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Too Much Can Cause Harm

Exceeding the government set tolerable upper limits can be a problem. "There is a reason for the tolerable upper limits that needs to be respected. Research has shown at which levels nutrients can cause potential problems, and these numbers take into account all sources of vitamins and minerals from food, fortified food and supplements," says Frechman.

When the level goes beyond the safe upper limits, vitamins can act like drugs, says Roberta Anding, MS, RD, spokesperson for the American Dietetic Association, director of sports nutrition at Texas Children’s Hospital, and dietitian for the Houston Texans pro football team. Excessive calcium intake, more than 2,500 mg a day, can interfere with kidney function, cause kidney stones and constipation, and interfere with the absorption of iron and zinc. One 2,500 mg calcium supplement is like drinking eight glasses of milk, which goes beyond the reasonable food level, explains Anding.

The Risks of Fortified Foods Combined With Supplements

Fortified foods are another way people get additional nutrients. Historically, fortified foods were the way Americans filled some nutrient voids. Public health concerns over nutrient deficiencies led to production practices like adding iodine to salt, grains enriched with B vitamins and iron, and milk fortified with vitamins A and D.

But the combination of whole foods, supplements, and fortified foods raises safety concerns with experts. Eating fortified foods while also taking supplements can cause a person's diet to exceed safe upper levels and potentially lead to a toxic buildup.

Six Brazil nuts, which weigh about 1 ounce, contain 544 micrograms of selenium, says Frechman. That's a whopping 780% of the Daily Value of this trace mineral, which is only needed in small amounts.

The bottom line to remember is that if you are taking supplements, know the tolerable upper limit of the vitamins and minerals you're taking, and check all labels to make sure your food choices are not enriched with the same nutrients. Your best option is to consult a doctor or dietitian before you begin taking any supplement.