For those who rattle to work on a slug of black coffee and a cocktail of vitamins and minerals, the latest slur on supplements is bad news indeed. In research published by the Cochrane Library last week, Danish scientists revealed how people taking antioxidant supplements, including vitamins A and E, may be inadvertently shortening their lifespan and increasing their risk of an early death.

In their review of 67 studies on 230,000 healthy people, the team from Copenhagen University found “no convincing evidence” that antioxidants help to keep diseases such as cancer at bay and that some may “increase mortality” by up to 16 per cent. “The review underlines the fact that many peopledo not need to take these supplements,” says Bridget Aisbitt, a scientist at the British Nutrition Foundation. “They can get what they need to stay healthy from food.”

In the UK, around one third of the population pops at least one vitamin pill a day, and the supplements industry is worth in excess of £330 million a year. But many of the pills are surrounded by controversy and it is not the first time that supplements have come under attack this year.

In February, the Copenhagen researchers published their first findings in the Journal of the American Medical Association. Then, scientists at the University of Washington reported how taking daily supplements of vitamin E for ten years was linked to an increased risk of lung cancer. In January, New Zealand researchers suggested calcium supplements, often prescribed after the menopause to counter the loss of bone density, raised the risk of a heart attack in older women.

Part of the problem highlighted in the latest report, says Aisbitt, is that the single large doses supplied in many supplements seem to throw the body's oxidative systems off kilter. Antioxidants such as vitamins A, C and E, beta carotene and selenium, are considered essential for health. They are known to mop up the free radical compounds in the body that cause disease and to boost the immune system. But there is no scientific confirmation that more is better or that the levels in supplements are beneficial. “Oxidation happens all the time and antioxidant vitamins do their best to keep this in check,” she explains. “But it is a very delicate system and ingesting too many antioxidant vitamins in one go could possibly disrupt the balance of things, although nobody has yet discovered precisely how or if this happens.”

Others are less convinced that we should discard our vitamin tablets. Dr Michele Sadler, an independent nutritionist who is scientific adviser to the Health Food Manufacturers' Association, says that although this study “throws up important questions, it does not prove unequivocally that vitamins are harmful”. While the authors claim to be assessing antioxidants for the prevention of mortality, she says, “they used two statistical methods that produced different results, so the evidence is far from clear-cut”. While antioxidant supplements “cannot be expected to undo a lifetime of unhealthy living”, Sadler believes that they “can play a role in promoting overall wellbeing, particularly if dietary intake is low, and are safe provided people stick to the intake on a product's label”.

Working out how much of a vitamin or mineral you should be taking is fraught with complexity. Most food and supplement labels list a nutrient's recommended daily allowance (RDA), a figure set at EU level that, far from being a formula for tip-top health, is simply the minimum intake needed to prevent a nutritional deficiency. In the UK, dietary reference values (DRVs), set by the Government's Food Standards Agency, provide more specific recommendations for both men and women that take into account age and potential health issues at various times of life. But even these suggested levels are far below what is generally supplied in a supplement. Instead, manufacturers often supply doses much closer to the “safe upper levels” for nutrients, figures that were reached after the Government's Expert Group on Vitamins and Minerals analysed studies on intakes that actively promote health, rather than just prevent disease. So, while the RDA for vitamin C, for example, is just 60mg, consumers in the UK can buy it in 1000mg tablets.

It is these huge doses that are cause for concern. Many nutritionists have witnessed a growing trend for people to abandon meals in favour of a handful of tablets that they believe will provide the nutrients they need but without the calories or, indeed, the hassle of cooking. “Tablets can never replace a healthy diet,” Aisbitt says. “Foods contain a complex matrix of different components that can't be replicated by supplements. They provide fibre, phytonutrients and other nutrients that are essential to health. You just don't get that in a pill.”

Catherine Collins, the chief dietician at London's St George Hospital in London, concedes that some people do need supplements at times in their lives, such as during pregnancy or old age. “But for the vast majority there's absolutely no benefit in taking high-dose vitamin supplements and the latest research is extremely troubling,” she says. “People take them like they eat sweets. At best they are a waste of money and at worst a serious risk to your health.”

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