections. It's also a powerful antioxidant that can neutralize harmful free radicals, and it helps make collagen, a tissue needed for healthy bones, teeth, gums, and blood vessels. The question is, do you need lots of vitamin C to keep you healthy? No. Vitamin C's cold-fighting potential certainly hasn't panned out. Small trials suggest that the amount of vitamin C in a typical multivitamin taken at the start of a cold might

ease symptoms,but for the average person, there's no evidence that megadoses make a difference, or that they prevent colds. Studies of vitamin C supplements and heart disease, cancer, and eye diseases such as cataract and macular degeneration also show no clear patterns.