We all know that vitamins and minerals are essential to good health. If taking 100% of the Recommended Dietary Allowance (RDA) of, say, vitamin C is good enough to keep us going through the day, then why shouldn't taking 1,000% be enough to melt our fat, cure our blues, and let us leap tall buildings in a single bound?

Meanwhile, the $19 billion-a-year dietary supplement industry continually reminds us that we can get our vitamins from a pill. Which invites yet another question: Why should we bother choking down bushels of brussels sprouts when we could get the same effect by sprinkling supplement shavings over our Boston cream pie?

The broad consensus from nutrition experts is that while vitamins are indeed essential, big doses are usually pointless and can even be harmful. And no pill is likely to ever adequately substitute for a healthy diet.

Vitamins and minerals are substances your body needs for normal growth and functioning. Some facilitate crucial chemical reactions, while others act as building blocks for the body.

Nutritionists call vitamins and minerals "micronutrients" to distinguish them from the macronutrients such as proteins, carbohydrates, and fats that make up the bulk of our food. While micronutrients are vital for the proper processing of macronutrients, they're needed in smaller quantities. Think of it this way: If macronutrients are the gas in your engine, then micronutrients are like the motor oil, coolant, and battery fluid.

Micronutrient deficiency can lead to acute diseases with exotic names like scurvy, pellagra, and beriberi. Deficiency diseases were common in the U.S. until the 1940s, when the FDA-mandated fortification of common foods like bread and milk. These diseases are still common in many poorer countries.

It's easy to get enough micronutrients from your food if you maintain a healthy diet. But most people fail that test; they'll eat two or three servings of fruits and veggies per day rather than the recommended five. A multivitamin is a sort of nutritional safety net for many people. But it's just a safety net. So-called "whole foods" like veggies and whole grains contain fiber and a host of other important nutrients that can't be adequately delivered through pills. In fact, scientists are still finding new "trace elements" in whole foods that may someday be labeled essential to health -- but aren't found in any pill.

We're sending the wrong message if people believe they've got everything under control and if they're taking vitamins while eating a horrible diet.

It's easy to become overwhelmed when looking at the dietary supplement shelves of a health food store or even your local supermarket. While many of the health claims are unproven or downright bogus, some supplements may be useful for some groups.

Major multivitamin makers typically produce different varieties for men, women, children and older folks. Picking a pill that fits your group makes sense as the optimal level of various nutrients varies by age and sex. For example, premenopausal women need more iron than children or the elderly.

But the elderly have a harder time obtaining adequate amounts of vitamin B-12 from natural sources, so the need for supplementation may increase with age.

Folate, or folic acid, is key to preventing birth defects such as spina bifida. All women of childbearing age should ensure they get 100% of the RDA of folic acid through fortified food or a multivitamin.

Calcium supplements are also important for certain age groups. The Institute of Medicine recommends that adolescents get 1,300 milligrams of calcium a day. One cup of milk or calcium-fortified orange juice contains about 300 milligrams of calcium.

Other sources of calcium include cheese, tofu, yogurt, vegetables, and beans. A typical calcium supplement may contain 500 milligrams or 600 milligrams of calcium. People over 50 should get 1,200 milligrams a day of calcium to ward off osteoporosis (thinning of the bones).

Federal dietary guidelines recommend that the elderly, the homebound, and people with dark skin boost their vitamin D intake with both fortified foods and supplements to reduce the risk of bone loss. Vitamin D helps with absorption of calcium; often calcium supplements will also contain vitamin D.

Special groups such as smokers, pregnant women, or people recovering from traumatic injury may need additional supplements. Decisions to take supplements beyond a multivitamin are best made with your doctor or registered dietitian.

The evidence is strong that a healthy diet can ward off chronic diseases like cancer and heart disease. What's less clear is if big intakes of particular micronutrients can boost that preventive effect further.

There is promising evidence that the mineral selenium could prevent a variety of cancers. But beyond selenium, the data aren't promising. For example, there's no solid evidence that taking large doses of antioxidants like vitamins B or C have any beneficial effect.

As you seek the proper multivitamin or dietary supplement, it's best to keep your guard up. The supplement industry is relatively unregulated, and you can injure or even kill yourself with "natural" products bought at your neighborhood supplement store.

Many health claims attached to multivitamin formulations are doubtful, but harmless. Some men's multivitamins contain extra lycopene, a substance once thought to prevent prostate cancer. If indeed lycopene did anything, supplements don't have enough to make a difference. Multivitamins aimed at women are often spiked with green tea or ginseng extract; the effect of these on weight control is yet unproven.

More dangerous are recommendations of vitamin mega-doses to treat obesity, depression, carpal tunnel syndrome or other problems. At best, mega-doses are a distraction from real treatments for these problems, experts say. At worst, they can cause injury or death. So-called fat-soluble vitamins -- that is, vitamins A, D, E, and K -- accumulate in the body, making overdosing a real threat. Vitamin overdoses have been associated with liver problems, weakened bones, cancers, and premature mortality.

Until recently, water-soluble vitamins such as B and C were considered non-toxic, even at high doses. But now evidence is emerging that B-6 mega-doses can cause serious nerve damage.

Despite the warnings, the quest for a magic pill plunges ahead. Wouldn't a sensible diet and exercise do the trick even without the supplement?