Fortifying your knowledge of vitamins

Take a mul-tivitamin for insurance.” Until recently, that was the usual advice from physicians, scientists, and other experts. Now, it’s under scrutiny: That’s because if you’re looking for hard evidence that a multivitamin will lower your risk of heart disease, cancer, cataracts, or other major disease, you may be disappointed. In 2006, the US National Institutes of Health held a State-of-the-Science Conference. Its conclusion: “Evidence is insufficient to prove the presence or absence of benefits.”

“Insufficient” is putting it mildly. By the time the scientists narrowed the field to trials that pitted a multivitamin against a placebo and excluded poorly done studies, they ended up with less than half a dozen — two of them done in China and two in France. The single US study looked only at vision. But it’s one thing to lack proof that multivitamins prevent disease. It’s quite another to find evidence that a multi might cause harm. After all, a handful of recent studies have suggested that people who take multivitamins — or a few of the nutrients they contain — might have a slightly higher risk of cancer. Here’s what to do until scientists sort out the confusion.

How To Read A Multivitamin Label

Please refer to the illustration of a made-up multivitamin label on this page. It lists how much of each vitamin and mineral we need to get every day (from food plus supplements), according to the US National Academy of Sciences’ Institute of Medicine (IOM). The IOM’s recommendations vary slightly by age and gender. In most cases, what is listed is the highest value for adults, excluding pregnant and breastfeeding women. The label also lists the Daily Value (DV) for each vitamin or mineral. It is the US Food and Drug Administration’s (FDA) advice on how much to shoot for each day (from food and supplements combined). The DVs, which haven’t been updated in decades, are still used on multivitamin labels. In some cases, the values date from 1968 and don’t reflect the recent research. Here is the current thinking on these nutrients (please refer to the values as indicated in the label):

• Vitamin A (retinol). The Daily Value (5,000 IU or International Units) is outdated. You need only 3,000 IU a day of vitamin A. Too much retinol (typically listed on labels as vitamin A palmitate or vitamin A acetate) may increase the risk of hip fractures, liver abnormalities, and birth defects. Beta-carotene, which the body converts to vitamin A, doesn’t cause those problems, but very high doses (33,000 to 50,000 IU a day) may increase the risk of lung cancer in smokers. Current advice: Don’t get more than 4,000 IU of retinol or 5,000 IU of beta-carotene from your multi (less retinol would even be better). Instead, load up on beta-carotene-rich fruits and vegetables like carrots, cantaloupes, sweet potatoes, and broccoli, which may help prevent some cancers.

• Vitamin C. The DV (60 mg) is lower than the IOM’s recommendations (75 mg a day for women and 90 mg a day for men). Roughly 250 to 500 mg saturates the body’s tissues, so more than that is probably excreted in urine. Taking more than 1,000 mg of vitamin C at one time in a supplement may cause diarrhea.

• Vitamin D. This vitamin helps you absorb calcium and may reduce the risk of cancer, diabetes, and falls. Some people may get too little vitamin D from sunshine or from their food. The IOM recommends 200 IU a day for adults 50 and under, 400 IU for people 51 to 70, and 600 IU for anyone over 70. Some vitamin D experts though say that everyone should get at least 1,000 IU a day. Those amounts include what you get from the sun, from salmon and other fatty fish, and from fortified foods like milk, breakfast cereals, and some brands of yogurt, margarine, and orange juice. They also include the vitamin D that’s added to many calcium supplements. Most multivitamins have 400 IU of vitamin D (the DV).

• Vitamin E. Doses of 30 to 800 IU a day haven’t protected against heart disease or stroke, and 400 IU a day or more may slightly raise the risk of dying. Studies are underway to see if 400 IU a day prevents prostate cancer. To play safe, stick to no more than 100 IU.

• Vitamin K. The IOM recommends 120 mcg/day, yet most multivitamins have much less than the DV (80 mcg). In recent studies, taking extra vitamin K doesn’t strengthen bones, as earlier studies had suggested. You can get K from leafy green, some calcium supplements, and vitamin K supplements. Vitamin K can interfere with blood-thinning drugs like Coumadin, so people who take them should check with their doctor before taking a multi with vitamin K.

• Thiamine (B-1), riboflavin (B-2), niacin (B-3), B-6. The higher-than-DV levels in many multivitamins are harmless. Two exceptions: More than 100 mg a day of vitamin B-6 can cause (reversible) neurologic damage. And as little as 50 mg a day of niacin can cause flushing. Super-high doses of niacin (3,000 mg a day or more) may cause liver damage, though you won’t find that much in a multivitamin.

• Folic acid. If you could become pregnant, look for a multi with the DV (400 mcg) to reduce the risk of birth defects. Others should probably take less until studies clarify whether high intakes (roughly 1,000 mcg a day or more from supplements and food combined) raise the risk of cancer. Since most multis have 400 mcg, one option is to take your “daily” multi every other day.

• Vitamin B-12. Most multivitamins have at least 6 mcg (the DV). That’s more than the 2.4 mcg the IOM recommends for adults, but it’s perfectly safe. (So are the higher doses — 600 to 800 mcg — that are found in a few multivitamins.) Ten to 30 percent of older people are unable to absorb the B-12 in the form that’s added to supplements and fortified foods. A B-12 deficiency can cause irreversible nerve damage and may masquerade as Alzheimer’s disease.

• Biotin, panthotenic acid. Ignore. You’d have to eat a bizarre diet to run short.

• Calcium. Calcium may help prevent colon cancer and (with vitamin D) may reduce the risk of osteoporosis. Shoot for 1,000 mg a day (if you’re 50 or younger) or 1,200 mg (if you’re over 50). But men should get no more than 200 mg from their multi, since 1,500 mg a day or more may raise prostate cancer risk.

• Iron. Many people, especially premenopausal women, are deficient. But taking too much can cause constipation or iron overload if you’re susceptible. Men and postmenopausal women should look for a multivitamin with no more than 10 mg of iron or should take a multi for premenopausal women every other day. The DV (18 mg) is fine for premenopausal women.

• Phosphorus. Unnecessary to take in a multi. Too much may impair calcium absorption, and we already get more than we need from our food.

• Magnesium. Many people get too little of magnesium from their food (among the best sources: whole grains and beans). A deficiency may increase the risk of diabetes and colon cancer. Look for a multi with at least 100 mg, just for insurance. The IOM recommends 320 mg a day for women and 420 mg for men. More than 350 mg a day from a supplement may cause diarrhea.

• Zinc, copper. Look for 8 mg (women) or 11 mg (men) of zinc and 0.9 mg of copper. There’s no harm in taking a multivitamin with the DVs (15 mg for zinc and 2 mg for copper), but don’t take more than 23 mg zinc. Getting more than 40 mg may make your body lose copper.

• Selenium. Many multis have less than the DV (70 mcg) or the IOM’s recommended level (55 mcg). A large study is under way to see if high doses (200 mcg) can lower the risk of prostate cancer. But a few studies have suggested that taking 200 mcg a day may raise the risk of skin cancer and diabetes, so it’s safest to take no more than about 100 mcg a day.

• Chromium. The IOM recommends only 20 to 25 mcg a day (women) or 30 to 35 mcg a day (men). Many brands have closer to 120 mcg (the DV), which is safe.

• Iodine, magnesium, molybdenum, chloride, boron. Ignore. There’s no evidence that people need more than what they get from their food.

• Potassium. Ignore. The amounts in multis are low. And while the potassium chloride that’s used in supplements may lower blood pressure and the risk of stroke, it won’t help prevent kidney stones and bone loss like the potassium citrate that’s found in fruits and vegetables.

• Nickel, silicon, tin, vanadium. Ignore. It’s not clear that they’re needed.