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The A to Z of vitamins
They may help prevent cancers and ward off colds, but not all vitamins are the wonder pills they are cracked up to be - in fact, some could shorten your life
By Roger Dobson
Published: 06 March 2007
More than £100m a year is spent on them and millions of people take them daily in the belief they boost health, but could vitamin pills really be unhealthy? According to a new report, some of those little capsules and tablets used to try and ward off diseases as diverse as cancer, Alzheimer's, acne, arthritis and the common cold may be bad for you. "Treatment with betacarotene, vitamin A, and vitamin E may increase mortality,'' say researchers from Copenhagen University Hospital.
While some research has shown beneficial effects for vitamins, minerals and other supplements, others have either found none, or produced conflicting results, or, in some cases, shown potentially harmful effects. The Danish researchers combined the results of a number of previous clinical trials to judge the overall effects of vitamin pills. The results show that in 47 trials involving 180,938 people, antioxidant supplements increased mortality by 5 per cent. When they looked at individual vitamins, E increased mortality by 4 per cent, betacarotene by 7 per cent, and A by 16 per cent. No effect was found for vitamin C or selenium.
The Danish researchers are not the first to find negative effects from taking vitamin pills: "Evidence is insufficient to prove the presence or absence of benefits from use of multi-vitamin and mineral supplements to prevent cancer and chronic disease," say researchers from Johns Hopkins University in Baltimore, Maryland. A study by a team at the same university found that taking high doses of vitamin E may increase the mortality rate: "We think that high-dosage vitamin E supplementation may increase all-cause mortality. Current evidence does not justify using it to reduce the risk for cancer, cardiovascular or Alzheimer's disease," the researchers say.
Vitamin A can also harm bones, and high levels have been linked to birth defects, blood clotting and over-stimulation of the immune system, according to a new report from Harvard University. "A vast amount of research has shown that you can cut your risk for chronic disease and disability by following a healthy diet, as well as exercising regularly and avoiding smoking. The evidence for taking vitamin and mineral supplements is much less extensive,'' says the report.
While the evidence for some supplements, including folic acid, is good, the advice for most people is to get as many vitamins as possible from a healthy diet. But if you feel you can't do that, which vitamins and minerals are worth taking?
WHAT ARE VITAMINS AND MINERALS?
Vitamins are vital nutrients the body needs to work properly. They provide energy, boost the immune system, keep skin healthy, help keep the brain and nervous system in good working order, and they play a part in control and repair of tissues. There are two types: fat-soluble and water-soluble. Fat-soluble - A, D, E and K - are mainly found in fatty foods such as dairy products, liver and oily fish, and are stored by the body when they are not needed. The others, including C and folic acid, are in fruit and vegetables, and are not stored in the body, which means they need to be eaten more often.
Minerals are essential nutrients found in varying amounts in a variety of foods, including meat, cereals, fish, dairy foods, vegetables, fruit and nuts.
VITAMIN A
Boosts the immune system and is important for healthy bones, teeth, skin and eyesight. Found naturally in green leafy vegetables and in orange-coloured fruit and vegetables, as well as liver, eggs and fish oils.
A study in The New England Journal of Medicine found that the men with the highest amounts of vitamin A in their blood had the greatest risk of breaking a bone. The Copenhagen report associates vitamin A with a 16 per cent increase in mortality.
Bottom line "Plenty of evidence from earlier research shows too much vitamin A can harm bones. A pregnant woman who takes too much vitamin A risks birth defects to the foetus. Excess vitamin A also compromises bone health and blood clotting, and it can over-stimulate your immune system,'' says the Harvard report. Try to get this vitamin from food sources.
BETACAROTENE
A carotenoid, it is a vitamin A precursor, which the body converts into vitamin A. Found naturally in a wide range of fruits and vegetables that are deep yellow, orange or dark green, such as carrots and spinach.
The Copenhagen study found a 7 per cent increased risk of mortality associated with taking the pill. Other studies have shown a link between betacarotene and increased risk of lung cancer in smokers. In one trial, 18 per cent more lung cancers were diagnosed and 8 per cent more deaths occurred in those taking betacarotene. In a second study, 28 per cent more lung cancers were diagnosed and 17 per cent more deaths occurred in those taking betacarotene and vitamin A supplements than in those taking placebos. Neither of these studies showed a benefit from taking supplements.
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作者:admin@医学,生命科学 2011-04-07 18:13
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