To prevent the depletion of stored carbohydrates during high intensity activity lasting 90 minutes or more, many athletes may do what is known as “carbohydrate loading.” Since this technique began in the early 1930’s, many variations of it have evolved over the years to eliminate glycogen depletion in endurance activities.

The earliest, most extreme versions involved exercising to exhaustion 7 days prior to an event. Then, you would maintain a diet consisting of only 10% of calories from carbohydrates for the following 3 days. The final 3 days leading up to the competition, you would eat a diet consisting of 90% of calories from carbohydrates thereby overshooting your normal muscle glycogen levels by almost 100%, leaving you plenty of stored immediate energy for the event. This early technique had its drawbacks including increased water retention, weight gain, flatulence, diarrhea, and possible heart complications due to its stressful nature.
The modern versions are much less stressful on the body and there is less overshoot of stored glucose but it can still be effective. Basically, you do not run to extreme exhaustion prior to the competition nor do you reduce the amount of carbohydrate intake before running. Simply eat a normal balanced diet consisting of 55% of calories derived from carbohydrates and, 3 days prior to the event, increase that to 70%. Also, your training should be decreasing in intensity as you get closer to the event. Any work you do the week of the event will not make a difference in your performance; it is the weeks and months that precede it when your body is making the necessary adaptations to facilitate long-term aerobic activity. A 20% increase in carbs combined with decreased use due to the tapering of your exercise regimens should suffice to elicit an increase in performance while avoiding any harmful side-effects.