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Taking Too Many Vitamins? Side Effects of Vitamin Overdosing

WebMD explains why taking too many vitamins can be unhealthy and even dangerous and how to be sure you’re getting the right amount of vitamins and minerals.

Nowadays, everything from bottled water to orange juice seems to have souped-up levels of vitamins and minerals in it. That may sound like a way to help cover your nutritional bases, especially if your diet is less than stellar.

But are you in danger of getting too much of these important nutrients? And can these overloads hurt you?

Yes, if you're routinely taking megadoses. For instance, too much vitamin C or zinc could cause nausea, diarrhea, and stomach cramps. Too much selenium could lead to problems including hair loss, gastrointestinal upset, fatigue, and mild nerve damage.

Most people aren't getting megadoses. Still, if you eat a fortified cereal at breakfast, grab an energy bar between meals, have enriched pasta for dinner, and take a daily multivitamin, you could easily be over the recommended daily intake of a host of nutrients.

When it comes to vitamins and minerals, more is not necessarily better. Here's what you need to know to avoid overdoing it.

Look Beyond Your Plate

Chances are, the unfortified foods you eat aren't a problem. "It's pretty hard to overdo it from food alone," says Johanna Dwyer, DSc, RD, a senior research scientist with the National Institutes of Health's Office of Dietary Supplements.

A few rare cases in medical journals have described, for example, an overload of vitamin A in a person who ate polar bear liver, a meat with extremely high amounts of this vitamin. Signs of a surplus of vitamin A may include nausea, blurred vision, and dizziness.

And if you eat handfuls of Brazil nuts every day, you could be way over the Tolerable Upper Intake Limit (the maximum per day that is unlikely to cause harm, as determined by the Institute of Medicine) for selenium. Just one ounce of Brazil nuts contains 544 micrograms of selenium. The Tolerable Upper Intake Limit is 400 micrograms per day -- and less if you're younger than 14.

Since polar bear liver and sacks of Brazil nuts are probably not on your menu, you'll want to think about the supplements you take and fortified foods or drinks.

Supplements: Check the Dose

"Most people don't realize there's no real advantage to taking more than the recommended amounts of vitamins and minerals, and they don't recognize there may be disadvantages," Dwyer says.

"If you're taking a supplement, stick to one that's no more than the Daily Value," Dwyer says.

Talk with your doctor about any supplements you're taking, including vitamins and minerals, and the dose you're taking, too. That way, your doctor can help you keep doses in a safe range.

"If you're taking a basic multivitamin, there's no need to fear taking too much," says Andrew Shao, PhD, senior vice president of scientific and regulatory affairs for the Council for Responsible Nutrition, a trade group for the supplements industry.

"Most multivitamins have such a wide margin of safety that even when you're combining them with fortified foods, it's still not going to cause you to keel over," Shao says.

Subtle Signs You're Getting Too Much

"I have not seen someone off the street who was taking a toxic level of vitamin A or D -- those are very unusual," says David Katz, MD, MPH, director of the Yale University Prevention Research Center in New Haven, Conn., whose medical practice specializes in nutrition. "What I'm more likely to see is a person with a dosing level of supplements that's higher than optimal."

Scientists don't yet know if routinely getting a little bit too much of a vitamin or mineral (as opposed to a megadose) is a problem, Katz says.

"There might be hints of concern, but they would be very subtle signs," he says.

These fairly mild symptoms may include difficulty sleeping or concentrating, nerve problems such as numbness or tingling, or feeling more irritable -- depending on the nutrient that's going overboard.

Katz tells WebMD that a bigger concern is that we're "garnishing the food supply with overfortification."

He says manufacturers have shifted their focus from what they've taken out of food -- such as its fat, sugar, or salt -- to what they're putting in, whether it's vitamin D, probiotics, or omega-3 fats -- whatever nutrient is in vogue.

"When more and more foods are enhanced, it becomes impossible for consumers to know what dose they're getting over the course of a day," says Katz. "Clinicians have to realize we might be introducing new dietary imbalances because of this practice."

Three Nutrients to Watch

Dwyer says vitamin D, calcium, and folic acid are three nutrients you may get too much of through a combination of food and supplements.

Adults who regularly far exceed the 4,000 international units (IUs) daily safe upper limit for vitamin D might be setting themselves up for kidney stones down the road -- a health problem that may also occur with excessive intake of calcium, whose upper limit range is 2,000-2,500 milligrams daily.

Folic acid is added to enriched grain products -- white flours, pasta, rice, breads, and cereals -- to help prevent birth defects in babies due to folic acid deficiency in pregnant women..

While folic acid fortification has successfully cut the number of birth defects by 25% to 50%, it might have created other health concerns in people getting too much. (There's no need to worry about foods naturally rich in folate.)

It's not that hard to get more than 1,000 micrograms of folic acid a day (the safe upper limit for adults) from fortified foods and supplements on a regular basis. Doing so might hide the signs of a vitamin B12 deficiency in older adults. Vitamin B12 deficiency can sometimes lead to permanent nerve damage if left untreated.

What's more, some recent studies have hinted that high levels of folic acid may be linked with a greater risk for lung and prostate cancers. These studies do not prove cause and effect, however.

"Most people can now get enough folic acid without having to rely on supplements," Dwyer says.

In fact, she says, "most people have no problem [with getting too much vitamins or minerals] if they start with food, which is the healthiest and safest way to get them."

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