Every year we spend £300 million on vitamin supplements, but do they actually do us any good? Some believe they offer the promise of preventing or even curing some of the world's biggest killers, such as heart disease and cancer. Others claim that taking large doses of some vitamins may in certain cases be harmful. So what are the facts?Nearly 40 years ago, one of the greatest scientists of the 20th century and double Nobel Prize winner, Linus Pauling, revolutionised the way people thought about vitamins. He claimed that by taking huge doses of vitamin C you could prevent or even cure the common cold.He predicted that if everybody followed his advice, the common cold could even be eradicated. Many scientists dismissed his theory as quackery, but the public loved it and it helped launch a huge industry. But the latest evidence shows the great man was mistaken. Vitamin C can help you once have got a cold, but for most people it does nothing to prevent you from catching one in the first place.Even if large doses of vitamin C do not prevent the common cold, some claim that it can still offer a more profound benefit. It is one of a group of vitamins called anti-oxidants that some believe can prevent illnesses such as cancer, Alzheimer's and heart disease.In 2004, scientists in the United States claimed that people could be missing any of the potential benefits of taking one of the world's most popular anti-oxidant vitamin supplements, vitamin E, because their bodies might not be absorbing it. But our own investigation suggested that the American scientists' conclusion could be mistaken.While most safety experts believe that vitamins C and E can be taken safely even in quite large doses, there is worrying evidence that one form of another common vitamin, vitamin A, could be linked to osteoporosis, a debilitating bone disease.If the theory is right it means that a person's diet, or some supplements that they take every day to improve their health, could actually be slowly and silently weakening their bones.