While it is clear that vitamin and mineral deficiencies impair athletic performance, there is no evidence that vitamins taken in excess of normal daily requirements will improve physical performance. For example, in one study of 82 athletes from four sports (basketball, gymnastics, rowing, and swimming), 7 to 8 months of daily supplementation with a high-dose vitamin/mineral tablet failed to have an effect on muscular strength, or aerobic and anaerobic fitness. Heavy exercise is associated with an increased need for many nutrients, including iron, zinc, copper, chromium, vitamin B6, riboflavin and vitamin C. However, it is the position of the American Dietetic Association that these demands are usually met when the athlete eats more food to meet the increased energy needs.
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First, Eat Well
It's much easier to take a vitamin/mineral pill than to change the way you eat. Supplements provide the illusion of caring for your health, but unless you're eating three squares a day, you're not getting the benefits of food itself. Remember that the word supplement means "something extra."
No matter what an ad might imply, supplements do not replace food and cannot make up for nutritional deficiencies of a diet based on beer and pizza.
So, before reaching for a vitamin bottle, go straight to the source: your favorite fruits and vegetables. Not only will you get to enjoy the delicious flavor of the real thing, you'll also get the benefits of added energy, fiber, vitamins, minerals, antioxidants and phytochemicals (a scary sounding word which means health-promoting chemicals found by the hundreds in plants).
Fiber, the noncaloric plant material that aids digestion, helps prevent obesity, and helps ward off colon cancer and heart disease. In addition, the skins of many fruits contain cancer-fighting chemicals. For example, the skins of grapes contain a chemical called resveratrol which can prevent cells from turning cancerous. Also, those phytochemicals appear to prevent cancer, reduce tumor size, lower blood cholesterol levels and enhance immune function. You can't do all of this with a pill alone. Just as an adequate diet is not enhanced by supplementation, an inadequate diet is not redeemed by supplementation.
If you want to take vitamins, fine, but first stock up on fruits, vegetables and whole-grain bread products. The National Cancer Institute urges all Americans to include a generous intake of fruit (2-4 servings per day) and vegetables (3-5 servings per day). If athletes take care to select those that are dark green or yellow in color, they should meet their requirements for most of the antioxidant nutrients (including phytochemicals) from diet alone. One exception could be vitamin E. The Daily RDA for vitamin E is 15 IU, which may be an amount insufficient for most athletes. Recently, it has been suggested that vitamin E intakes of 100 to 400 IU per day may be reasonable for athletes engaged in moderate to heavy exercise.
http://www.power-nutrition.com/healthy eating/vitamins.html