Does a normal healthy person on a well balanced diet need vitamin supplements and if so why?

While a person consuming a well-balanced diet can certainly do alright without a daily multivitamin, long-term supplementation with a multivitamin has been shown to have positive effects. The reason for this probably lies in the fact that even a well-balanced diet could be low in various nutrients from day to day. While getting only half of the RDA for, say, Vitamin A every few days isn't going to cause any immediate symptoms of a deficiency, the effects may show themselves in over time in a large enough group of people eventually. There are dozens upon dozens of nutrients which we need, and getting these daily is optimal. Remember though, that a multivitamin is a supplement to an already balanced diet- not a replacement. Fruits and vegetables contain far more beneficial substances than just your standard laundry list of vitamins and minerals. No multivitamin could ever fully replace the benefits of these foods. Additionally, your body cannot completely absorb all of the nutrients from a multivitamin at once- you'll still need to derive some of your vitamins and minerals from food, even if the multivitamin label makes it seem as though the pill contains everything under the sun. Consider a multivitamin simply as added protection to fill-in whatever nutritional gaps even the best of diets might produce.

According to the American Medical Association, "supplementation is no longer a matter of choice, as people who do not supplement their diets are at higher risk for degenerative diseases"; and Dr. John Klippel, president of the Arthritis Foundation, stated "There are lots of choices that people need to make and supplements are one of them". A 1991 study done by the British, and a more recent one done by the University of Texas based on USDA data, show that the food grown today is less nutritious than that grown 60 years ago, due to soil depletion.