Toin,
It sounds to me like indigestion, probably caused by eating too much or too soon (or both) before the race. Once you start running the water in your system has two jobs to do, 1 = combat dehydration and 2 = help to digest food. If there isn't enough water available then the body prioritises according to the most urgent need which will be to combat dehydration, so any undigested food just stays put in the stomach. It’s a bit like running with a bowling ball in your stomach! You also feel very dehydrated.
If you have eaten well on the day before the race then you will still have a full or almost full tank when you get up next morning, and that energy should last for around 2 hours of running. (in very round figures a full tank is appx 2000 calories and you burn up appx 1000 per hour at 7.5 min mile pace). This means that you don't need to eat too much on race morning, just a couple of hundred calories of something very easy to digest at least 2 hours before the start. My pre race “meal” is usually a couple of slices of bread & honey as soon as I get up, sometimes just a few spoonfuls of honey if I have an early start. Energy gels are ideal.
If you are used to eating plenty before exercise then this idea may require a leap of faith so you would obviously need to try it first in training, but this is definitely what works for me. I have successfully completed many long distance triathlons using this strategy.
One very important thing to remember is to eat and drink plenty shortly after finishing – make sure you have something within the first hour.
Martin.
