Eric Wooltorton
CMAJ
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 multivitamins 8) 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).
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 cancer
1). 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
References
- Fairfield KM, Fletcher RH. Vitamins for chronic disease prevention.
JAMA 2002;287:3116-26.
[Abstract/Free Full Text]
- Food Standards Agency. Vitamins and minerals A-Z. London (UK): Food
Standards Agency. Available:
(accessed 2003 June 13).
- 3. Oakley GP. Eat right and take a multivitamin. N Engl J
Med1998;338:1060-1.
[Free Full Text]
- Fawzi W, Stamper MJ. A role for multivitamins in infection? Ann Intern Med 2003;138:430-1.
[Medline]
- Willett WC, Stampfer MJ. What vitamins should I be taking doctor? N
Engl J Med 2001; 345:1819-24.
[Free Full Text]
- Kadir RA, Economides DL. Neural tube defects and periconceptional
folic acid. CMAJ 2002;167 (3): 255-6.
[Free Full Text]
- Nutrient requirements. In: Compendium of pharmaceuticals and
specialties. Ottawa: Canadian Pharmacists Association; 2003. p. L81-6.
- Jacobson MF. Vitamin confusion. Nutrition Action Healthletter
[Washington: Centre for Science in the Public Interest] 2003;30(Jan/Feb).
- Vieth R, Fraser D. Vitamin D insufficiency: no recommended dietary
allowance exists for this nutrient. CMAJ 2002;166(12):1541-2.
[Free Full Text]
- Warning on vitamin use. BBC News World Edition Aug 30, 2002
Available: (accessed 2003
June 13.)
