"I am glad that you alerted me to the danger of "overdosing" with calcium. My wife needs to know this! "
In a study of 2,195 middle-aged people (published last year in the Journal of the American Dietetic Association), some supplement users had intakes of certain nutrients that exceeded tolerable upper intake levels (ULs)—guidelines set by the Institute of Medicine’s (IOM) Food and Nutrition Board to help protect consumers from the potential adverse effects associated with high intakes. Eighteen percent surpassed the tolerable upper intake level for niacin (high doses can cause flushed, itchy skin). Nearly 5 percent exceeded the UL for magnesium (too much from supplemental sources may cause diarrhea). Three percent exceeded the upper limit for vitamin C (excessive amounts may bring about gastrointestinal distress).
Whether adverse effects actually occurred, the scientists aren’t sure, says Sujata L. Archer, Ph.D., R.D., the study’s lead author and a research assistant professor at the Feinberg School of Medicine at Northwestern University in Chicago. “These upper limits were established to help consumers monitor excessive intakes of nutrients. Exceeding them does not always result in adverse effects.”
Yet it can—and does. It’s been shown that too much vitamin D can harm the kidneys, and high doses of vitamin A may damage the liver and nerves. Excessive iron may increase risk of vascular disease and cancer. In high doses, folic acid, a water-soluble B vitamin, may mask the symptoms of a vitamin B12 deficiency—and too little B12 can cause permanent neurological damage.
But unless you’re consuming nutrient doses well beyond the recommended amounts, you’re unlikely to hit upper levels, says Maret Traber, Ph.D., a vitamin E expert and professor of nutrition at Oregon State University in Corvallis: “It’s when people decide to take handfuls of different things, or consume gobs of one nutrient, that they get into trouble.” Indeed, in Archer’s 2005 study in the Journal of the American Dietetic Association, subjects who exceeded upper intake levels took single nutrient supplements—often on top of a multivitamin/mineral.