These aren't simple questions, in part because everyone's diet is different. Some of us get all the vitamins and other "micronutrients" we need from the foods we eat, while others fall short on vitamin D, potassium, and more.
The picture gets even murkier because the science behind nutrition is so inconclusive. Adding micronutrients may help protect against disease, yet high doses of some vitamins can be dangerous. "These are extremely potent molecules," warns Dr. Cutberto Garza, professor of nutrition science at Cornell University. "In their absence we don't survive, but we have no experience consuming [them] in quantities higher than what's found in food."
That's why the safest advice is to eat right. Strive for up to nine servings of fruit and vegetables each day, along with ample amounts of whole grains and nuts. (A serving may mean just a few celery sticks, a peach, two-thirds of a cup of juice, or two tablespoons of peanut butter.) "We know that increased consumption of fruits and vegetables is protective against cardiovascular disease and a lot of cancer," says Agriculture Dept. nutritionist Ronald L. Prior.
To be safe, add a multivitamin product containing the recommended daily allowances (RDA) of most micronutrients and minerals. Such products aren't heavily regulated by the Food & Drug Administration. But in contrast to many herbal supplements from small companies, where consumers can't be sure what they're getting, vitamin products from major manufacturers probably do contain what the label claims. The vitamins and minerals don't vary much by manufacturer, but products may be targeted. Men need less iron, about 8 milligrams a day. Pregnant women need the most: 27 mg. Women also need more calcium, and the elderly need more B vitamins.
In general, experts say, taking a multivitamin is safer than consuming just a few vitamins and minerals. "Overloading the system with one vitamin may make things worse in other areas," explains Bernadette Latson, a program director at Texas Southwestern Medical Center at Dallas. Some studies suggest that too much B-6 can cause nerve damage, excess D can cause kidney problems, and excess A could lead to a slew of troubles. "There are some serious dangers," warns Dr. Sidney M. Wolfe, director of Public Citizen's Health Research Group.
To complicate things, many scientists believe that popping a daily multivitamin isn't sufficient. For one thing, says Dr. Godfrey P. Oakley Jr., visiting professor of epidemiology at Emory University, "we know that if we tell people to eat right, almost no one will do it." Even if your diet is good, many studies suggest that doses of some micronutrients higher than the RDA can be beneficial.
Take the B-vitamin family, which includes B-6, B-12, and folic acid. "It's anybody's guess if some of the things we attribute to normal aging -- such as declining mental function or poor memory -- are due to that, or to deficiencies in some of the B vitamins," explains Latson. Higher levels of B vitamins can lower blood levels of homocysteine, which in turn has been linked to heart disease. And the Centers for Disease Control & Prevention recommends that every woman who might become pregnant take 400 micrograms a day of folic acid (found in fruits, spinach, lentils, and other foods) to prevent birth defects.DECLINING BIRTH DEFECTS
Indeed, public-health officials were alarmed enough about folic acid deficiencies in the late 1990s that they required flour to be enriched with the vitamin. As a result, the number of "neural tube" birth defects has declined. So too, have deaths from heart attacks and strokes, says Emory's Oakley. He's convinced that higher levels of fortification, or taking more supplements, would make many of us healthier. However, "there may be trade-offs," warns Cornell's Garza: Consuming higher levels may stimulate tumor growth in individuals or cause other adverse effects.
Lately much attention has focused on vitamin D. Humans produce plenty of it in their own skin -- but only by being out in the sun for at least 15 minutes a day. "The general population does not realize that if you're not getting enough sun, you have to take supplements," says University of Michigan pharmacologist Robert U. Simpson. The vitamin is vital for preventing rickets and bone disorders. But Simpson and other researchers have discovered vitamin D receptors in the heart and other tissues, suggesting a cardiovascular benefit. Simpson himself takes 800 international units (IU) of vitamin D a day during winter. Nutritionists say it's time to rethink the official recommendations. "There is a great deal of enthusiasm for enhancing the amount of vitamin D," says John Erdman of the University of Illinois and chair of the Institute of Medicine's standing committee for evaluation of "dietary reference intakes."
In other cases, scientists don't know enough to set a recommended daily allowance. One example: chromium. The average American every day gets 20 to 35 micrograms of this element, which is found at low levels in some fruits and vegetables, such as broccoli. That's not nearly enough, believes Agriculture Dept. nutritionist Richard Anderson. Because chromium makes insulin more effective, higher levels of the mineral can help prevent or control diabetes. The same with cardiovascular disease, says Anderson. He recommends a diet with more complex carbohydrates. "As insurance, you should take [a supplement of] 200 to 250 micrograms of chromium per day," he suggests.
Some of the doctors' recommendations are controversial, and almost none of them is definitive. What to do when even the experts disagree? The first priority is to strive for a better diet. If you add supplements, don't take levels more than a few times the RDA. For maximum safety, stay well below the upper levels -- the highest amounts thought to cause no harm -- estimated by the committees that make the recommendations. You can find links to all these levels, as well as to the full reports explaining dietary reference intakes, at www.nal.usda.gov/fnic/etext/000105.html. For most essential substances, aiming for 100% of the RDA should assure good nutrition -- and peace of mind. By John Carey