Carbs needed to fuel efforts: calculation/examples

bahzob

Below are some workings/examples of a calculation of how much carbohydrate is needed as fuel to sustain a given level of power for a certain time. Reason for doing this is general interest and to prepare a feeding approach for events I plan to do in the future. (From experience, sometime bitter, getting feeding right for these is important, eating too little or eating too much can affect performance, potentially quite drastically.)

I'd be grateful for any feedback checking my maths/assumptions and/or other examples of feeding especially from anyone who has done hard/fast 6+ hour events.

The calculation is simple enough

Carbs needed = Carbs stored as glycogen - Carbs burned during exercise.

Variables/Assumptions
Carbs Needed: This is purely from the aspect of carbs as fuel for an exercise session.

The body has limited capacity to absorb carbs from food/drink. This is around 1-1.5g per kg body weight per hour. Rate will vary by person and also be affected by food type mix, (fats will slow rate down, peak rates require customised mix of sugars/protein.).

This means that in some situations endurance efforts may be limited by the carbs needed as fuel. Part of the purpose of this exercise is to identify these.

“Carbs stored as glycogen” comes from total body glycogen which is in a range from 450-550g, say an average of 500g. Not all this will be used up however since a proportion is muscle glycogen which is distributed over the the body and not all muscles will be used. Nor will all the remaining proportion (stored centrally in the liver) be entirely used for exercise. So it's assumed 90% of total body glycogen I.e. 450g, is actually available

“Carbs burned during exercise” = “Total calories burned” * "carb % factor" representing proportion of these derived from carbohydrate, assuming 4kcal/gramme of carbohydrate.

“Total Calories burned” is calculated from kJ (= Watts * duration), assuming 1kJ is effectively = 1kCal taking account of metabolic inefficiency.

“carb % factor” of calories burned from carbohydrate varies as a function of power and will be in range 30% at low “fat burn” wattage to 75% at high tempo pace to 98%+ at threshold and above(numbers come from a lab respirometer test)

So as an e.g. for 4 hour time trial averaging 250W
kJ=250*4*3600/1000 = 3600kJ which is assumed to require 3600kcal

Effort is at high tempo level so around 25% will be fuelled from fat .

So calories needed from carbs = 3600*75% = 2700kcal.

Glycogen can supply 450g * 4kcal/g = 1800kcal.

This leaves a balance of 900kcal required from food/drink

Which equates to 225g of carbohydrate at 4kcal/g.

Which is an average of 56g per hour, which is comfortable albeit requiring some attention to feeding regularly.

Just for fun/illustration below are some more examples. All assume body is rested, has full store glycogen of which 450g is available to fuel efforts and are simply concerned with the carbohydrate needed for the effort concerned.

Short workouts of an hour or so: Need no carbs, there are enough stored as glycogen to sustain 60 minutes at 450W+

2-3 hour workouts: Need little carbs, 3 hours at 250W requires around 60g of carbs total.

3+ hours hard: http://www.trainingpeaks.com/av/5N6EKHMS2OBNJ3GXQ2NIOC3YKA shows the effort it takes to be in a long breakaway at the top level. It requires an average of 272W (over 4.5w/kg) for 3+ hours. Though hard fuelling should not be too much of an issue. Around 170g of carbs will be needed, around 50g an hour. Key thing here I guess is recovery before and after, given this was stage 12 of a 21 stage race.

5 hours intense: http://www.trainingpeaks.com/av/LKIDHLHMVJPXSKHRPXUDKHFK6E shows the effort of David Lopez on the double Alpe stage of this years tour. Average from the start of the first climb was 262W for more than 5 hours, requiring a total of more than 5000kcal. Fuelling must have been an issue as this requires a steady average of 90g of carbs per hour. Looking at the plot the second ascent of ADH is done at around 70W less than the first, which means it was probably done in a lower effort zone which apart from being less tiring also means more fat % contribution. Those at the front didn't have this option as shown by Chris Froome when he missed out his feeds and bonked.

6 hours intense: http://andrewbye.blogspot.co.uk/2008/07/and-now-for-some-numbers.html shows what it takes to finish in the top 10 of the Marmotte, more than 1000kcal/hour for over 6 hours. That's pretty much exactly at the theoretical 1.5g/kg carb absorption limit. This may not be just coincidence, though that said I'd guess that riders like this one and the pros above may well have greater glycogen stores and be more metabolically efficient (either by birth or training or combination of the two) which would explain why they can go further/faster.

7 hours hard: For comparison with the above taking an hour+ longer to do the Marmotte cuts the average power down to under 200W. This more than compensates for need to fuel for a longer time on the road, reducing carbs/hour to around 70g. Still at this sort of level feeding is still a key issue, especially the need to do this constantly from the off.

12 hours: I only have personal data for these, done a couple, one on a road bike other on a TT. Power for both was similar, around 200W average. This requires just under total of 9000kcals and around 100g of carb an hour. This may explain the power profile of these rides with the first few hours at 220W then slowly subsiding to under 200W, so slowly moving down to a zone with with a greater fat % contribution. Riders who manage 300 mile 12s are probably averaging 240W+, so I guess again that these riders must have a combination of greater glycogen stores and a more efficient carb burning metabolism achieved via genes, training or both.

SBezza

bahzob wrote:

12 hours: I only have personal data for these, done a couple, one on a road bike other on a TT. Power for both was similar, around 200W average. This requires just under total of 9000kcals and around 100g of carb an hour. This may explain the power profile of these rides with the first few hours at 220W then slowly subsiding to under 200W, so slowly moving down to a zone with with a greater fat % contribution. Riders who manage 300 mile 12s are probably averaging 240W+, so I guess again that these riders must have a combination of greater glycogen stores and a more efficient carb burning metabolism achieved via genes, training or both.

BINGO, (well nearly), the body is unlikely to process 100gms of carbs per hour, so eating this much is likely to give you stomach issues later in the race, you eat enough to help minimise glycogen use, but you will never take in what you use. It could be, for the more aerobically fit that the 240+ watts (not that that is 100% true IMO), is just at the top end of endurance/lower end of tempo and with regular feeding isn't so much about glycogen availability, but the ability to resist fatigue at this level for a long period. Glycogen stores will not vary greatly between fit and unfit riders ( well not a huge amount anyhow), and like I have said in previous threads, you can train yourself to be more efficient at using fat as a fuel source hence lowering the usage of glycogen. Getting fitter however is a better idea so that 240 Watts (or whatever you need to do a 300 mile 12 Hour - + a lot of very good luck) is a lower effort compared with your FTP. As a gauge I used just short of 10,000 calories in my last 12 hour, and I didn't take on anything near this amount in fuelling, I took on probably on average 75 gms per hour (might be slightly high), so 300 calories per hour, so some 500 calories per hour less than I was using, and I was still doing top end of endurance at the end.

Overall though everyone is an individual, and will burn glycogen and fat in very different percentages even if they have the same FTP and exercise at the same percentage of FTP. The figures above are good ball park figures, but you shouldn't assume you will be same.

BenderRodriguez

Good post bahzob!

Some interesting data and links there.

bahzob

SBezza wrote:

bahzob wrote:

12 hours: I only have personal data for these, done a couple, one on a road bike other on a TT. Power for both was similar, around 200W average. This requires just under total of 9000kcals and around 100g of carb an hour. This may explain the power profile of these rides with the first few hours at 220W then slowly subsiding to under 200W, so slowly moving down to a zone with with a greater fat % contribution. Riders who manage 300 mile 12s are probably averaging 240W+, so I guess again that these riders must have a combination of greater glycogen stores and a more efficient carb burning metabolism achieved via genes, training or both.

BINGO, (well nearly), the body is unlikely to process 100gms of carbs per hour, so eating this much is likely to give you stomach issues later in the race, you eat enough to help minimise glycogen use, but you will never take in what you use. It could be, for the more aerobically fit that the 240+ watts (not that that is 100% true IMO), is just at the top end of endurance/lower end of tempo and with regular feeding isn't so much about glycogen availability, but the ability to resist fatigue at this level for a long period. Glycogen stores will not vary greatly between fit and unfit riders ( well not a huge amount anyhow), and like I have said in previous threads, you can train yourself to be more efficient at using fat as a fuel source hence lowering the usage of glycogen. Getting fitter however is a better idea so that 240 Watts (or whatever you need to do a 300 mile 12 Hour - + a lot of very good luck) is a lower effort compared with your FTP. As a gauge I used just short of 10,000 calories in my last 12 hour, and I didn't take on anything near this amount in fuelling, I took on probably on average 75 gms per hour (might be slightly high), so 300 calories per hour, so some 500 calories per hour less than I was using, and I was still doing top end of endurance at the end.

Overall though everyone is an individual, and will burn glycogen and fat in very different percentages even if they have the same FTP and exercise at the same percentage of FTP. The figures above are good ball park figures, but you shouldn't assume you will be same.

Do you have any power data from your 12 that you could share?

SBezza

I do have power data, but I am not prepared to share it, it wouldn't prove anything useful IMO, you have no idea of my weight, my FTP, what my zones are, how aero I am etc.

If you want advice for a 12 hour I will gladly give you pointers, but I never share my power files, though I am sure if you look around you might find it somewhere.

I saw on the wattage forum you ask the same question, my questions for you would be as follows

Where did you do your 251 miles
What weight are you (or should say size)
What was your average power (and did it stay pretty well constant)
What percentage of FTP (or training zone) did this represent
How long of the 12 hours were you stopped for

Grill

Wilko set the 24hr record on a 500ml bottle of energy drink and a bar every hour, so about 60g carbs.

12hrs for me is 80gm carbs and 400-500 cals an hour.

bahzob

Here are the answers, any advice you can offer would be appreciated.

Where did you do your 251 miles> Welsh TT
What weight are you (or should say size): 71kg/1.73m
What was your average power (and did it stay pretty well constant): see the graph below. Pacing wasn't too great, you can see how this dipped between the 2 and 3 hour mark
What percentage of FTP (or training zone) did this represent: FTP was 305 so avg of 180=59%
How long of the 12 hours were you stopped for: about 9 minutes




The timing and extent of the in power in the early stages are what surprise me and lead me to think I cocked up eating during that time. I know if I get this right I can sustain better power as this example from sub 4 hour 100 a couple of months earlier shows (the dip at the end of this one was due mostly to severe pain in the gonads)

SBezza

One idea would be to go to a "faster" course, that would probably be the easiest way to go further. You are about the same weight and height that I am, so I would assume with some tinkering with position you could probably get more aero. With your FTP, what you achieved in the first 2 hours should be manageable for the full duration, this will come with training, no point having a great FTP if you can only keep a certain %age of it for a short duration, work on getting the fitness to keep at 70%+ of FTP for the full duration (or as near 12 hours as possible), your drop off wasn't great, but with your FTP you should be able to sustain 200 watts at least without dropping off hardly any. Last one, don't stop for too long, just over 2 mins was too long for me (I had planned no stops whatsoever). The TSS for my 12 hour was 670+ so it shows you how hard as ride it was, and that was with perhaps a slightly over inflated FTP.

I doubt the first 2 hours of not eating right played a huge part, even this effort should be fuelled in huge part by fat, and if you were eating and drinking during this time, even if not at 60gms+ of carbs you would be able to fuel yourself for a fairly long period. Not training to sustain 200+ watts for a long period probably has more effect, but without knowing what sort of training you were doing in the lead up to the 12 Hour, and what sort of nutrition you normally use of these rides it is very hard to say. Given your FTP and the starting wattage, I don't think it is excessive by any means and is certainly something you should be able to do for many hours even if not fuelled right.

bahzob

SBezza wrote:

One idea would be to go to a "faster" course, that would probably be the easiest way to go further. You are about the same weight and height that I am, so I would assume with some tinkering with position you could probably get more aero. With your FTP, what you achieved in the first 2 hours should be manageable for the full duration, this will come with training, no point having a great FTP if you can only keep a certain %age of it for a short duration, work on getting the fitness to keep at 70%+ of FTP for the full duration (or as near 12 hours as possible), your drop off wasn't great, but with your FTP you should be able to sustain 200 watts at least without dropping off hardly any. Last one, don't stop for too long, just over 2 mins was too long for me (I had planned no stops whatsoever). The TSS for my 12 hour was 670+ so it shows you how hard as ride it was, and that was with perhaps a slightly over inflated FTP.

I doubt the first 2 hours of not eating right played a huge part, even this effort should be fuelled in huge part by fat, and if you were eating and drinking during this time, even if not at 60gms+ of carbs you would be able to fuel yourself for a fairly long period. Not training to sustain 200+ watts for a long period probably has more effect, but without knowing what sort of training you were doing in the lead up to the 12 Hour, and what sort of nutrition you normally use of these rides it is very hard to say. Given your FTP and the starting wattage, I don't think it is excessive by any means and is certainly something you should be able to do for many hours even if not fuelled right.

Thanks for the advice, its very helpful. I know my position is weak point and needs work. It's good to hear that you think I should be able to sustain more power if I do more focussed training and knowing this will help do the rides necessary. My training prior to the 12 was more focussed on sustaining higher power for shorter durations.

Stops were due to me riding unsupported + a 5 minute wait for cows to cross a road. One reason I chose the Welsh was because of the support the event provides, but next year I will rope in some support and try elsewhere.

I have also, very kindly, been sent a WKO file of a 280 which is also a great help and Ric Stern pointed me in the direction of this article http://www.bikeradar.com/blog/article/the-12-hour-time-triallings-ultimate-test-26910/ from Jeff Jones' ride which makes great reading.

Cheers again, with winter coming up it will be a great help/motivator.

SBezza

If you want to sustain a high endurance effort over 12 hours, you do need to work on endurance. I know some may say that if you raise your FTP you should be able to translate that over all durations, but I think that is foolish. You need to train the body for the events you want to really target, and that doesn't mean pootling around at a low endurance level for 5/6/7 hours etc, rides of these durations need to be ridden at a decent endurance pace IMO.

Getting help will help no end, and in more ways than just handing up food and drink.

bahzob

SBezza wrote:

If you want to sustain a high endurance effort over 12 hours, you do need to work on endurance. I know some may say that if you raise your FTP you should be able to translate that over all durations, but I think that is foolish. You need to train the body for the events you want to really target, and that doesn't mean pootling around at a low endurance level for 5/6/7 hours etc, rides of these durations need to be ridden at a decent endurance pace IMO.

Getting help will help no end, and in more ways than just handing up food and drink.

I agree and that will form the bulk of my training through winter. And I will definitely get help on my next event.

bahzob

Over on the wattage forum someone posted a pretty amazing file from a 12 hour race (not a TT) averaging close to 300W for 12 hours and burning 12000+ calories.

The chap that did it offered the following advice which is pretty extreme but obviously seemed to work.

"I was in great shape back then. My FTP was probably set too low for that file as I didn't test it during that period. The secret to a good 12h is pre-race nutrition. I did a 14 day no carb diet riding 3-8 hours per day eating just protein. I definitely got a training adaptation from this as the power I was putting out increased substantially over the 14 days. I then took 2.5 days off the bike before the race and binged on high GI carbs. My legs blew up like balloons full of glycogen! I put on several kg's in those 2.5 days due to all the water associated with glycogen (3g water per 1g gylcogen). However, during the race this pretty much lasted me about 8h, after which I was totally depleted and relied predominately on fat metabolism to get me through. You can see that from the power file."

SBezza

Yep I saw that, now what worked for him might be worth trying, depends on how important the race was, as no doubt it is a risky strategy, you can carb load differently however and this is always advisable for a 100 or 12 hour IMO. As for the power, well he obviously is in a different league to most I would imagine.

I can't see how you can see on the power file about glycogen depletion though, as it seems like a fairly natural degradation in power and HR over that duration, nothing I can see like a sudden drop in power. I would imagine he was taking on plenty of food and fluids during the race and although glycogen would obviously have become depleted I highly doubt he was doing the last 4 hours on pure fat reserves, he would have slowed a hell of a lot IMO.

It is difficult to tell from that file along, as the TSS and IF are pretty much pointing to a very incorrectly set FTP, I doubt he was near threshold at all for that duration, though if it was a road race I can understand VI being a bit high, and hence this inflating TSS a fair margin. Be nice to have that power for a 12 hour TT though

bahzob

I'm thinking that I will plan a couple of weeks over the winter to try out the approach and see how it affects me. There will be relatively little risk and it will be interesting to see what happens during the time, especially given his observation that he saw power increase which is quite counter intuitive. I can see it improving how your endurance power since it may force higher fat utilisation, but overall power should be down. Whatever, it will a add a bit of variety into the next few months.