+15 jaar member
- Lid geworden
- 16 feb 2005
Hoe weet je of wat zijn de bijverschijnselen van teveel vitamines.
de 2 vitamines waar je zeker niet teveel van moet binnekrijge zijn A en E
bij vit D blijkt dit serieus meegevallen en zijn er nog niets dan voordelen gevonden bij "megadoseringen"
ik ken iemand die teveel vitamina A slikte en daardoor is blind geworden
van E weet ik niet
The antioxidant myth: a medical fairy tale
05 August 2006
From New Scientist Print Edition.
CRANBERRY capsules. Green tea extract. Effervescent vitamin C.
Pomegranate concentrate. Beta carotene. Selenium. Grape seed extract.
High-dose vitamin E. Pine bark extract. Bee spit.
You name it, if it's an antioxidant, we'll swallow it by the bucket-
load. According to some estimates around half the adults in the US
take antioxidant pills daily in the belief they promote good health
and stave off disease. We have become antioxidant devotees. But are
they doing us any good? Evidence gathered over the past few years
shows that at best, antioxidant supplements do little or nothing to
benefit our health. At worst, they may even have the opposite effect,
promoting the very problems they are supposed to stamp out.
It's little surprise that antioxidants have acquired a reputation as
miracle health supplements. As long ago as the 1950s, scientists
discovered that many diseases - including heart disease, strokes,
cancer, diabetes, cataracts, arthritis and neurodegenerative
disorders such as Parkinson's and Alzheimer's - were linked to damage
caused by highly destructive chemicals called free radicals.
Free radicals are compounds with unpaired electrons that stabilise
themselves by oxidising other molecules - including proteins,
carbohydrates, lipids and DNA. In the process they often create more
free radicals, sparking off a chain of destruction. Oxidative damage
accompanies most, if not all, diseases and has even been proposed as
a direct cause of some including lung cancer, atherosclerosis and
Free radicals are an unavoidable hazard of being alive. We live in an
oxygen-rich atmosphere, and radicals, particularly reactive oxygen
species (ROS), are natural by-products of respiration. "One per cent
of the oxygen we consume turns into ROS," says biochemist Barry
Halliwell from the National University of Singapore. "It doesn't
sound like much but humans are big animals and we breathe a lot. Over
a year a human body makes 1.7 kilograms of ROS." Exposure to X-rays,
ozone, tobacco smoke, air pollutants, microbial infections,
industrial chemicals and intensive exercise also trigger free radical
In the 1980s, however, a potential weapon against free radical damage
appeared on the horizon. Scientists had known for a long time that
people whose diets are rich in fruits and vegetables have a lower
incidence of heart disease, diabetes, dementia, stroke and certain
types of cancer - the very diseases that are associated with free
radical damage. Now there was an explanation. Fruits and vegetables
are a rich source of antioxidants that can neutralise free radicals
by donating electrons to them.
Green plants are full of antioxidants for good reason. They are
especially vulnerable to oxidative stress since they produce pure
oxygen during photosynthesis. To protect themselves they manufacture
an assortment of potent antioxidants.
And so a hypothesis was born: dietary antioxidants are free-radical
sponges that can stave off the diseases of old age. It was a great
idea. "Putting two and two together, scientists assumed that these
antioxidants were protective, and that taking them as supplements or
in fortified foods should decrease oxidative damage and diminish
disease," says Halliwell, who pioneered research into free radicals
and disease. "It was simple: we said free radicals are bad,
antioxidants are good."
The concept helped spawn a colossal supplements industry. According
to the US National Institutes of Health (NIH), more than half of US
adults take some form of vitamin or mineral supplement at a total
cost of $23 billion a year. The bewildering range of supplements on
the shelves makes it hard to say how much of this expenditure goes on
antioxidants, but the NIH says it is probably a "large proportion".
And their popularity just keeps on growing. SPINS, a market research
firm based in San Francisco, estimates that the antioxidant market
has grown by 18 per cent in the past year alone.
The best known antioxidants are vitamin E (also known by its chemical
name tocopherol), vitamin C, and two broad classes of plant chemicals
called polyphenols (including flavonoids) and carotenoids (including
beta carotene and lycopene). Most supplements touted as antioxidants
contain at least one of these, often as a pure chemical and sometimes
as a concentrated plant extract.
Since the early 1990s scientists have been putting these compounds
through their paces, using double-blind randomised controlled trials -
the gold standard for medical intervention studies. Time and again,
however, the supplements failed to pass the test. True, they knock
the wind out of free radicals in a test tube. But once inside the
human body, they seem strangely powerless. Not only are they bad at
preventing oxidative damage, they can even make things worse. Many
scientists are now concluding that, at best, they are a waste of time
and money. At worst they could be harmful.
The first antioxidant to produce disappointing results was beta
carotene. Once a star among supplements, beta carotene pills were
recommended to smokers to protect them against lung cancer. This was
largely based on the observation, made in the 1970s, that people who
ate a lot of carrots - which contain large quantities of beta
carotene - had some protection against cancer.
In 1992 researchers at the US National Cancer Institute set about
testing beta carotene. They recruited more than 18,000 people at high
risk of developing lung cancer, either because they smoked or had
been exposed to asbestos, and gave around half of them beta carotene
supplements. The trial was supposed to run for six years, but the
researchers pulled the plug two-thirds of the way through after
discovering, to their surprise and horror, that those taking
supplements were faring worse than the controls. Their lung cancer
rate was 28 per cent higher, and the overall death rate was up 17 per
cent. "It was a shock. It not only did no good but had the potential
to do harm," Halliwell says.
The researchers couldn't be sure that these increases were not caused
by chance, and beta carotene capsules are still widely sold as an
antioxidant. Further trials, though, have strengthened the evidence
that beta carotene supplements not only fail to protect people
against cancer but can also increase the risk of lung cancer in
smokers. In May of this year an expert panel convened by the NIH
concluded that there was no evidence to recommend beta carotene
supplements for the general population, and strong evidence to
recommend that smokers avoid it.
It's a similar story with the world's most popular antioxidant.
Vitamin E shot to fame in the early 1990s, after two large studies
involving more than 127,000 people in total found that those with a
diet high in vitamin E were significantly less likely to develop
cardiovascular disease. The first study followed 87,245 female nurses
for eight years; it found that the top 20 per cent with respect to
vitamin E consumption had a 41 per cent lower incidence of
cardiovascular disease than the bottom 20 per cent (New England
Journal of Medicine, vol 328, p 1444). The second study, involving
39,910 male health professionals, found a similar reduction in heart
disease risk (New England Journal of Medicine, vol 328, p 1450).
The researchers, based at Harvard Medical School and Harvard School
of Public Health, even had a plausible mechanism. Evidence was
emerging that one of the causes of heart disease was free radical
damage to LDLs, tiny packages of lipid and protein that circulate in
the bloodstream, delivering fatty acids to cells. It turned out that
adding vitamin E to blood samples in the test tube made LDL more
resistant to oxidation. Perhaps this was how vitamin E prevented
heart disease. "At the biochemical level, the rationale sounded so
good - at that time," says Roland Stocker, a biochemist at the
University of New South Wales in Sydney, Australia.
Use of vitamin E supplements soared. In 1990, almost nobody took
vitamin E; by the end of the decade an estimated 23 million US
citizens were knocking back daily doses.
On the back of these positive results, other researchers set up large
studies using vitamin E supplements. The results, however, have been
almost universally disappointing. Only one experiment - the Cambridge
heart antioxidant study (CHAOS) - found a positive effect, a 77 per
cent reduced risk of heart attack. Several others found no protective
effect and one even concluded that vitamin E increased the risk of
Time for a rethink?
Other trials designed to test whether vitamin E supplements could
prevent cancers, such as the ATBC study in Finland, also came in
negative. Vitamin E also did not halt the progression to Alzheimer's
disease in people with mild cognitive impairment.
What is more, when scientists went looking for evidence that vitamin
E protected LDL against oxidation in the body, not just in the test
tube, they found none - except in people with vitamin E deficiency
(Journal of the American Medical Association, vol 285, p 1178). In
fact, despite good evidence that vitamin E is a powerful antioxidant
in the test tube, there is now serious doubt that it acts the same
way in the body. "Vitamin E is not an antioxidant. In fact it must be
protected against oxidation," says Angelo Azzi, a biochemist at Tufts
University in Boston, Massachusetts. He points out that vitamin E
exists in eight different forms in nature, all of which function as
antioxidants in the test tube. Yet the body only uses one form, alpha
tocopherol, which is pulled out of the bloodstream by a highly
specialised protein in the liver. All the other forms are excreted.
Azzi argues that evolution is unlikely to have gone to such great
lengths simply to obtain an antioxidant from the diet. "There are
millions of antioxidants," he says.
Vitamin E is clearly doing something in the body - it is an essential
part of the diet and deficiency leads to neurological problems - but
whatever it's doing, it's not an antioxidant.
There is even some evidence that vitamin E supplements can be
harmful. Last year, a team led by Edgar Miller of the Johns Hopkins
Medical Institutions in Baltimore made headline news when they
amalgamated the results of 19 separate trials and concluded that high
doses of vitamin E increase overall mortality (Annals of Internal
Medicine, vol 142, p 37) - though this conclusion remains
controversial. "It's flawed," asserts Azzi. "We re-analysed the data
and there is no change in mortality." "Most people agree that there
is no good evidence that large doses are harmful," adds Stocker.
Vitamin C is another disappointment. "People are still trying to
defend it, but you don't get an effect on free radical damage unless
you start with people with a vitamin C deficiency," says
Halliwell. "I think it is a lost cause." In fact, results from a vast
US trial probing the links between diet and health, called the
Women's Health Study, suggest that vitamin C supplements may
accelerate atherosclerosis in some people with diabetes.
One class of antioxidants that remains relatively unresearched is
polyphenols. What little evidence there is comes from epidemiological
studies, some of which suggest that polyphenols prevent disease and
others of which do not. While polyphenols act as antioxidants in the
test tube, it is not clear that they are absorbed into the
bloodstream, and if they are, they are swiftly metabolised. For
example, 95 per cent of a flavonoid called resveratrol - the one
found in red wine - is destroyed by our digestive system before it
The conclusion is becoming clear: whatever is behind the health
benefits of a diet rich in fruits and vegetables, you cannot
reproduce it by taking purified extracts or vitamin
supplements. "Just because a food with a certain compound in it is
beneficial, it does not mean a nutraceutical [with the same compound
in] is," said Paul Coates, who works in the Office of Dietary
Supplements at NIH.
Yet the fact remains that people eating diets abundant in vitamin C,
vitamin E, polyphenols and carotenoids are less likely to suffer
heart attacks, vascular disease, diabetes and cancer. One explanation
is that these people have a generally healthier lifestyle - they
exercise more and smoke less, for example. For now, no one knows for
There are some ideas. Halliwell still believes that antioxidants are
at least partly responsible. He argues that because the polyphenols,
carotenoids and vitamins in fruit and vegetables are bound into
tough, fibrous material, they hang around in the stomach and colon,
where they can neutralise free radicals. The gastrointestinal tract,
especially the stomach with its highly acidic environment, is
constantly generating reactive oxygen species from food. Supplements
may not replicate this effect because they are digested too quickly.
Andrew Shao from the Council for Responsible Nutrition, a supplement
industry trade association based in Washington DC, argues along
similar lines. He says that pulling a nutrient out of context and
testing it in a clinical trial is not appropriate. "Antioxidants
should not be expected to perform as drugs," he insists. "That's
simply not how nutrients work. They work in concert with each other."
There's yet another, more intriguing explanation. Among the leading
sources of dietary antioxidants are tea and coffee, and there is some
evidence that green tea in particular is linked with health benefits
including reduced risk of cancer and cardiovascular disease. Oddly,
though, Halliwell has discovered that tea and coffee are also
bursting with reactive oxygen species in the form of hydrogen
"Every time you drink a cup of coffee it's a dilute bowl of hydrogen
peroxide," says Halliwell. The hydrogen peroxide is there because of
the presence of the antioxidants - "antioxidants" is really just
another way of saying reducing agent, which can react with oxygen in
the water to produce hydrogen peroxide. Think platinum blond, and you
get the picture of what you might be drinking.
But if free radicals are bad for us, how come coffee and tea might be
beneficial? One possibility is that they can help nudge our own
internal antioxidant systems into action. "There has been a
considerable rethink as to what free radicals are doing," says
Malcolm Jackson, a biochemist at the University of Liverpool, UK. He
believes that in the right quantities radicals can be positively
health-enhancing, prompting our cells to fire up their own internal
defence machinery: a battery of radical-busting enzymes such as
catalase and superoxide dismutase. "Cells are very good at protecting
themselves against minor stresses, as long as they are not
excessive," says Jackson. "The question is: should we be quenching
free radicals at all?"
If it turns out that antioxidants in food work because they generate
health-promoting quantities of free radicals, that would be an ironic
turnaround. It may also explain why supplements and extracts don't
seem to work or may even be dangerous: the doses are too high, and
produce too many free radicals.
For now, the advice is simple. "Stick to flavonoid-rich foods, red
wine in moderation, tea, fruits and vegetables," says
Halliwell. "Don't start taking high-dose supplements or heavily
fortified foods, until we know more."
Hoe weet je of wat zijn de bijverschijnselen van teveel vitamines.
Veel B-vitamines kunnen diarree veroorzaken. Dit is goed te verhelpen door ze alleen nog maar bij een maaltijd in te nemen.
nee voor D nog meer dan C, D3 dan wel