Is there a way to understand how long it takes to burn through your glycogen?
Yes. This is not exact math, but it will give you an idea:
Assume 1 kJ of work = 1 kCal of food consumed. This is not exactly right, but it’s close enough and it makes the math easy.
Also assume that your riding at a high enough intensity where you are burning only glycogen. Depending on how sugar vs fat dependent you are, this could either be close to true, or only partially true. This also makes the math easier to follow.
- If you have eaten enough carbs before, you might have 2000 kCal of muscle and liver glycogen stores.
- If you can eat and absorb another 250 kCal of food during your ride, that’s another source of glucose
- let’s say you ride at 200W. That burns through 720 kJ in 1 hr. So about 720 kCal worth of food.
- Within 3 hours, all your original glycogen stores will be gone. You’ll have used 2160 kCal of energy.
- In those 3 hrs you’ll have consumed 750 kCal of food.
- so by the 3 hr point, you’ll be 590 kCal ahead. (2000 kCal to start + 750 kCal of food = 2750 kCal. Minus 2160 burned = 590 kCal left.
- if you continue to consume 250 kCal per hour, by end of hour 4, you’ll have 120 kCal left in your system (590 kCal from above + 250 of food = 840 kCal. Minus 720 kCal used = 120 kCal left.
- in hour 5 you will consume another 250, but burn 720… so by about 15 minutes into hour 5, you will run out, and bonk.
This is example math. Your body will slow down before hour 5 in an attempt to preserve glycogen, so it won’t let you continue to ride at 200W. So you’re glycogen will last longer than this.
But it gets the point across that in long rides, you have to be careful not to run out. This can be accomplished by:
- Pacing. Riding at a slower pace where you burn more fat, less carbs
- Having a robust fat metabolism - being less sugar dependent so a higher fraction of your energy at a given power output comes from fat
- nutrition: eating enough
- being fit. A higher FTP means ata given % of FTP, you’ll be riding at a higher absolute power, hence finish the race sooner.
Check out the short video in section #4 in the following link for more:
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Wow, that is 100% me last year in a 60 mile, 7000 ft of climbing, gravel race. I was probably eating about 250 kCals/hr and bonked at about 4.5 hours!
I get a shot at redemption in a couple months and I’m going to eat like crazy! It’ll also help that I’ve increased my FTP a bit 
Good stuff, thanks for that @DaveWh !
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Check out this thread also.
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The problem with all the guesswork on how much glycogen gets burned is two-fold:
- there is quite some (scientific) debate on the actual existance of something like “glycogen sparing” due to carb intake during a ride. At least for the muscles, liver glycogen sparing seems to be to occur
See for example here:
- the relationship carb/fat burning vs intensity is often not known. And even if lab tested, does the relationship still hold true after 1 hour. Or when pre-meals were different? These lab tests are just a tiny snapshot in the time conitnuum.
A crude field test may be the observation of heart rate drift. Ride at a constant power output (probably in a range that is more sustainable, hence no short efforts) and observer HR and breathing. The point where HR goes up and breathing becomes more elaborate is the point where you run out. I must say I’ve observed this quite often when doing very long continuous tempo/SST workouts outdoor.However, I saw the effect more pronounced when not eating carbs during the session. Which adds to the confusion on point #1.
Of course, you would still not know the percentage coming from fat and carbs. But you will know how it feels when you run out of glycogen. Not everyone “bonks” actually (in the classical sense), this can be quite individual.
https://fellrnr.com/wiki/Heart_Rate_Drift#Heart_Rate_Drift_and_Glycogen_Depletion
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I really like your maths as they are insightful regarding how much glycogen you have around at the start.
You clearly indicate your assumptions. I would like to add to why it is probably not so easy to calculate when you’ll bunk:
- lactate (cori cycle) and pyruvate are converted to glucose
- adrenaline leads to liver gluconeogenesis and release of further fatty acids from tissue
- gluconeogenesis is induced from fatty acids and protein through glucagon and cortisol
Also of course, being light - so if there are long climbs in the event you can burn fewer calories going up them to achieve better pacing.
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It’s something I’ve often wondered. Does the ratio of Fat/Carbs depend on your actual heart rate or on the power you are producing. ie does a couple of coffees prior to the ride which elevates your HR with respect to what it normally would be lead to greater carb utilisation at a given power output?
These often mentioned 2000kcal are whole body (all muscles + liver). HOwever, during cycling only leg muscle glycogen is used. Glycogen from other muscles like arm muscles are not mobilized.
About 55% of total muscle mass is located in the legs. This means any calculation would have to assume only half of the amount of total glycogen available for the effort.
On average for someone trained glycogen content is about 6g per kg muscle. Therefore, lean body mass x .55 x 6g gives the amount available in the leg muscles.
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If you want to dig in a little bit deeper, these may be of interest for you. I must admit, after seeing what we actually know and how much it varies, I can’t take these fat vs carb measurements serious.
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Just got to say, this forum is amazing. Helpful, insightful comments with references. Interesting topic with excellent answers. Thanks.
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Don’t forget the importance of proper pacing, which imo is more important than fueling. You can’t eat enough to compensate for burning too many matches.
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Yeah, this is a simplified example, but shows the point about how glycogen is limited, and in long rides, you risk running out.
For a given rider, the ratio of fat vs carb is largely driven by power you are riding at. There are things you can do over time (training, nutrition) to change this ie move the fatmax point ideally up and to the right (see chart in link below - the “diesel” rider). There’s probably other things that can influence this also, but this is my main understanding.
There have been some great recent posts on this, and related topics. Example here:
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Here is another thread with information around that topic: Questions on nutrition, carb intake, glycogen stores and fat conversion
This article was linked in one of the answers:
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