Between supplements and fortified foods, it’s not difficult to overdose on vitamins and minerals.
About half of American adults take vitamin or mineral supplements in an effort to ward off chronic conditions or offset a possible deficiency, a strategy that may benefit certain individuals. For example, the elderly are advised to take supplements of calcium and vitamin D to help prevent bone loss because the aging body manufactures less vitamin D, as well as a vitamin B12 supplement. Women of childbearing age are advised to consume an additional supplement of 400 micrograms (mcg) of folic acid per day.
However, while a daily multivitamin pill can help fill any nutritional gaps in your diet, certain vitamins and minerals may be hazardous when taken over time in high doses. Not only can high doses of individual supplements or doubling up on multivitamins prove dangerous, but foods that are fortified with vitamins and minerals—such as cereals, breads, and some snack products—can be harmful when eaten in large amounts, especially if you are already taking a supplement.
For example, iron is often added to cereals and grains. This can pose a problem for individuals who are at risk for the genetic disorder hemochromatosis (a disease of iron metabolism). So before taking a supplement, check with your physician about the proper dosage and be sure to inform him or her about other supplements or medication you are taking. Fortunately, it’s difficult, if not impossible, to consume unsafe levels of vitamins and minerals through unfortified foods.
To make sure you’re taking supplements that have been tested by the U.S. Pharmacopeia for strength and purity, look for “USP” on the label.
Defining Safety Limits
The Food and Nutrition Board of the National Academies of Science developed the Dietary Reference Intakes (DRIs) in 1995 to reflect evolving scientific knowledge about vitamins and minerals. DRIs encompass three values for nutrients: the Estimated Average Requirement (EAR), the Recommended Dietary Allowance (RDA), and the Tolerable Upper Intake Level (UL).
The EAR specifies the value that meets the needs of half the people in a specified group, while the RDA is designed to meet the needs of nearly everyone in a group. Both of these are below the level at which toxicity may occur, whereas the UL represents the maximum amount designated to be safe for most people.
Taking vitamins and minerals in excess of ULs can cause adverse effects ranging from the relatively benign, such as diarrhea with high vitamin C intake, to serious, such as liver damage with high vitamin A intake. Excessive intake usually must continue for several months at high levels before such reactions are apparent, but it is important not to consistently exceed the UL.
Below are some nutrients that can have serious health consequences when taken in high doses.
Vitamin A
Too much vitamin A may interfere with bone formation and increase the risk of fracture. A study of 72,000 postmenopausal women published in the Journal of the American Medical Association in 2002 found that those with a high intake of vitamin A from retinol were at increased risk for hip fractures; vitamin A from beta-carotene did not affect fracture risk.
More recently, a study of more than 2,000 older men published in The New England Journal of Medicine in 2003 linked high blood levels of vitamin A with fractures. High levels of vitamin A are also associated with birth defects and liver problems.
Men should aim for 900 mcg (3,000 IU) of vitamin A per day and women 700 mcg (2,333 IU) a day. Nobody should consume more than the upper limit of 3,000 mcg (10,000 IU) daily, the highest level likely not to produce adverse effects.
Few Americans need vitamin A supplements, but if you take multivitamins that contain vitamin A, at least 20% should be in the form of beta-carotene.
Folate
The concern with too much folate, which is found in fortified breads and cereals as well as in leafy green vegetables, is how it interacts with vitamin B12. High levels of folate can mask symptoms of pernicious anemia, a blood abnormality caused by a B12 deficiency, and make it more difficult to diagnose the disorder. B12 deficiency surfaces frequently after middle age.
People with insufficient levels of vitamin B12 may experience generalized symptoms of anemia, such as fatigue, pallor, headaches, and shortness of breath during physical exertion, in addition to symptoms more specific to B12 deficiency, including alternating bouts of constipation and diarrhea, weight loss, numbness and tingling in the hands and feet, burning of the tongue, poor balance, confusion, depression, and dementia. Undetected, advanced vitamin B12 deficiency may result in permanent nerve damage.
Adults should aim for 400 mcg of folate daily. Although the UL is 1,000 mg, people should be aware that getting more than 400 mcg could mask pernicious anemia in individuals with B12 deficiency. To avoid the risk of pernicious anemia, people who take multivitamins should also take a daily supplement containing 1,000 mcg (1 mg) of B12.
Vitamin D
Unlike other vitamins and minerals, the body can manufacture vitamin D in response to small amounts of sunlight. Other sources of vitamin D include fortified dairy products and oily fish, such as mackerel, salmon, and sardines.
Amounts of vitamin D in excess of the UL can cause nausea and vomiting, poor appetite, constipation, and weight loss. Megadoses of vitamin D can also lead to high blood calcium levels, which can cause mental changes and nausea and increase the risk of kidney stones.
Moreover, a 2004 study published in the International Journal of Cancer examined the rates of prostate cancer among nearly 200,000 Scandinavian, middle-aged men. Researchers found that men with both the lowest and the highest blood levels of vitamin D had an increased risk of prostate cancer.
Previously, it had been thought that only low serum levels of vitamin D posed a risk, leading some scientists to believe vitamin D supplementation would reduce the risk of prostate cancer. In this study, however, researchers speculated that taking vitamin D supplements for a long period of time might increase the risk of prostate cancer. This conflicting information highlights the fact that doctors and researchers don’t yet know all of the potential adverse effects of vitamin and mineral supplementation.
Vitamin D deficiency is common in adults; those 50 and older should get 400 IU of vitamin D through food and/or supplements while those 70 and older require 600 IU. The UL for vitamin D is set at 2,000 IU.
Iron
Most men and postmenopausal women have adequate iron stores and do not need supplemental iron. But excess iron—from iron-enriched breads and cereals, supplements, or foods high in iron, such as liver and red meat—is a potential problem for people with hemochromatosis, an often undiagnosed genetic disorder that affects one of every 200 to 300 people. Hemochromatosis may lead to diabetes, liver disease, heart failure, and impotence.
Adults over 50 should aim to consume 8 mg of iron daily. As with all nutrients, the upper limit for iron is set at the amount that would likely pose no risk for most people (45 mg). However, even this amount is likely too much for people at risk for hemochromatosis.
