Vitamin by vitamin, here's what you need, according to the Institute of Medicine's Dietary Reference Intakes (DRIs, formerly known as Recommended Daily Allowances). Numbers are different for children, the elderly and pregnant and lactating women.
Vitamin A: It comes in two forms. Retinol, found in egg yolks, dairy foods and meat, is used immediately by the body. Beta carotene, found in yellow, orange and green leafy vegetables, is converted to vitamin A in the body. Vitamin A enhances vision, strengthens bones, teeth and skin. Recommended daily intake of 700 micrograms for women, 900 for men (3,000 international units, or IU). Too much vitamin A is highly toxic -- regularly taking more than 10,000 IU a day can cause nerve and liver damage, dry lips and nails, hair loss and might increase the risk of bone fractures.
The three Bs -- B6, B12 and folic acid: Studies show that B vitamins might help prevent heart disease and some cancers. All three help us build new proteins and folate helps synthesis of DNA. B12 works with folate to maintain the nervous system. Too little folate is a major cause of neural tube birth defects, such as spina bifida. All women of childbearing age should take a folate supplement. Folate may also lower the risk of breast cancer in women who drink alcohol. The recommended amounts are likely to change in the next few years as results come in from ongoing long-term studies, and a debate still rages between groups such as the March of Dimes, which believes food fortification with folic acid should increase in order to reduce the incidence of birth defects, and others who say not enough is known about effects on the overall population. For example, some early studies suggest that folic acid may stimulate the early stages of prostate and colon cancer. "There's an urgent need for more information," says Cornelia Ulrich, a nutritional scientist at the Fred Hutchinson Cancer Research Center in Seattle. Current recommended intakes are: 1.3 milligrams of B6, 2.4 micrograms of B12 and 400 micrograms of folic acid.
The other Bs -- thiamin (B1), riboflavin (B2) and niacin (B3): Thiamin, riboflavin and niacin work together with other B complex vitamins to help turn food into energy. These vitamins also help sustain the nervous system, skin and digestive tract. Low thiamin levels can result in difficulty concentrating, depression and muscle weakness. (Alcoholics are often severely low in thiamin.) Riboflavin works as an antioxidant and deficiencies have been associated with migraines, cataracts and arthritis. Niacin improves circulation and lowers cholesterol levels in the blood. The DRIs are 1.2 milligrams for thiamin, 1.2 milligrams for riboflavin and 15 milligrams for niacin. (These are averages for men and women.)
Vitamin C: Vitamin C has been on the public radar for a long time as an immune system enhancer and cold-fighter. It works as an antioxidant and helps build collagen (necessary for healthy bones, teeth and blood vessels). There has been a lot of debate about whether extra-large doses do any good -- and studies show no clear link between huge amounts of C and decreases in heart disease, cancer or eye problems. Because the vitamin is water soluble, you probably pee out anything beyond what your body can use. DRI: 75 milligrams for women, 90 for men, more for smokers. Citrus fruits, tomatoes, broccoli and spinach are good sources.
Vitamin D: This one has been getting lots of attention as accumulating studies show that vitamin D helps protect against some cancers, including breast, prostate and colon. It also helps with bone strength. Most people who live in northern latitudes become deficient in D during winter months. Current DRI is 5 micrograms (200 IU), but many experts think we need at least twice that much. Keep an eye open for news about this one. Vitamin D is fat-soluble and better absorbed when taken with food.
Vitamin E: Study results are mixed on whether vitamin E supplements can help reduce the risk of heart disease, and the American Heart Assn. refuses to recommend it. Large, long-term studies may help shed light on this question. One complicated thing about this fat-soluble vitamin is that it comes in eight forms, but our bodies absorb some better than others. "This highlights the challenges" of evaluating vitamins, says Brent Bauer, director of the Complementary and Integrative Medicine Program at the Mayo Clinic in Rochester, Minn. "Just because something works in the diet doesn't mean we can extract one part of it, take it in a pill and get the benefits." Look for "mixed tocopherol" on the label. Recommended intake is 15 milligrams (22.5 IU). Anything more than 1,000 milligrams (1,500 IU) could be toxic.
Vitamin K: This fat-soluble vitamin is essential for blood clotting and important for building bone. Supplements are recommended for people who take anticoagulants and for post-menopausal women, but it is abundant in leafy greens and cooking oils. Recommended intake is 90 micrograms for women, 120 for men.
Calcium: Calcium is essential for strong bones, and it also assists with blood clotting, muscle contraction and communication within the nervous system. Adequate vitamin D and magnesium boost calcium absorption. Any adult who is not getting 1,000 milligrams a day through dairy products, broccoli and other greens should take supplements, and people older than 50 should up their intake to 1,200 milligrams. Calcium comes in several forms. Calcium citrate is easiest to absorb without food, but often costs more than calcium carbonate or other forms. Calcium interferes with iron absorption, so these two supplements should be taken separately.
Selenium: Selenium is an antioxidant, found in meat, fish and fortified foods such as rice and bread. It occurs in high amounts in Brazil nuts. The mineral seems to help prevent cancer, heart disease and arthritis, according to some studies, and HIV patients with low selenium levels appear to be more likely to die. The recommended intake is 55 micrograms, and most of us get enough in our diets.
Zinc: This mineral keeps our immune systems strong, helps wounds heal, maintains our sense of taste and smell, and helps us grow in the womb and through childhood. Diets rich in meats and seafood (especially oysters) are rich in zinc, which also occurs in beans, nuts, whole grains and fortified cereal. Pregnant women and vegetarians should consider taking zinc supplements, and experts debate whether zinc may also shorten the length of colds. Taking too much iron can block absorption of zinc. The DRI is 8 milligrams for women and 11 milligrams for men.
Iron: Iron is a crucial player in delivering oxygen to our cells. Low levels of the metal -- called anemia -- lead to fatigue, difficulty working and a weakening of the immune system. Iron is abundant in red meats, organ meats and shellfish, so vegetarians can become deficient, but it also appears in sources such as oatmeal, beans, pumpkin and sunflower seeds, spinach and fortified cereal. Menstruating women need 18 milligrams a day of iron and pregnant women need 27 milligrams. Men and post-menopausal women need just 8 milligrams. Pregnant women, women who might become pregnant, women with heavy periods and athletes need to make sure they get enough iron, but there is some evidence that taking extra iron can damage heart health in people who aren't already anemic. In supplements, look for "ferrous" rather than "ferric" salts to boost absorption. To avoid severe illness from iron toxicity, do not exceed more than 45 milligrams a day, and make sure to keep iron supplements away from children.
-- Emily Sohn
and Susan Brink
Freelance writer Sara Solovitch contributed to this report.