Article
 The
role of multivitamins and minerals in preventive healthcare
by Dr. Joe Schwarcz, Director, McGill
Office for Science and Society
Introduction
More than 250 years have passed since James Lind published
his classic "Treatise on Scurvy," and it's been almost 100
years since Casimir Funk isolated the first vitamin, thiamine.
Since then, thousands of research papers have been published
on vitamins, and 12 Nobel Prizes have been awarded for research
in the field. In spite of all this activity, and in spite
of the great popularity of dietary supplements with consumers,
general agreement on the benefits of vitamin and mineral
supplementation is difficult to reach. Some of the main
questions today regard dosing, possible risks, different
needs for different population groups and the efficacy of
multivitamins in the promotion of health and prevention
of chronic disease.
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Examples of
randomized controlled trials on multivitamins
Linxian General Population
Trial4
- population: 30,000 men and women
aged 40-69, in the Linxian province of China
- health: unusually high cancer rates
- intervention: various combinations
of vitamin A (10,000 IU), zinc oxide (45 mg), riboflavin
(5.2 mg), niacin (40 mg), vitamin C (180 mg), beta-carotene
(25,000 IU), selenium (50 µg) and vitamin E (60
mg)
- results: only the combination of
beta-carotene, vitamin E and selenium showed any
benefit, with a 9% decrease in death rate after
5 years
- shortfalls: diet low in fruits, vitamin
blood levels low by Western standards, so that intervention
may have just corrected subnormal intake
Supplementation en Vitamines
Et Mineraux Antioxydants5
- population: 12,000 French men and
women
- intervention: vitamin C (120 mg),
vitamin E (30 mg), beta-carotene (10,000 IU), selenium
(100 µg) and zinc (20 mg) supplement over 8 years
- results: small reduction in cancer
incidence in men but not in women
- shortfalls: reduced prostate cancer
risk in men who had normal prostate-specific antigen
levels (PSA) at the beginning of the study, but
increased risk in men who had a higher PSA at outset
Age-Related Eye Disease Study
(AREDS)6
- population: 4,600 American men and
women with some degree of macular degeneration
- intervention: supplement containing
vitamins C (500 mg) and E (400 IU), beta-carotene
(25,000 IU), zinc (80 mg) and copper (2 mg)
- results: reduction in the progression
of age-related macular degeneration
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Roughly half the population takes dietary
supplements, with multivitamins making up the single largest
class of such products. Some take them with the belief that
they improve health or prevent chronic disease, others take
them as nutritional "insurance." Are these views justified?
What advice can be given to patients who ask about multivitamins?
When is it appropriate to recommend their use?
This article will first examine some of the
difficulties involved in making recommendations about multivitamin
use. We will then look at where clear benefits have been
established for specific populations and where they have
not. We will also explore the possibility of preventing
heart disease and cancer with multivitamins and, finally,
the risks of multivitamin use will be examined along with
potentially harmful interactions with drugs.
Studying the
effects of multivitamins
Unfortunately, there's no clear definition of what
constitutes a multivitamin supplement. Attempts to investigate
the benefits of such products are complicated by variations
in the combinations and doses of individual micronutrients
as well as by individual patient variables. For example,
multivitamin takers are more likely to engage in proactive
health behaviour, a confounding factor in observational
studies which make up the majority of studies dealing with
multivitamins.
In an observational study, subjects are not
assigned a particular intervention, such as taking vitamins,
but are questioned on self-administered choices. For example,
the Nurses' Health Study, administered by the Harvard University
School of Public Health, has followed the health status
of over 100,000 nurses since 1980, when they were first
asked to fill out questionnaires about diet and supplements.
By 1998, multivitamin use, and especially folic acid content,
was found to be associated with a significantly reduced
risk of colon cancer.1 Such associations, however, can't
tease out other lifestyle factors, and they can't prove
cause and effect.
Randomized placebo-controlled trials using
single nutrients or simple combinations of nutrients haven't
always borne out the results suggested by observational
studies. Beta-carotene is a case in point. Several observational
studies had linked an increased beta-carotene intake with
a reduced risk of cancer -- not surprising, given that the
nutrient is both an antioxidant and a vitamin A precursor.
Inspired by such observational studies, a randomized controlled
trial using supplements was designed.2 Shockingly, instead
of offering any benefit, the supplements increased the risk
of lung cancer, albeit only among smokers and asbestos workers.
This finding was significant enough to justify the current
recommendation that smokers avoid beta-carotene supplements,
but not beta-carotene rich foods.
On the other hand, observational studies that
suggested a reduced risk of birth defects with an adequate
intake of folic acid have been corroborated by randomized
controlled trials. The results were impressive enough to
support the general recommendation that all pregnant women
supplement their diet with 400 mg of folic acid daily, and
for governments to launch folate fortification programs
for wheat products. This program has worked remarkably well,
with birth defects attributed to folic acid deficiency now
significantly reduced.
Only a few randomized controlled trials have
been carried out using multivitamins (see Table),
but because of confounding factors, their relevance for
Western populations is unclear.
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