They are exculpation, insurance and saintliness in handy pill form. They supposedly guard against anything from wrinkles to cancer. But are dietary supplements - from the humble multi-vitamin to the mega-dose antioxidant - really as benign as they seem?
A study published last week in the Journal of the National Cancer Institute found that men with prostate cancer who took more than seven multivitamins a week were 30% more likely to get an advanced and fatal form of the disease. This comes after a large - though hotly contested - review published in the Journal of the American Medical Association in February found that people who took antioxidant vitamin tablets (particularly vitamins A and E and beta-carotene) were more likely to die earlier than those who did not.
Such research seems to fly in the face of common sense. We all know vitamins are "good" for us. Indeed, thanks to a national obsession with TV diet "gurus", many people believe they are de facto deficient in key nutrients and can only be healthy if they supplement their food with pills or potions. Even those not downing spirulina algae with breakfast probably own at least one pack of multi-vitamins.
However, says Catherine Collins, chief dietician at St George's hospital, London, "The whole idea that you must meet some vitamin and mineral target every day of your life is a marketing myth: you can eat lots of fruit and veg one day and not much the next, but over a week you will still get the right amount of nutrients." What's more, she says, "There is very little scientific evidence of any benefit whatsoever in taking a daily multi-vitamin, even in old people." Indeed, taking daily vitamins could be positively harmful if you eat a poor or highly restricted diet, then assume you are nutritionally covered by popping a pill.
"You cannot exist on a poor diet and then shore yourself up with a multi-vitamin," says Collins.
Supplements, she says "have become pseudo-medicines", prescribed by trusted therapists and frequently taken where conventional medicine has failed. Many people, for instance, routinely pop supplements such as echinacea pills to ward off colds, or evening primrose oil to counter hot flushes or premenstrual tension (PMT), despite a complete lack of scientific evidence that these supplements actually work.
"The supplements industry treats the body like a blunt, crude tool," explains Collins "You put in a certain vitamin, and it solves a problem. But the body does not work like this at all. It has a far more complex and sophisticated mechanism for absorbing vitamins and minerals from food."
According to Dr Toni Steer, nutritionist with MRC Human Nutrition Research in Cambridge, supplements cannot compete with real food because when we eat fruits and vegetables the vitamins and nutrients interact with other chemicals to produce positive effects on the body. "If these same vitamins are pulled out and isolated in pill form," she says, "there is no guarantee at all that they will have the same effect."
There is also a trend for taking "mega-doses" of specific vitamins in order to target particular health issues (for instance, swallowing hefty quantities of vitamin B6 for PMT). Again, this could be misguided. "The idea that taking high quantities of vitamins will give you a health boost - like putting premium petrol in your car - is complete nonsense," says Collins.
People assume that vitamins are harmless, even in high doses, because they are "natural". But swallowing a gram of vitamin C is the equivalent of consuming a whole bush of broccoli or four litres of orange juice: hardly a "natural" amount. "With vitamins and minerals," says Collins, "more is not necessarily good." Some, such as vitamin C, will simply be excreted if taken in large doses. (The body can use a maximum of 500mg of vitamin C a day.)
Others may be even more problematic. Most multi-vitamin pills, for instance, contain vitamin A. But if high doses of vitamin A (above 3,000 micrograms) are taken by pregnant women, they can cause birth defects. Since there is no upper limit on how much of any vitamin a product may contain, some vitamin tablets, says Collins, actually contain 30%-40% more than the recommended daily allowance stated on the packet. (This is why pregnant women are advised to take only pregnancy-specific multi-vitamins.)
Another recent trend is to take daily doses of antioxidants such as beta carotene, vitamin A and C or selenium in order to protect yourself against cancer, heart disease or signs of premature ageing. Found naturally in fruit and vegetables, antioxidants protect the body against cell damage. Dietary studies show that people who have a high level of antioxidants in their diet have a lower risk of heart disease and certain cancers. This is why we are told to eat at least five portions of fruit and vegetables a day. However, studies also seem to suggest that taking antioxidants in pill form may not have the same effect and may even be harmful.
"There's still conflicting evidence about whether taking certain vitamin supplements can affect a person's risk of cancer," says Dr Alison Ross, science information officer at Cancer Research UK. "These products don't seem to give the same benefits as vitamins that naturally occur in our food." The British Heart Foundation is similarly cautious. "Research does not support the claim that taking extra antioxidants in the form of supplements will benefit the heart," says its spokesperson.
What does and doesn't work
Daily multi-vitamin
Common claim: provides most of your recommended daily allowances of key vitamins.
Reality check: while they may plug gaps in diet, they cannot replace fruit and veg.
Vitamin C megadose
Common claim: 1g doses will ward off or even cure the common cold.
Reality check: the human body can absorb only 500mg vitamin C and will excrete the excess. Vitamin C reduces the average length of a common cold from five days to four and a half.
Antioxidants (beta carotene, vitamins A, E, C and selenium)
Common claim: daily intake will lower risk of cancer, heart disease and reduce the signs of ageing.
Reality check: only true if consumed in fruit and vegetables.
Evening primrose oil
Common claim: can reduce symptoms of PMT, breast pain, hot flushes and eczema.
Reality check: no current evidence to show it is effective in treating these conditions.
Echinacea
Common claim: will boost your immune system, warding off the common cold.
Reality check: studies show it has little effect.
What does work:
Folic Acid Pregnant women should take folic acid supplements (400 micrograms a day) in first 12 weeks of pregnancy. Known to prevent neural tube defects such as spina bifida.
Vitamin D Made by the body when it is exposed to sunlight, but now many of us keep out of the sun and it may therefore be deficient. Vitamin D deficiency is linked to osteoporosis, cancers of the breast, colon and ovary, multiple sclerosis and insulin-dependent diabetes.
Glucosamine sulphate There is evidence that it can relieve symptoms of osteo-arthritis in moderate sufferers.
