Much depends on race conditions, time, and your sweat rate.

Assuming it is not high heat/ humidity, I will drink a bottle of water on the bike and maybe take a swig of Gatorade at the first station on the run. Sometimes I take a gel on the bike, but usually I don't. Two to three hours before the race I have a breakfast of oatmeal mixed with a banana and dried cherries. I'll usually eat another banana before I start to put on my wetsuit. I'm careful to make sure I'm hydrating the day before the race.

In general, I avoid sports drinks, gels, and blocks. I find it all a little too sweet.

What’s your weight and estimated finish time?

I burned 2,250 calories in my last Oly event (with a time of 2:20ish) in conditions that were not bad and almost bonked despite taking in probably 300ish calories (and my same old prerace fueling routine which never failed me before). So I don’t think it’s always as simple as some others have suggested. It made me rethink my strategy and that I may need some more calories before and during.

How did you burn so many calories?

I’m not sure how to answer that....just what my Garmin devices told me; they could be off a little but those numbers are not out of line for what I may burn in workouts. 325ish for swim, 11 in T1, 1075ish for bike and T2 and 835 on the run. I weigh mid 190s.