How much Carbs do you really need?

I completed Ironman 70.3 Turkey over the weekend, and thought I woudl share some findings. For the race, I had:
Breakfast - rice porridge max 250g + 2 cups of coffee
nothing else before the swim
on the bike I had my carb drink mix approx 750ml which would have given me 90g carbs total (bike time 2:15 ave power 220w IF of 0.69) and 500 of precision hydration 500
on the run I had water, coke and red bull at the aid stations - 1 gulp of each x 3 aid stations x 4 laps. run time was 1:28 with an IF of 0.92
My finish time was 4:19 which was good enough for 9th AG (35-39). does this sound right? based on everything I ever hear I should have needed double the amount of carbs for the bike and way more water to fuel and hydrate sufficiently.

In the week leading up to the race, I tend to eat well (more than I woudl normally) but nowhere close to a carbo-loading strategy. Is the current thinking on nutrition advice overly complicated? do we really need that much to perform effectively? Is it more likely a very individual thing and we should be doing a lot more individual experimentation to find out what our own limits are? i would say my approach worked, but woudl be interested to hear thoughts.

Generally the ‘max’ is ~90g/hour. It takes a ton of work to where your gut can handle that. Try to increase the amount you can handle over time, but don’t think that you are doing something wrong by not hitting that maximal number.

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.


really detailed view, thanks! the bike burnt 1688Cal, and the run was 1100Cal so around 3000 for the event.

I did Ironman Italy back in September off the same strategy I employed here. As the bike was much longer I went through more carb mix. approx 1.5l. I didn’t drink enough water on that occasion and tested out salt tabs vs the Precision hydration bottle, but wasn’t in too much of a hole and could drink my way out with water at all the aid stations. Biked a 4:40 and ran a 3:23 and finished in 9:11 so again, not needing to go too crazy on gel bottles and carb drinks and getting through.

the thought that is playing around in my head is on the fat metabolism. looking at the numbers, this must be playing a more significant role than given credit for. I would love to know what proportion of fat I am burning working at 0.7 IF

I am doubting that this should be a goal though. i am wondering if the minimum effective dose is more important than training the gut to the maximum dose.

Thanks for sharing. No doubt we vary as individuals so testing ourselves is always going to give us the most useful data.

Obviously intensity varies as well so longer duration events that are lower intensity will have less of a demand for carbs.

I think knowing the max uptake per hour for the average person is good enough for most amateurs because it means there’s no risk of glycogen depletion, which is more harmful to performance than eating slightly more than necessary,

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Some glycogen depletion is both inevitable and desirable though. If you’re racing at reasonably high intensity you simply can’t take on carbs quickly enough to replace what you’re burning. And pushing your carb intake to the max has an impact on performance as your body is diverting resources to your digestive system, as well as increasing the risk of gastric distress. For most events there’s a happy medium of consuming enough carbs to take the risk of bonking out of the equation, but not so many that you’re getting close to the limit of what your body can handle.

My view is that the hardest events are in the 6-12 hour range. Shorter than this and like the OP you can likely get away with fairly conservative carb intake. Longer than this and the intensity drops sufficiently that you’re getting more energy from fat plus your body has more bandwidth for digesting.

When, in racing, would glycogen depletion be desirable?

You said a happy medium intake would eliminate the risk of bonking but also that depletion is inevitable?

Surely carb intake is dictated by intensity as much as/more than duration. The shorter the race distance, the higher the likely intensity.

Also, I didn’t say that over consumption of carbs had no negative affects, just that those affects aren’t as bad as running out of glycogen.

You have 2000 calories in glycogen stores when you’re at the start line, maybe a little more if you’ve done a good carb load over the preceding days. That’s a pretty big buffer. By glycogen depletion I simply mean finishing the race with less glycogen than you started, not full depletion I.e. bonking. Some glycogen depletion is nearly always desirable e.g. In a 1 hour TT I’m going to burn through well over 1000 calories, the majority from carbs. There is no way I can replace those carbs and I’d be foolish to try, given that my body has more than enough stores to see me through the race with plenty in hand, and that trying to take on even 300 calories is likely to lead to give me gastric issues when racing at that intensity (research shows that there is some benefit in short events in drinking energy drink or just swishing it in your mouth even when you don’t need the calories, but that’s another topic!). So with a choice between glycogen depletion of 7-800 calories vs potentially having to stop and chuck my guts up by the side of the road, it’s a no brainer.

If your interpretation of glycogen depletion is running the tank to empty and bonking then I agree, any alternative including slowing down to increase the amount of calories you can absorb is preferable.

I see the confusion now. I meant total depletion (aka bonking) not partial.

I feel like the parameters of your answers are changing now though. I thought we were talking about long events such as the OPs 70.3, hence my suggestion of maintaining a high rate of consumption to avoid depletion (bonking), but now you’re referencing shorter intense efforts where surely the demand for nutrition is going to be quite different.

Similar principle on a 70.3. You start with stores of 2000, let’s say you’re also burning 2000 calories of carbs (400 per hour for 5 hours). I think it’s better to aim to replace half of those by consuming 200/hour which is pretty moderate and easy to do, and still leaves you with plenty in reserve, rather than trying to push your limits by taking on 350-400/hour which risks impacting performance.