Clearly worked well for you!
The way I figure out carb needs is this:
- if you’ve been eating well and had a decent breakfast you should have about 2000 calories of glycogen stored in the body
- At endurance racing intensity assume you get half your calories from fat, half from carbs. There’s quite a bit of individual variability in this, and it also depends on intensity (the higher the intensity, the higher % of carbs you burn, but then those higher intensities are also only possible for shorter events where fuelling is less of an issue) but it’s good enough to start with
- If you have a PM and a decent running watch with weight and HR zones well set, then you can get a decent estimate of the calories you’re burning per hour.
- Multiply the hourly calorie burn by the duration of your race to get total calorie needs. Divide by 2 to get your carb needs. Subtract 1000 which is half of your original glycogen stores (we’re keeping the other 1000 in reserve, firstly because you don’t want to finish on empty, and secondly to act as a bit of a buffer as we’ve got a few estimates and assumptions in there). What’s left is the number of carb calories you need to be replacing during the race. Try it, see how it goes, adjust as needed.
For you, at 220W you’re burning ~800 calories/hour, let’s assume same calorie burn for whole race, so total 3500 cals. Half of that 1750. Subtract 1000 and you’re left with 750 which I guess isn’t too far from what you actually took on. 360 on the bike, maybe 300 from red bull and coke on the run. You could have taken on a few more calories to be on the safe side, and still been well within the limits of what your body can tolerate.
It gets trickier for longer races like IM. Assume you were burning calories at the same rate for 10 hours (it would be a bit lower of course), suddenly you’ve used up 4000 calories of carbs, so need to take on 3000, which by the time you’ve finished the swim means taking on in excess of 300 cals/hour on the bike and run. That’s harder to do and much easier to get gastric distress if you get the mix of carbs and water a bit wrong. Heavier athletes also obviously burn through more calories. And if it’s not a steady state race like a triathlon but something like a road race where you’re repeatedly going above threshold (or if you’re a triathlete who gets overexcited on the hills…) then you’re burning through your carb stores much more quickly during those high intensity sections.