Prior to commencing any form of exercise (especially endurance sports) it is extreemly important to ensure you are properly fueld up.
Because if you use more energy completing the exercise (say a three hour triathlon) than the energy you have stored as glycogen in your liver and muscles, you will become catabolic.
When you are in a catabolic state your body is dissasembling itself (muscle tissue first) to provide your body with energy for the exercise you are undertaking.
Methods of ensuring you have enough energy to perform include.
Simply ensure you eat enough low GI carbohydrate over the few days prior to exercising or competition.
Choose as sports drink that gives you the energy you require, however does not significantly raise your blood insulin levels.
Learn more at my sports drink page.
Ensure you have appropriate levels of the essential amino acids in your bloodstream prior to exercising.
Studies at the Loughborough University showed that loading with sodium phosphate tribasic prior to exercise can increase performance.
I recommend "Race day boost" by Hammer nutrition. www.hammernutrition.com/products/race-day-boost.rdb.html
If you know of a product shiped in the UK please let me know.