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How Low Carbohydrates Can Increase Aerobic Capacity: The Glycogen Threshold

How Low Carbohydrates Can Increase Aerobic Capacity: The Glycogen Threshold
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We often discuss concepts like the aerobic threshold, the anaerobic threshold, the lactate threshold, etc. But have you heard of the Glycogen Threshold? To put it simply, low carbs availability in your muscles can serve as strong signals to increase your muscle’s aerobic capacity.

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Hello everyone, welcome to fall! I hope you had a great summer of riding and are enjoying your hard-earned fitness as the days continue to shorten. In this article, I would like to introduce a relatively new concept known as the ‘Glycogen Threshold’. I will briefly discuss the cellular mechanisms behind it, as well as discussing how & why it might help your training.

The Role of Carbohydrates in Cell Signaling

As cyclists, we tend to love our carbohydrates: pastries, pastas, and pizzas, oh my! And the principle of carb-loading before a big ride or race is well known by us all. With that in mind, the theory of deliberately training with reduced carbohydrate (CHO) availability is a heavily debated topic in sports nutrition.

Training in a fasted/carb-depleted state can significantly impact fat oxidation during steady-state cycling (Hulston, et al., 2010), as well as stimulate mitochondrial biogenesis, eg, the creation of more mitochondria, commonly referred to as “the powerhouses of the cell” (Bartlett, et al, 2013). In fact, I have already elaborated on the concept of “training low”, or training with reduced carbohydrate availability previously.

Increasing mitochondrial biogenesis and fat oxidation is highly sought after in the endurance community, since our bodies are only capable of holding so many carbs. The feeling of hitting the wall/bonking on a ride when you run out of carbs can be miserable.

What happens during Carb Depletion?

There are a few ways to intentionally cause carb depletion, but they can be summarized below:

  1. Two-a-day training – the first training session of the day is followed by reduced CHO intake so that the second training session is performed with reduced muscle glycogen.
  2. Fasted training – we’ve covered this topic previously, but the idea is to wait until after your training session to eat breakfast
  3. Sleep low, train low – like two-a-days, the idea here is to perform an evening training session followed with reduced CHO intake overnight preceding a fasted training session in the morning

Regardless of how carbohydrate depletion is achieved, the result is similar: reduced muscle glycogen serves as a powerful signal to our cells. Our cells respond to these low levels of carbohydrate by employing two vital cellular messengers, PGC-1α and AMPK, both of which are vital energy regulators for your cells. These messengers indicate that the cell is critically low on CHO and in response, the cell must be prepared to effectively utilize fats as a fuel source. These sorts of adaptations which increase fat utilization are commonly sought after by athletes & their coaches.

Note: It is important to point out that key (HIIT) training sessions and competitions should be performed with high carbohydrate availability!

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The Glycogen Threshold

Researchers have proposed a ‘glycogen threshold’, where a critical absolute level of glycogen depletion during (or following) training is especially potent for skeletal muscle adaptations – mainly for increases in mitochondrial biogenesis and increases in fat oxidation. To put it simply, low carbs availability in your muscles can serve as strong signals to increase your muscle’s aerobic capacity.


It is interesting to note that glycogen levels can be depleted from a wide range of exercises, from short, high-intensity efforts, to longer, aerobic exercise.

This image shows that the proposed ‘glycogen threshold’ (the gray band) can be reached via a wide variety of training sessions – from 9 min of HIIT to 4 hours of endurance riding. Muscle glycogen concentration is displayed on the vertical axis, while exercise duration is displayed along the horizontal axis. Image taken from Impey, et al., 2018.

Future Studies

The existence of a glycogen threshold does not mean that you won’t achieve adaptations for endurance training if you perform training with higher levels of muscle glycogen. Rather, this threshold suggests that enhanced adaptations associated with low levels of muscle glycogen are especially prominent once a certain amount of depletion has been reached. However, further research is required to understand exactly how low muscle glycogen concentration needs to be to optimize skeletal muscle adaptations – does the threshold change for different athlete populations? More research is also needed to understand the carbohydrate cost of essential training sessions – think about workouts like 4×8, 5×5, etc. – so that athletes ensure they have enough carbs to successfully complete those tough sessions. Nobody wants to be bonking during a hard workout!

How Can You Apply This?

In the real world, it is likely that you might practice one (or several) of the methods mentioned previously to reduce your carbohydrate availability. I recommend trying out a fasted ride maybe once or twice per week – be sure it’s for one of your easier/aerobic rides and not a high-intensity ride/workout, or a Zwift race!

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Try it on a Zwift race

Conclusion

In today’s article, we explored a bit more behind what makes fasted training, and low carbohydrate availability, such a potent stimulus. In my next couple of articles, we will continue exploring some of the key cellular regulators that help you adapt & increase your fitness level. That’s all for this month. Stay safe, ride fast, and I’ll see you next month!

References

Impey SG, Hearris MA, Hammond KM, Bartlett JD, Louis J, Close GL, Morton JP. Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis. Sports Med. 2018 May;48(5):1031-1048. doi: 10.1007/s40279-018-0867-7. PMID: 29453741; PMC ID: PMC5889771.

Bartlett, JD, Louhelainen, J., Iqbal, Z., Cochran, AJ, Gibala, MJ, Gregson, W., … & Morton, JP (2013). Reduced carbohydrate availability enhances exercise-induced p53 signaling in human skeletal muscle: implications for mitochondrial biogenesis. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 304(6), R450-R458. https://pubmed.ncbi.nlm.nih.gov/20351596/

Hulston, CJ, Venables, MC, Mann, CH, Martin, C., Philp, A., Baar, K., & Jeukendrup, AE (2010). Training with low muscle glycogen enhances fat metabolism in well-trained cyclists. Medicine and Science in Sports and Exercise, 42(11), 2046-2055. https://pubmed.ncbi.nlm.nih.gov/23364526/


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