Dietary supplements are one of our biggest growth industries - greedily embraced as an antidote to increasingly hectic lifestyles, abnormal eating habits, and sporadic efforts to keep fit.

Not so long ago, anything more exotic than cod liver oil was laughed off as mumbo-jumbo. But by last year we were spending £360m annually on the likes of pine bark, royal jelly, green-lipped mussels and ginseng in the hope of warding off everything from hangovers and spots to depression and lung cancer. However, behind skyrocketing sales and the bewildering expansion of product ranges, concerns are mounting over the safety of these " natural" products, which are classified as foods, and therefore undergo less rigorous testing than medicines.

Our knowlege of how and why vitamins and minerals work - especially in highly refined doses isolated from normal dietary intake - is still sketchy. Many studies have shown that, at best, supplements have no effect at all on our health. Others suggest they may be harmful. Most recently, a shadow has fallen over vitamin C supplements, regularly taken by 10m Britons, including Baroness Thatcher and Michael Portillo, and thought to boost the immune system and tackle free radicals, which play a role in heart disease and cancer.

Last week, results from a study at the University of Southern California showed the carotid arteries of middle-aged people taking daily 500mg doses thickened two and a half times faster than normal - among smokers the rate was five times faster. The study was relatively small (573 people) but the results are potentially alarming. Although no one suffered major illnesses during the trial, atherosclerosis causes poor blood flow, associated with heart disease and strokes.

The government recommends a daily 40mg dose of vitamin C, 12 times less than the amount used in the American study: 10 mg a day is enough to prevent scurvy. However, many people take much higher doses to ward off colds and flu.

Dr James Dwyer, the epidemiologist who led the American study, says: " When you extract one component of food and give it at very high levels, you just don't know what you are doing to the system. It may be dangerous. If a physician has pre scribed vitamin C, it is appropriate to be taking it. But if you are healthy and taking it in the hope of preventing cardiovascular disease, the American Heart Association does not recommend it."

Gail Goldberg, senior nutrition scientist of the British Nutrition Foundation, is surprised by the results "considering other evidence about vitamin C protecting against heart disease and high blood pressure". However, she points out that little is known about how refined vitamin C may act compared to vitamin C in fruit. "There are phytochemicals in fruit, for example, which may be a protective factor, but we don't know yet what they do or how they work.

"The interaction of vitamins and minerals is complicated, so it's difficult to take supplements in isolation. Many require other compounds for them to work properly: iron cannot be absorbed without vitamin C, and calcium cannot be absorbed without vitamin D. Vitamin C is an antioxidant like vitamin E and beta carotene, which are some of the most popular food supplements around. In America, US$2bn is spent annually on these alone."

However, the body makes its own antioxidants, and there is little evidence that taking large amounts of refined antioxidants has any benefit. A recent major study in Italy showed no heart benefits among 11,000 people taking 300 daily units of vitamin E over three and a half years, while a Finnish study in 1994 had to be halted after smokers taking beta carotene showed an alarming increase in cases of lung cancer. It is already known that high doses of vitamin C (more than 3,000mg) can cause diarrhoea and blood in stools, while high doses of vitamin E can interfere with blood clotting.

Dwyer says more research needs to be done on his findings, but he believes arterial thickening could have been due to the supplements causing a build-up of collagen in the artery walls, or causing the absorption of excess iron, which can damage the heart.

Goldberg says people taking vitamin C supplements should not panic or stop taking them." Most are formulated to make it very hard to take a dangerously high dose, and vitamin supplements have an important role to play for anaemic people, vegetarians, the pregnant and elderly."

She says many foods, such as breakfast cereals, margarine and flour, have been safely fortified for years, helping to cut down on deficiency diseases such as rickets which were still rife 50 years ago. But she adds: " Healthy people can obtain all the vitamins and minerals they need from a normal balanced diet, including five portions of fruit or vegetables a day. It's also important to cut out smoking, which drastically lowers the amount of vitamin C in the body.

"Trying to balance a poor diet with supplements or use them as food replacements is potentially dangerous, and never likely to be successful."