More than 50 percent of Americans take some type of supplement, according to a 2009 article published in "U.S. News and World Report." However, some of these individuals exceed the upper limit for one or more nutrients. This can cause problems because some vitamins and minerals compete for absorption or inhibit the absorption of others.
Vitamin and mineral supplements come in two main varieties: multivitamin and mineral supplements that contain most or all of the necessary vitamins and minerals people need to stay healthy, and single vitamin or mineral supplements that provide large amounts of just one nutrient.
Many people take a multivitamin as a "safety net" in case they do not get enough of some vitamins through their daily diet. Others take specific vitamins, either as recommended by a doctor, as in the case of iron for those who are anemic, or through self-medicating in the hopes of lessening their chances of getting ill after hearing about the benefits of a particular vitamin or mineral. One example is calcium and vitamin D supplements for those wanting to prevent osteoporosis. However, mega-doses of specific vitamins and minerals can be toxic or interfere with the absorption of others, so you should discuss your supplement use with your doctor.
Competing for Absorption
There are a number of vitamin-mineral and mineral-mineral interactions that can cause problems for those who take high-dose supplements. Vitamin C can inhibit copper absorption, and too much copper can lead to vitamin C deficiency. Zinc inhibits both copper and iron; magnesium, copper, iron and calcium all compete for absorption, so too much of one can lead to low blood levels of the others.
Multivitamins and mineral supplements are a good way to get balanced amounts of vitamins and minerals because most supplements are formulated to take interactions into account. "Perspectives in Nutrition" recommends taking a supplement that doesn't provide more than 100 percent of the Daily Value, or DV, for any nutrient.
When taking supplements of individual vitamins and minerals, such as zinc, Dr. Jeremy Kaslow of DrKaslow.com recommends also taking supplements of complementary vitamins and minerals so the levels of each are kept in the right proportions. A doctor or registered dietitian can help you figure out the right amounts, as well as whether you need the supplement in the first place.