Harvard Women's Health Watch

Response to readers: What is the upper limit of magnesium intake?

Response to readers

What is the upper limit of magnesium intake?

A reader wrote in with a question about the mineral table on page 7 in the May 2003 Harvard Women's Health Watch. She noticed that, in some cases, the recommended amount of magnesium seemed to exceed the nutrient's safe level of intake. She asked, "What should the tolerable upper limit of magnesium really be? Many women take it with their calcium, so it is important for us to know."

The Dietary Reference Intakes (DRIs) for minerals that we published in May 2003 (and the ones for vitamins in the April 2003 issue) were developed by the Institute of Medicine, which advises the government on health policy. DRIs reflect the latest research on the links between essential nutrients and human health. One of the DRIs, Recommended Dietary Allowance (RDA), is the average daily amount considered sufficient to meet the nutrient needs of 97.5% of healthy people. Another DRI, the Tolerable Upper Intake Level (UL), is the maximum daily amount unlikely to cause adverse health effects in otherwise healthy people.

For women, the magnesium RDA varies with age and pregnancy status. The RDA is 310 mg/day for women ages 19–30 (350 mg/day for those who are pregnant); for women over 30, the RDA is 320 mg/day (360 mg/day for those who are pregnant). The UL for all women is 350 mg/day, which seems to conflict with the pregnancy recommendations. But the magnesium UL applies only to intake from supplements, not to intake from food. In other words, you can eat as much magnesium as you want in food, but you should watch your intake from nonfood sources. Too much daily magnesium from pills or supplements—it's also in some laxatives and antacids—can cause stomach cramps and diarrhea.

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