DV - Daily Value

IU - International Units

NAS - National Academy of Sciences

Note: Percentage Daily Value is often written as "percent DV". It means the same as "percent USRDA" (U.S. Recommended Daily Allowance). Labels use them interchangeably. The Daily Value for each vitamin or mineral is the Food and Drug Administration's advice on how much to shoot for each day.

Vitamin A

The old DV (5,000 IU) is outdated. You need only 3,000 IU of vitamin A. Too much retinol (listed on labels as vitamin A palmitate or acetate) may increase the risk of hip fractures, liver abnormalities and birth defects. Beta-carotene, which the body converts to vitamin A, does not cause those problems, but very high doses (33,000 to 50,000 IU a day) may increase the risk of lung cancer in smokers. Don't get more than 2,500 IU of retinol or 15,000 IU of beta-carotene from your multivitamin. Instead, load up on beta-carotene-rich fruits and vegetables like carrots, cantaloupe, sweet potatoes and broccoli, which may help prevent cancer.

Vitamin D

It helps you absorb calcium. Yet many people over 50 get too little D from sunshine, especially in the winter, or from their food - the major sources are milk, fatty fish like salmon and fortified cereals. The NAS recommends 200 IU a day for adults 50 and under, 400 IU a day for people aged 51 to 70 and 600 IU for anyone over 70. But those amounts include what you get from the sun and food. Look for a multivitamin with 400 IU (the DV).

Thiamin (B-1), Riboflavin (B-2), Niacin (B-3), and B-6

While there is no reason to get more than the Daily Value for these B-vitamins, the high doses in some multivitamins are harmless. But that does not mean the sky is the limit. More than 100 mg a day of B-6 can cause (reversible) neurological damage. Super-high doses of niacin (3,000 mg a day or more) may cause liver damage.

Vitamin B-12

People older than 50 may lack the stomach acid needed to extract B-12 from food, so they should get at least 2.4 mcg a day from a supplement or fortified food like breakfast cereal. Vegans (who eat no meat or dairy) also need to take B-12. Most multivitamins have at least 6 mcg (the DV). A B-12 deficiency can cause irreversible nerve damage and may masquerade as Alzheimer's disease.

Iron

Many children and pre-menopausal women are deficient, but too much can cause iron overload (hemochromatosis) in susceptible people. To play it safe, men and post-menopausal women should look for zero to 8 mg of iron in a multivitamin. (The 9 mg in many brands is close enough). The Daily Value (18 mg) is okay for pre-menopausal women.

Magnesium

Americans get too little from foods (among the best sources: Whole grains and beans). A deficiency may increase the risk of diabetes. Look for a multi with at least 100 mg, just for insurance. The NAS recommends 320 mg (for women) and 420 mg (for men). More than 350 mg a day from a supplement (not food) may cause diarrhea.

Selenium

Many leading brands (like Centrum) have considerably less than the DV (70 mcg) or the latest recommended level (55 mcg). A large study is under way to see if a high dose (200 mcg a day) can lower the risk of prostate cancer. Taking more than 800 mcg a day of selenium can make your nails or hair brittle (or fall out), so the NAS set the highest safe level at 400 mcg.

Iodine, Manganese, Boron, Molybdenum and Chloride

Ignore. There is no evidence that people need more than what they get from their food.

Potassium

Ignore. Amounts in multivitamins are trivial. The NAS hasn't yet set a recommended level, but the Daily Value is 3,500 mg a day. Instead, eat plenty of fruits and vegetables, which are potassium-rich, to help keep your blood pressure down.

Expiration Date

Make sure that it is at least a few months away.

Vitamin C

The old DV (60 mg) is lower than the new recommended levels (75 mg for women; 90 mg for men). Roughly 250 to 500 mg a day is enough to saturate the body's tissues. If you follow the National Cancer Institute's advice to eat at least five to nine servings of fruits and vegetables every day, you should get that much. Taking more than 1,000 mg at one time in a supplement may cause diarrhea.

Vitamin E

The NAS's new recommendations are essentially the same as the DV (30 IU). Studies are under way to see if higher doses (400 IU to 600 IU a day) reduce the risk of prostate cancer and cataracts. In several large studies, vitamin E did not protect against heart disease or stroke.

Vitamin K

The NAS now recommends 120 mcg a day, and recent studies suggest that 150 to 250 mcg a day may be ideal to reduce the risk of hip fractures. Yet most multivitamins do not even have the DV (80 mcg). Leafy greens are the best sources, but some calcium supplements (like Viactiv chews) also supply vitamin K, as do vitamin K supplements, of course. Vitamin K interferes with anti-coagulant drugs like coumadin, so check with your doctor if you are taking them.

Folic Acid

Look for the DV (400 mcg) to reduce the risk of birth defects (for women who could become pregnant) and possibly heart disease or colon cancer (for everyone).

Biotin and Pantothenic Acid

Ignore. You would have to eat a bizarre diet to run short of either.

Calcium

To reduce the risk of osteoporosis (and possibly colon cancer), shoot for 1,000 mg if you are 50 or younger, or 1,200 mg if you are over 50. But doses of 2,000 mg a day or more may increase the risk of prostate cancer, so don't go overboard. A day's worth of calcium will not fit in most multivitamins. So if you do not eat three or four servings of low-fat milk, yogurt, cheese or calcium-fortified orange juice every day, take a 300 mg calcium supplement for each serving you miss.

Phosphorus

Unnecessary. The less you get in your multivitamin, the better. Too much may impair calcium absorption and we already get more phosphorus than we need from our food.

Zinc and Copper

Look for 8 mg (women) or 11 mg (men) of zinc and 0.9 mg of copper. But there is no harm in getting the DV for each (15 mg for zinc and 2 mg for copper). Taking more than 40 mg a day of zinc may make your body lose copper. Higher levels may depress your immune system. So even though sucking on zinc lozenges may help shorten a cold, swallowing zinc pills may be counter-productive.

Chromium

The NAS recommends only 20 to 25 mcg a day for women and 30 to 35 mcg a day for men, but many brands have closer to 120 mcg (the DV), which is safe..

Nickel, Tin, Silicon and Vanadium

Ignore. It is not clear that humans need them at all.