How long does it actually take to burn off your glycogen?

Training
2

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:

https://scientifictriathlon.com/testing/

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