The simple truth on diet is that there really isn't a magic bullet which will turn an also ran into an Olympic Champion.  The fantastic claims that most supplement makers make are usually based on anecdotal accounts and/or faulty science; these claims are rarely substantiated by reputable nutritionists. 

So if the supplement makers are wrong, then how can diet improve or enhance athletic performance.  Here a number of tips gleaned from Nutrition and Athletic Performance in the Winter 2000 edition of the Canadian Journal of Dietetic Practice and Research: 
 

• Eat enough calories to replace what you burn during exercise. A 150 lb male will burn about 100 calories per mile run.  Therefore, if he runs 6 miles a day, he will need an additional 600 calories more than a non active person just to maintain weight.  If weight loss is desired, then a weight loss program should be undertaken before the competitive racing season. 

• Strive to eat a diet consisting 60-70% carbohydrates, 10-15% protein, and 15-25% fat.  An endurance athlete’s diet need not vary much from a normal healthy diet; the key difference would be that the athlete’s diet would contain more calories.

• Keep hydrated. This involves drinking water or a diluted sports drink before, during, and after exercise.  One trick is to weigh yourself before and right after exercising.  You should drink 450 to 675 ml of fluid for every pound of body weight lost during exercise.

• Be wary of vitamin supplementation. Endurance athletes who maintain a healthy diet with enough calories shouldn't need vitamin supplementation.  Moreover, some fat soluble vitamins and minerals can build up in the body to toxic levels.  Vitamin supplementation should only be undertaken under the advice of a medical professional.

• Women may have special needs. Despite the above warning about vitamin supplementation, women who exercise heavily should be careful to get enough iron and calcium.  A blood test can identify deficiencies in these areas.  Supplementation may be needed to correct these deficiencies.

• Eat a small meal or snack a few hours before exercising.  A little snack a few hours before exercising can provide the fuel and fluid needed for a good workout.  Carbohydrate rich foods low in fat and fiber are good choices.

• Eat something soon after exercising. Eating a snack consisting primarily of carbohydrates and a little protein within 30 minutes of completing your workout will speed recovery for your next workout.