my experience in this is as follow: when you race you tend to work harder, thus you burn more energy, if you have not conditioned yourself at that level of burning you tend to take in more liquid, should you only have trained on water, you will loose electrolytes and energy a lot faster, if you take supplements during the race but haven't trained on it, your intake of liquid being higher might cause you to take in too much sugars etc. and you might become nauseous or feel such in the race, this is especially prevalent in multiple stage events.
