Do mega-doses of vitamins like Vitamin C help prevent diseases?

Q: I’ve heard that mega-doses of vitamins, like Vitamin C, can help prevent diseases. Is that true?

A: Vitamins are complex organic compounds that are needed in small amounts by the body for normal growth and metabolism. An important part of a balanced diet, vitamins occur naturally in some foods and may be added to processed foods to increase their nutritional value. Common vitamin supplements like Vitamins A, C, D, E and many others are sold as over-the-counter products in many stores. Sometimes vitamins are packaged as supplements for hair and fingernails, such as Vitamin B7 or Vitamin H, and it is difficult to tell what they contain and in what amounts.

A mega-dose of a vitamin is one that is many times higher than the recommended amount. It is commonly believed that taking mega-doses of certain vitamins will act like medicine to cure or prevent certain ailments. For instance, vitamin C is suggested as a cure for the common cold, and vitamin E is widely promoted as a beneficial antioxidant to help prevent heart disease.
Research has shown that neither of these claims has been shown to be true. Large-scale studies have consistently shown little benefit in taking mega-doses of supplements. In fact, there is some evidence that taking high-dose supplements to prevent or cure major chronic diseases, such as heart disease and cancer, may be harmful to your health.

The truth is that anything that is consumed in excess can be harmful to your health. Think about something as simple as water. We all know we need water to stay alive, but drinking too much water can actually be fatal.

When your healthcare provider asks you what supplements or over-the-counter medications you’re taking, answer them truthfully. Vitamins and other supplements can interact with some medications and can even influence the results of medical tests, leading, in some cases, to potentially incorrect diagnoses.

Vitamins and minerals are essential to health, but that doesn't mean mega-doses will keep you out of the hospital or make you live longer. In most cases, it's preferable to get these nutrients from a balanced diet. High doses of certain vitamins and minerals may be appropriate for certain people, though. Talk to your doctor about supplements if you are a woman of childbearing age, are a vegetarian or vegan, have limited exposure to the sun, are an athlete in training or suspect for any reason you may be malnourished.