These days, the shelves at pharmacies and health food stores are loaded with an alphabet soup of vitamins and supplements, plus dozens of new herbal concoctions. How do you know which ones you really need?

Don't look to the marketing claims on the label for your answer. Many of the products out there rely more on hype than on science and some could even be harmful. Here are a few things to keep in mind when you're sorting through the supplements:

• The best vitamins are in the produce aisle. Most adults don't need to take multivitamins or supplements at all, as long as they're eating well. By eating a plant-based diet, with plenty of whole grains and at least five fruits and vegetables a day, you'll get the nutrients you need, plus cancer-preventing phytochemicals and other helpful natural compounds that you won't find in a pill.

That said, there are occasions when you should consider supplementing your diet:

• Do take a daily multivitamin if you don't eat well, or if you smoke or drink alcohol (behaviors which can increase your nutritional needs.) Though it's best to change unhealthy behaviors, getting select nutrients in pill form is better than not getting them at all, or getting less than you need.
• Do consider calcium supplements. Calcium helps prevent osteoporosis, an epidemic in this country. If you don't get enough calcium from dairy products and leafy green vegetables, consider taking a supplement, but be sure to take the right kind. Avoid those made from bone meal, oyster shell, or dolomite, which may be contaminated with heavy metals. Instead, choose supplements with calcium citrate or calcium carbonate. You can increase the benefit by taking smaller amounts of calcium throughout the day, rather than a lot at once, to improve absorption.
• Pregnant or thinking about becoming pregnant? Be sure to get at least 600 micrograms of folic acid (a.k.a. folate) per day to help prevent neural tube birth defects (other adults should get at least 400 mcg). While you may get enough of this and other vital nutrients through food, ask your doctor about taking vitamins as extra nutritional insurance for yourself and your baby--folate supplements of 400 mcg/day are generally recommended prior to conception and throughout pregnancy.
• Choose supplements from major pharmaceutical houses. If you do opt to take supplements, keep in mind that they're not regulated and you may not be getting quantity or quality of nutrient touted on the label. In addition, there have been more than a few cases of contaminants appearing in supplements, particularly in amino acids and mood-enhancers, such as tryptophan. These can cause a host of problems, even some as severe as paralysis. You improve your chances of getting a quality product by selecting a major manufacturer's brand.
• Be wary of megadoses. Taking too much of a vitamin or supplement can be dangerous. For example, fat-soluble vitamins, such as A and D, can be toxic in large amounts (do not exceed doses of 10,000 IU for vitamin A or 50 mcg for vitamin D), causing liver and kidney damage and other problems. Taking too much of a trace mineral, such as zinc, copper, selenium, or magnesium, can impair your absorption of other nutrients. Several studies have linked iron in both food and supplements to higher cancer rates. And taking megadoses of B vitamins can crank up your appetite, causing overeating. Your best bet is to follow the U.S. Government Dietary Reference Intake figures (based on the Recommended Daily Allowances), found on the labels of most multivitamins, which list the optimal amount of nutrients you should ingest during a day.