Vitamin A

Vitamin A does much more than help you see in the dark and helps form and maintain healthy skin, hair, teeth and bones. Vitamin A works together with vitamins D, B, E, zinc, phosphorus and calcium. It also acts as an antioxidant, which may help protect against cancer and other diseases. 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 whether insufficient vitamin A caused cancer. Several studies have dashed this hypothesis, as have randomized trials of supplements containing beta carotene, a precursor of vitamin A.

Although it's relatively easy to get too little vitamin A, it's also easy to get too much. Intake of up to 10,000 IU, twice 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.

Optimal Intake: The current recommended intake of vitamin A is 5,000 IU for men and 4,000 IU for women. Many breakfast cereals, juices, dairy products, and other foods are fortified with vitamin A. Many fruits and vegetables, and some supplements, also contain beta-carotene and other vitamin A precursors, which the body can turn into vitamin A. 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 vitamin supplement that has all or the vast majority of its vitamin A in the form of beta-carotene. Another reason to avoid too much preformed vitamin A is that it may interfere with the beneficial actions of vitamin D.

Vitamin C

Vitamin C has been in the public eye for a long time. Even before its discovery in 1932, nutrition experts recognized that something in citrus fruits could prevent scurvy, a disease that killed as many as 2 million sailors between 1500 and 1800. More recently, Nobel laureate Linus Pauling promoted daily megadoses of vitamin C (the amount in 12 to 24 oranges) as a way to prevent colds and protect the body from other chronic diseases.

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

Optimal Intake: The current recommended dietary intake for vitamin C is 90 mg for men and 75 mg for women (add an extra 35 mg for smokers). There's no good evidence that megadoses of vitamin C improve health. As the evidence continues to unfold, 200 to 300 mg of vitamin C a day appears to be a good target. This is easy to hit with a good diet and a standard multivitamin. Excellent food sources of vitamin C are citrus fruits or citrus juices, berries, green and red peppers, tomatoes, broccoli, and spinach. Many breakfast cereals are also fortified with vitamin C.

Vitamin D

Vitamin D plays a critical role in maintaining bone health. When blood levels of calcium begin to drop, the body responds in several ways. It promotes the conversion of vitamin D into its active form, which then travels to the intestines (to encourage greater calcium absorption into the blood) and to the kidneys (to minimize calcium loss in the urine).

For bone health, an adequate intake of vitamin D is no less important than calcium. Vitamin D is found in milk and vitamin supplements, and it can be made by the skin when it is exposed to sunlight in the summertime.

Optimal Intake: The current recommended intake of vitamin D is 5 micrograms up to age 50, 10 micrograms between the ages of 51 and 70, and 15 micrograms after age 70. Optimal intakes are higher, though, with 25 micrograms (1000 IU) recommended for those over age 2. Very few foods naturally contain vitamin D. Good sources include dairy products and breakfast cereals (which are fortified with vitamin D), and fatty fish such as salmon and tuna. For most people, the best way to get the recommended daily intake is by taking a multivitamin, but the level in most multivitamins (10 micrograms) is too low.

Vitamin E

This antioxidant protects tissues against free radical damage, thus playing a role in the prevention of cancer and cardiovascular disease. The primary function of vitamin E is to maintain cell membrane integrity from oxidative damage, thereby promoting healthy circulation and acting as a protector against environmental pollutants. A deficiency of vitamin E can result in decreased birth weights and anemia in children and neuropathy in adults.

Vitamin K

Vitamin K, which is found mainly in green, leafy vegetables helps make six of the 13 proteins needed for blood clotting. Its role in maintaining the clotting cascade is so important that people who take anticoagulants such as warfarin (Coumadin) must be careful to keep their vitamin K intake stable.

Lately, researchers have demonstrated that vitamin K is also involved in building bone. Low levels of circulating vitamin K have been linked with low bone density, and supplementation with vitamin K shows improvements in biochemical measures of bone health.

If you eat a healthy diet, do you need to take vitamins? Not long ago, the answer from most experts would have been a resounding "no". Today, though, there's good evidence that taking a daily multivitamin makes sense for most adults.

What's changed? Not only have scientists determined why we need pyridoxine (vitamin B6), but they are also accumulating evidence that this vitamin and others do much more than ward off the so-called diseases of deficiency, things like scurvy and rickets. Intake of several vitamins above the minimum daily requirement may prevent heart disease, cancer, osteoporosis, and other chromic diseases.

This summary will focus on vitamins with newly recognized or suspected roles in health and disease. It will present some of the evidence about vitamins' possible new roles, point out how to get more of these in your diet, and assess the value of taking a daily multivitamin.

Antioxidants

Our cells must constantly contend with nasty substances called free radicals. They can damage DNA, the inside or artery walls, proteins in the eye, just about any substance or tissue imaginable. Some free radicals are made inside the body, inevitable byproducts of turning food into energy. Others come from the air we breathe and the food we eat.

We aren't defenseless against free radicals. We extract free-radical fighters, called antioxidants, from food. Fruits, vegetables, and other plant-based foods deliver dozens, if not hundreds, of antioxidants. The most common are vitamin C, vitamin E, beta-carotene and related carotenoids. Food also supplies minerals such as selenium and manganese, which are needed by enzymes that destroy free radicals.

During the 1990s, the term antioxidants became a huge nutritional buzz word. Antioxidants were promoted as wonder agents that could prevent heart disease, cancer, cataracts, memory loss, and a host of other conditions.

It's true that the package of antioxidants, minerals, fiber, and other substances found in fruits, vegetables, and whole grains help prevent a variety of chronic diseases. Whether high doses vitamin C, vitamin E, or other antioxidants can accomplish the same feat is an open question.

The evidence accumulated so far isn't promising. Randomized trials of vitamin C, vitamin E, and beta-carotene haven't revealed much in the way of protection from heart disease, cancer, or aging-related eye diseases. Ongoing trials of other antioxidants, such as lutein and zeaxanthin for macular degeneration and lycopene for prostate cancer, are underway.