Reduce your training volume and intensity in the week leading up to the competition and increase your carbohydrate intake to maximise glycogen stores. That means sticking to light workouts, technique work, and stretching, and eating plenty of rice, pasta, bread and potatoes, and all the fruit you can eat.
Make sure you get up early enough on the big day so you can have a good high-carb low-fat breakfast at least three hours before your first event. Then make sure you have plenty of small easily digested high-carb meals and snacks on hand to keep you fuelled during the day.
If you can you should try out your competition day diet a few times beforehand, as you don't want to risk going into an event with an upset stomach. And don't be afraid to use sports drinks and bars if you do find solid food causes you gastric distress during exercise. Just don't try anything new on the big day unless you want to risk having a Paula Radcliffe moment
AIS Sports Nutrition Factsheet: Nutritional Preparation for Tournaments and Multiple Heat Competitions.
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