Too much of a good thing? Toxic effects of vitamin and mineral supplements

Reason for posting: Vitamin deficiency syndromes are uncommon in Western countries, but many patients consume over-the-counter vitamin and mineral supplements with the hope of improving their health and preventing disease.¹ Although the benefits of vitamin and mineral supplementation are commonly highlighted in both the professional and lay literature, their harmful effects often receive little attention. A recent review of 36 vitamins and minerals by the UK Food Standards Agency discussed the potential harms that can come from supplementation with some of these agents.²

The agents: For otherwise healthy individuals, daily consumption of a multivitamin is often recommended to round out a well-balanced diet.^3,4,5 Although some specific supplements are routinely recommended for disease prevention (e.g., folic acid for women of child-bearing age to prevent neural tube defects),⁶ it is beyond the scope of this column to review the potential benefits of vitamin and mineral supplementation.

Scientific bodies around the world regularly review and recommend the daily vitamin and mineral intake levels (now expressed as Dietary Reference Intakes), taking into account age, sex, physiologic status (e.g., pregnancy) and concurrent disease states. Recommended intake levels are summarized in the purple pages of the Compendium of Pharmaceuticals and Specialties.⁷ Consumption of doses at or near the recommended levels (as is often, but not necessarily, the case for multivitamins⁸) is unlikely to cause harm, and some vitamins, such as thiamine and riboflavin, are relatively benign even at doses several times their recommended levels. However, high doses of some vitamins, especially when taken regularly, can be toxic (Table 1).

Table 1

Minerals taken as supplements can also be toxic. For example, magnesium can cause diarrhea at doses above 400 mg/d; phosphorus can cause diarrhea at doses above 750 mg/d, and mild nausea and vomiting at lower doses; iron can cause constipation, nausea and vomiting, reduced zinc uptake, and iron overload in hemochromatosis; zinc can cause nausea and vomiting, immunosuppression and impaired copper uptake; and selenium at doses above 0.91 mg/d can cause brittle hair and nails, peripheral neuropathies and gastrointestinal upset.^1,2

What to do: Regular screening enquiries about the use and dose of vitamin and mineral supplements may help to optimize a patient's nutrient intake and avoid potential harm from inappropriate use of supplements (e.g., intake of beta carotene by smokers may increase the risk of lung cancer¹). The European Union recently moved to adopt strict labelling standards for vitamin and mineral supplements, including having manufacturers list the percentage of a person's daily intake that is represented by one dose of the product, as well as toxicity warnings when they are appropriate.¹⁰ It is unknown whether Canada will adopt similar standards.

Eric Wooltorton CMAJ