I think these are the confused subjects here. The body functioning at 25% general efficiency generates 1kj of work for 1kcal burned, not for 4.
Where this isn’t aligning is we’re talking about energy onboard/stored vs work done. If you have available to you 1,800 kj of energy (which is approx 450kcal) but you’re operating at 25% efficiency, you’re only going to accomplish 450kj worth of work with that stored energy. The reduction in efficiency happens when you convert that stored energy/glycogen to actual work done.
All of that to say - 56g or 2 gels per hour will absolutely not cover anywhere close to the caloric expenditure of running a race, but it may help supplement your energy stores enough to keep you from bonking…
Can I drag this away from the technical to the romantic? I’ve ordered my first pair of spikes and I’m really excited! For me they represent a lot of history and a specialism that most runners will never experience.
I’ve grown surprisingly fond of training on the track and am going to tentatively take steps into the world of track meets soon. Provides a wonderful juxtaposition to my other passion of long runs over hill, fell, dale and bog.
My apologies! I was saying generally all the right things, but made a typo and you’re right, it would have been confusing. I’ve edited my prior message to correct the typo. Yes, it would take 4 calories burned by the body to generate 4 joules of work, a 1:1 (roughly) relationship.
Correct. But we’re never really aiming to replace 100% of our energy consumption during a race, we’re trying to ensure that we don’t run out of energy. If his prior marathons were completely unfueled, then 10 gels (one at start, one every 30min thereafter) might get him through.
Personally, I’d take 70-80g/hr rather than 50g (most gels have 20-25g) because more fuel is likely to yield better performance, and I know I can digest 80g/hr for 5 hours with no problem, and I don’t want to be scraping the bottom of the energy barrel if I can help it. More is better, so long as I can digest it well. But that’s my preference.
Separate point. Say you start with 1800 KJ of muscle glycogen reserves. After an hour, you’ve burned 600 calories running. I’ll assume a relatively easy run, so half of that will come from metabolizing fat. And then you eat two gels (50g total), getting 200 calories from sugar that’s easy to convert to glycogen so you can resupply the muscles.
Your net hourly glycogen depletion rate in this hypothetical scenario is 100 calories. And if you started with 1800 KJ worth of reserves, roughly 450 calories equivalent, you’re going to start running out of energy around 4.5 hours. Or a little less, most likely.
The goal is not to run out of glycogen. So in this hypothetical scenario, I’d try to fuel with 75g/hr of carbs to get 300 cals/hr, so in theory I’m not reducing my glycogen reserves over time, and in reality I’m only reducing them slowly.
Plus, you metabolize a lower percentage of your energy needs from fat when you’re operating at a higher intensity, or when you’re slightly dehydrated, or several other things… so if in doubt, fuel a little more rather than a little less.
I think you’re confusing things here, rather than the other way around. There’s generally enough glycogen stored in the body to generate 1800-2000kJ of work, so when we say we have 1800kJ available in stored glycogen, we have 1800. Not a quarter of 1800.
Enough glycogen to generate 2000kj of work would be approximately 8000kj (or 2000 kcal) worth of stored energy because you’re operating at 25% efficiency, thus the 1:1 for kcal stored:kj produced. You can’t equate the work you do (in kj) with the energy consumed (or stored) in kj at 1:1.
Sure, and you’re right… it makes the conversation simpler.
But I keep going back to one point: this ALL hinges on how large your muscle glycogen reserves are initially. And that, we have no way to help you with. Is it 1800 KJ? Or 2000? Or 2400? Or 1600 KJ?
You can gradually get to a number that’s useful to you in practical terms through experimentation. But there’s not much we can do in this conversation. IF your 1800 KJ number is correct, THEN your estimate that at least 45g/hr of carbs should be your minimum fueling level is reasonable assuming you start fueling immediately after the run starts.
I’d still fuel with more than that, but yes it’s reasonable. Again, back to trial and error. Try it and see how you feel after a five-hour marathon with that fueling strategy. Then try it at 75 g/hr and see what the differences are.
I think I’ve verified that as best I can; when I fall apart in marathons, combined with kj burn estimations from the fellrnr calculator and Garmin records.
Maybe I could do some maths on the lean muscle mass from my scales, but it’s widely reputed these are unreliable and don’t think there’s a way to know liver stores beyond average human numbers.
So on the fuel, 45g of carbs generates 45x16.7 = 751.5kJ or are we arguing that’s 188kJ?
If it’s ~750 I should be fine, if it’s ~190 then I’m still in trouble and I’ll be out of fuel at (est.) 3h20 instead of 2h20.
Sounds about right. Just remember that the percentage of energy you get from carbs rather than fat, increases as your intensity of effort increases, or with mild dehydration, or in several other ways. So you may want to fuel a little more if possible.
45g of carbs generates roughly 180kJ. It’s all been normalized for you, so that’s roughly 180kcals.
And then you realize it’s a range, not exactly 4kJ per gram. And that you aren’t 25% efficient. So you take kJ from power meter and figure out how to adjust intake based on what happens weighing yourself on the scale, day after day, week after week, month after month. And don’t be alarmed if taking in more calories actually results in better performance and weight loss.
No, 45g of carbs has 751 KJ of energy. Here, we’re ONLY talking about the actual amount of energy in the food.
The real issue is whether that amount of fuel is enough. He estimates his muscular glycogen reserves at 1800 KJ and says he depletes them by 800 KJ/hr while running. He’s asking whether putting in 750 KJ/hr of fuel will be enough.
Those 750 KJ/hr (a.k.a. 180 kcals/hr) reduce his energy deficit and prolong TTE. The question is, is that enough? I’m not sure it is, but it might be IF his numbers are right.
As noted several times, body efficiency varies within a range. And carb/fat utilization varies with intensity, hydration, and other factors. And honestly, I think doing all of this in joules is complicating things unnecessarily and introducing doubts like the question you’re asking here. But I understand that you want to work through the exercise, so have at it.
Not sure I can add anything of further value to this conversation. Hope somebody has more expertise on the matter and can help us all get this clear.
