Regular consumption will probably do you no good, with a
few exceptions
Some 20-30% of the population in developed countries take a daily vitamin
supplement. Does it do them any good? Our currentestimates of
vitamin requirements are based on the amounts neededto prevent
deficiency diseases; in most countries deficiency isno longer a
major problem. The question is whether higher levelsof intake
provide health benefits. There are two ways to answerthis question:
to identify biomarkers of optimum nutritional status,rather than the
absence of deficiency; or epidemiological studiesto identify
nutrients associated with a lower incidence of chronicdiseases,
followed by intervention studies. Neither approach hasyet provided
satisfactory answers, and a recent review finds littleconvincing
evidence in favour of supplements.1
A number of promising suggestions for biomarkers exist, including metabolic
markers of damage from radicals, immune responses,and damage to DNA.
None is responsive to only a single nutrient,and all are affected by
a plethora of non-nutritional factors. 23 To date we do not have any markers that can be
used to determineoptimumintakes.
The epidemiological approach has prompted a number of intervention trials,
most of which have been disappointing. There isclear epidemiological
evidence that people with a high plasmaconcentration of vitamin E
are less at risk from cardiovasculardisease. The Cambridge heart
antioxidant study showed a reductionin non-fatal but not in fatal
myocardial infarctions.4 Whilethe
benefits from reducing non-fatal infarctions are obvious,this is
hardly convincing evidence of the benefits of vitaminE
supplementation.
Similarly, there is evidence that high intakes of
carotene are associated with
lower incidence of lung, prostate, and othercancers, although
carotene may simply be a marker
of fruit andvegetable consumption. Carotenes are antioxidants and
might beexpected to reduce the damage from radicals that underlies
thedevelopment of cancer and cardiovascular disease. However, mostcompounds that act as antioxidants do so by forming stable radicalsthat persist long enough to undergo metabolism to non-radical
compounds. By definition they therefore form radicals that can
penetrate deeper into tissues and plasma lipoproteins, and potentiallycause more damage than the oxygen radicals they have replaced.
The results of two major intervention studies with
carotene,one in
Finland among smokers and the other in the United Statesamong people
who had been exposed to asbestos, yielded unexpectedand unwanted
results: more people receiving the supposedly protectivesupplements
died from lung (and other) cancer than people receivingplacebo.
56
Vitamin C is an antioxidant, and it also inhibits the formation of
carcinogenic nitrosamines from dietary amines and nitrites.It might
therefore be expected to have protective action againstthe
development of cancer and cardiovascular disease. The evidencewith
respect to cardiovascular disease is unconvincing.1
Theepidemiological evidence linking a high intake of vitamin C withreduced cancer incidence is confounded by the fact that the fruitsand vegetables that are sources of vitamin C are also rich ina
variety of other compounds that may be protective. There isa long
held belief that vitamin C (perhaps in very large amounts)is
protective against the common cold. A systematic review didnot
support this but did find some evidence of a modest benefitin
reducing the duration of symptoms of colds.7
An intake of vitamin D above what can be obtained from normal diets (possibly
in combination with supplementary calcium) delaysthe loss of bone
with increasing age, so supplements may be advisableto prevent
osteoporosis and osteomalacia.8 For most people,increased exposure to sunlight is probably more effective than
supplements, although we may have to balance the beneficial effects
on bone health against the increased risk of skincancer.
The benefits of folic acid supplements taken periconceptually in preventing
neural tube defect have been shown convincingly.9High intakes of folic acid also reduce plasma homocysteine, a
risk factor for cardiovascular disease independent of plasma lipids
and other risk factors, and low intakes of folic acid are associated
with increased risk of colorectal cancer. 1011 This has ledto mandatory
fortification of cereal products in the United Statesand elsewhere.
However, although folic acid lowers plasma homocysteine,there is no
evidence yet from controlled trials whether or notthis will reduce
cardiovascular disease or cancer. Until the resultsof intervention
trials in progress are available, the benefitsof folic acid
supplements other than to prevent neural tube defectsremain
unproved.12
The answer to the question of whether we should take a multivitamin tablet
every day must be that unless our intake is inadequateas a result of
a poor diet then supplements will probably do usno goodapart
from folic acid taken periconceptually and, possibly,vitamin D by
elderlypeople.
David A Bender, senior lecturer in biochemistry.
Department of Biochemistry and Molecular Biology, University College London,
London WC1E 6BT
Stephens NG, Parsons A, Schofield PM, Kelly F, Cheeseman K,
Mitchinson MJ. Randomised controlled trial of vitamin E in patients with
coronary disease: Cambridge heart antioxidant study (CHAOS). Lancet
1996; 347: 781-786[Medline].
Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study
Group. The effect of vitamin E and beta carotene on the incidence of lung
and other cancers in male smokers. N Engl J Med 1994; 330: 1029-1035[Abstract/Full
Text].
Omenn GS, Goodman GE, Thornquist MD, Balmes J, Cullen MR,
Glass A, et al. Effects of a combination of beta carotene and vitamin A on
lung cancer and cardiovascular disease. N Engl J Med 1996; 334:
1150-1155[Abstract/Full
Text].
Meleady R, Graham I. Plasma homocysteine as a
cardiovascular risk factor: causal, consequential, or of no consequence?
Nutr Rev 1999; 57: 299-305[Medline].
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-- Albert Einstein, letter to a friend, 1901
"I know of no safe depository of the ultimate powers of the society but the people themselves, and if we think them not enlightened enough to exercise control with a wholesome discretion, the remedy is not to take it from them, but to inform their discretion by education."
-- Thomas Jefferson, letter to William C. Jarvis, September 28, 1820
"What's the point of vaccination if it doesn't protect you from the unvaccinated?"
-- Sandy Gottstein
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