However, while part of getting all the nutrients you need sometimes means taking a vitamin and mineral supplement, experts warn there can be too much of a good thing.

"Unless you are suffering from a severe deficiency, if you have a disease or disorder that is causing you to be nutrient-deficient, the most you are going to need is a high quality multivitamin supplement, to be taken as insurance, and not used as your main source of nutrients," says Laurie Tansman, a clinical nutritionist at the Mount Sinai Medical Center in New York City.

Most people won't get into trouble taking a high-potency multivitamin. But problems can occur, experts say, when you dabble in individual mega-dosing -- excess amounts of certain vitamins or minerals that are recommended by some diet and fitness programs.

Among those causing the most concern are the fat-soluble vitamins A, D and E. Because they can be stored for long periods of time in the body -- mostly the liver -- a cumulative buildup can occur, turning your vitamin regimen from healthy to toxic.

"Vitamin D is one of the most toxic supplements there is, and in extreme cases and in very large amounts can lead to liver and kidney failure," says Jyni Holland, a clinical nutritionist at New York University Medical Center.

Take too much vitamin A, she says, and you could end up with hair loss, nausea, vomiting and significant joint pain.

Tansman says, "If you are pregnant, there is evidence to show that too much vitamin A may also cause birth defects. So you never want to take more than what your prenatal formula contains."

Likewise, too much vitamin E can thin the blood so much, says Holland, it could lead to internal hemorrhaging, particularly if you are taking any blood-thinning medication for a cardiac or hypertension problem.

While vitamin K has the reverse effect -- causing blood to clot -- it too can cause problems when used by those taking blood-thinning drugs.

Water-soluble nutrients such as the B vitamins and vitamin C are considered relatively safe, even in high doses, because they don't build up in tissues and rarely reach toxic levels on their own. But again, mega-dosing can produce health problems, experts say.

In very high levels, vitamin B6 has been associated with neurological symptoms such as nerve tingling, while vitamin B3 -- also known as niacin -- could be a problem if you suffer from heart disease.

"Even though B3 is often used as medication to treat high blood pressure, if you have a preexisting condition, particularly heart disease or hypertension, you should not be using mega doses of this or any vitamins unless directed to do so by your doctor," Holland says.

And while the body needs a steady supply of minerals, particularly calcium, in supplement form even this relatively safe mineral can have a toxic side.

"It has been linked to the formation of kidney stones, particularly if you have had this problem in the past," Holland says.

Tansman cautions that too often calcium supplements can be packed with vitamin D, creating the potential for a toxic overdose.

"Vitamin D helps the body absorb calcium, which is why it's often included in mineral supplements. But if you are mega-dosing on calcium, you may also be mega-dosing on vitamin D and setting yourself up for some toxic reactions without even realizing you are doing so," Tansman says.

Equally worrisome is the potential toxicity of overdosing on iron supplements, which are commonly used not only by menstruating women, but sometimes by body builders and athletes seeking to fight fatigue.

"What most people don't realize is iron is an oxidant. And when it's exposed to oxygen inside the body, it becomes a free radical, with much of the same destructive properties of other free radicals we try so hard to avoid," Holland says.

Free radicals are molecules that cause an oxidation process that eventually damages cells and can increase the risk of certain diseases, including cancer.

Holland believes that for those already at risk for cancer -- either due to environmental factors or a strong family history -- an iron overload just might set the stage for the disease process to start. This, she says, is particularly true for all men all the time and for women after menopause, a time when the need for iron drops dramatically and the risk of an overload is higher.

In addition, Tansman warns that because vitamin C enhances iron absorption, taking mega doses of this vitamin while you are taking high doses of iron supplements could increase your risk of a toxic reaction.

Although overdosing on supplements can be easier than you think, what won't harm you, experts say, is taking a multivitamin and eating lots of vitamin-rich foods.

Both Holland and Tansman agree this is still the best way to get all the nutrients you need.

More information

To learn more about vitamins, try this Oregon State University site. The Center for Food Safety and Applied Nutrition offers an overview of dietary supplements.