I would look at this differently, and in a simpler way.
Go for a run. See how many calories you burned. Whatever watch/app you use will tell you that. Assume you’d like to replace 50% of those calories on an easy run, or 75% in a race if you can absorb that much. Then 4 calories = 1g carbs. Start fueling with that from the start of the run.
You might find that you simply can’t absorb that much in carbs without GI distress, but most people will be fine (and you can increase your gut tolerance easily with practice). For example, I’m 240 lbs and raced a half-marathon at 622 cals/hr. Fueled with 90g/hr carbs from the beginning, arrived at the finish line destroyed by the distance but with plenty of energy and in a great mood (ergo, fueled well). It’s that easy. Modify to your personal needs.
[Nota bene: I ran the half in 3.15 hours, I’m still really slow. No doubt slimmer-but-faster folks can burn more calories per hour than I, but the theory and thinking is the same.]
You’re burning 800kJs or 800 calories per hour? 1 calorie (kcal) equals 4.2 kJs. I think that’s your issue. 1g of carbs supplies 17kJs or 4 calories. I’m guessing you’re burning 800cals/hr which would be 200g of carbs to replace that completely… aka GI issues.
This has always confused me, so I don’t want to say something wrong, so I’ll try to keep it brief and not confuse myself and everyone who reads this. From Wikipedia:
The energy contents of a given mass of food is usually expressed in the metric (SI) unit of energy, the joule (J), and its multiple the kilojoule (kJ); or in the traditional unit of heat energy, the calorie (cal). In nutritional contexts, the latter is often (especially in US) the “large” variant of the unit, also written “Calorie” (with symbol Cal, both with capital “C”) or “kilocalorie” (kcal), and equivalent to 4184 J or 4.184 kJ.[3] Thus, for example, fats and ethanol have the greatest amount of food energy per unit mass, 37 and 29 kJ/g (9 and 7 kcal/g), respectively. Proteins and most carbohydrates have about 17 kJ/g (4 kcal/g), though there are differences between different kinds. For example, the values for glucose, sucrose, and starch are 15.57, 16.48 and 17.48 kilojoules per gram (3.72, 3.94 and 4.18 kcal/g) respectively.
TLDR: there is a difference between kcal, calorie, Calorie, and kJ. I think that’s where the confusion is coming from.
I don’t think @JoeX is confused at all. A calorie is roughly equivalent to 4 joules. However, the body is roughly 25% efficient at consuming energy, so it takes burning 4 calories to create those 4 joules of actual work (edited to correct initial typo). For practical purposes in exercise or work done, one calorie burned by the body achieves roughly one joule of work. For all practical, real-world, daily purposes, calories and joules (or Calories, which are really kilocalories, and kilojoules) are equivalent.
His question is entirely different:
He’s starting out by describing the scenario as he understands it: based on the idea that he runs out of energy after 2:20 of easy running, and based on the estimate that his body carries 1800 KJ worth of glycogen reserves, he’s calculating that his net glycogen consumption rate is roughly 800 KJ per hour. This is the part that contains all the real science, and this is where he can really be right or wrong. What matters is whether his REAL net hourly consumption rate for glycogen is 800, or 600, or some other number.
Then, he’s trying to figure out fueling solutions based on that net glycogen consumption rate. That’ll be the easy part. The real meat of the issue is the calculations and estimates in the paragraph above. The units used to describe it are not important due to that 1:1 rough equivalence.
My initial plan was to switch to cycling focus but have spent most of my time just dabbling in any/all things just too keep active; strength, rowing, hiking, kayaking… It’s been quite nice but now starting to feel the itch to get back into routine.
My Fall/Winter plans are still tbd so I may just focus on shorter runs/workouts and hop into a few 5k/10k’s until I figure out plans for later this year. The shorter runs would allow me to mix in cycling too.
So that maths lines up with around 800kJ for an hour running on my Garmin and on a calculator on the fellrnr site. That calculator also shows how much less energy is needed for ‘normal’ weight people, blimey.
I think that gives me confidence Im fuelling well enough with two 26g gels an hour.
The rest is training and health. And water. I lose a kilo in the first hour of any run.
Thanks for helping me think this through.
FWIW I’ve run two half marathons in 1h50, but fell apart in two marathons…5hrs which I didn’t fuel. If I do it again I’ll be “one of those guys” with a hydration jacket covered in gels
I weigh 86kg where a normal weight would be 65kg. I’m not weak, I’m not unfit, but my body fat is only going to increase if the last five years are anything to go by.
I trained up to do a 30km run last autumn but bailed on it didn’t run farther than 21km
OK, I guess I am misunderstanding things because that’s not how I interpret Wikipedia’s explanation:
“Calorie” (with symbol Cal, both with capital “C”) or “kilocalorie” (kcal), and equivalent to 4184 J or 4.184 kJ"
and:
“Many countries and health organizations have published recommendations for healthy levels of daily intake of food energy. For example, the United States government estimates 8,400 and 10,900 kJ (2,000 and 2,600 kcal) needed for women and men, respectively”
I believe JoeX is doing the conversion backwards (or twice?). 800kJ / (17kJ/g carb) = 45g carb to have a net zero energy loss. This is not true based on my understanding. 45g of carbs will be about 180calories, not 800.
~50g carb/hr may be enough to not bonk, but it’s not replacing 100% of what you’re burning.
I believe that’s what you’re saying, but I might be missing something.
I did read it and I believe I understand it, but I’m just worried that you think by eating 50g of carbs you’re replacing 100% of what you burn. You are not, and I don’t want you to bonk and think that you’re replacing everything you burn.
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.
