One-third of adults in the United States supplement their diets with multivitamins, according to the American Dietetic Association. While taking a multivitamin may help ensure that you are meeting your nutrient needs, it can also increase your intake above recommended safe levels. Nutrients most often consumed in excess by multivitamin users include vitamin A, niacin, zinc and iron. Understanding the potential side effects of high doses of these nutrients can help you determine how a multivitamin fits into your diet plan.
Vitamin A
Vitamin A is a fat-soluble vitamin, which means any amount consumed in excess is stored in your liver. Consuming too much vitamin A leads to hypervitaminosis A, or high storage levels of vitamin A, which is toxic and can cause abnormalities in the liver, central nervous system disorders and reduced bone mineralization. High intakes of vitamin A from taking too many multivitamins can cause hypervitaminosis A. Keeping your vitamin A intake within safe levels prevents toxicities. Adult men over the age of 19 need 3,000 IU of vitamin A a day, and adult women 2,310 IU. The safe upper tolerable limit of vitamin A for both men and women is 10,000 IU.
Niacin
Niacin is a water-soluble vitamin, so it is not stored in the body. However, it is one of the vitamins most often consumed in excess by people who take multivitamins. Taking high doses of niacin causes flushing, itching, nausea and vomiting. It can also cause damage to your liver, leading to jaundice. Adult men need 16 mg of niacin a day, and adult women 14 mg. The safe tolerable upper limit is 35 mg a day.
Iron
While iron deficiency anemia is the most common nutritional disorder in the world, people who regularly take multivitamins may exceed their needs. Iron toxicity occurs because very little iron is excreted by the body, allowing it to accumulate in tissues and organs. Excessive doses of iron are especially concerning in children because it can cause death. Toxic iron levels in adults can cause nausea, vomiting, diarrhea or constipation. Adult men and women over age 51 need 8 mg of iron a day, and women between 19 and 50 years need 18 mg of iron a day. Iron needs in children vary depending on age, and range from 7 to 15 mg a day. Safe upper tolerable limits for iron in adults are 45 mg, and in children 40 mg.
Zinc
High intakes of zinc from taking too many multivitamins can also lead to toxicities. Acute toxic episodes cause nausea, vomiting, loss of appetite, diarrhea and headaches. Chronic zinc toxicity causes a decrease in copper levels, altered iron function, a decrease in immune health and a decrease in HDL, the good cholesterol. Healthy adult men need 11 mg of zinc a day, and women 8 mg. Safe upper tolerable limits for men and women are 40 mg.
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