This was a super interesting episode with lots of gems!
I am still a little confused about the whole glycogen and glucose thing.
Suppose I’m fueling well pre-ride and well rested. In this state my glycogen stores will be topped off. Now, on a fasted ride I’m guessing the body will release the glycogen and make it accessible as glucose. Fat will also be used, and the proportions of the two will largely depend on intensity and length. Am I correct so far?
Now, if we add in glucose late in the (fasted-so-far) ride, we can assume that glycogen is partially depleted. Now when I start ingesting glucose (or some other carb I guess), will my muscles prioritize the intake or continue using glycogen until depleted? Also, if my glycogen stores are partially depleted, will my body start converting carbs eaten during ride back into glycogen stores?
Similarly, starting well fueled, and starting carb intake very early in the ride, what will my body use first (or is it a combination?)?
Finally, a question I have not heard mentioned in any podcast or cycling nutrition info, is the large amount of glycogen stored in the liver! How does that factor in? What Is Glycogen? Storage, Function, Tests, and More
I know this is an epic topic and the above questions are simplified quite a bit (and probably misinformed), but I really want to get a better understanding of all this!
According to this article, ingesting glucose will greatly reduce the production of glucose from glycogen in the liver, but muscle glycogen will continue to be used. You can follow the reference to https://journals.physiology.org/doi/abs/10.1152/jappl.1988.65.4.1553 where they say straight up “The results of this study indicate that carbohydrate ingestion does not influence the utilization of muscle glycogen during prolonged strenuous exercise.” The second study speculates that some re-synthesis of glycogen might occur during rest periods, but it’s only speculation.
All the usual caveats with nutritional studies apply. N=5 male cyclists, for instance.
Another study. Forget about the incomprehensible title this showed the effects of sub max exercise on fasted, 45g & 90g CHO/hr on muscle and liver glycogen .Tl:Dr as far as I can see is the muscle glycogen depletion was the same but taking CHO spared liver glycogen and improved TTE.https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP281127
Thing is, ultra endurance shows that the leg muscles don’t run out of glycogen over many many hours and days. What’s going on there, as clearly there isn’t an infinite supply.
These are great questions. I believe the researchers still have many of these questions themselves! I will do my best to clarify where I can .
Exactly, nailed it.
When you have enough glycogen stored in your muscles, your body will preferentially use muscle glyocgen above exogenous carbohydrates (the carbohydrates you consume on the bike).
Muscle glucose uptake progressively increases as you progressively deplete your muscle glycogen levels. So the carbohydrates you consume during your ride become increasingly important later into the ride.
The highest rates of exogenous carbohydrate oxidation will occur around 1.5 to 2.5 hours into a ride depending on intensity and how well you fuelled in the days prior (your starting glycogen levels).
I bvelieve that some studies have indicated that high CHO intakes during a ride can reduce liver glycogen breakdown. So, consumig carbohydrate while riding could spare your liver glyocgen.
Ensuring your glycogen levels are topped off before your ride is important for peak performance. However, if you don’t have a full (glycogen) tank, your mid-ride nutrition becomes increasingly important.
If you are consuming more carbohydrates than is needed, some carbohydrates could be stored as glycogen in non-active tissue rather than being oxidised/used for immediate energy.
I love your curiosity @MxNat! I know that’s a lot, so I tried to keep this as concise as possible. I hope it helps
Oh and something that might be of interest to you: I spoke with Dr Podlogar on an episode of the Science of Getting Faster Podcast to discuss his research on consuming 90 g/hr vs 120 g/hr. I learned A LOT! If you’re interested, you’ll find it here: https://youtu.be/aClmBsQM3Xs?si=91R2Ebzy12fSWXYk
Thank you so much, Sarah! This definitely helps me understand this!
I’ve been dealing with the challenges of type 2 diabetes, and the relationship between the various energy stores has baffled me for some time.
I now have a continuous glucose monitor which is helpful for tracking changes in blood glucose levels, real time. With the info in your answer, it now makes sense that on a zone 2 ride, my BG can stay relatively high for the first 60-90 minutes, but if I’m doing any intervals at threshold or above, my levels drop much sooner. Since I’m measuring the levels in my blood stream, it doesn’t change very quickly as long as I’m burning muscle glucose.
Very interesting!
I’m still experimenting with fueling for rides. Since I’m fairly insulin resistant, it’s hard to pre-fuel without a huge BG spike. I also get spikes if I fuel with sucrose during the first 60 minutes or so. Do you know of any resources about the unique challenges of fueling with type 2 diabetes?
Thank you, and the whole TR team for all the work you do for athletes from all walks of life. My consistent training has not only made me faster, but also helped me lose weight and increase my ability to process insulin!
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