How Low Carb Can Increase Aerobic Capacity: The Glycogen Threshold - PezCycling News

How Low Carb Can Increase Aerobic Capacity: The Glycogen Threshold – PezCycling News

We often discuss concepts like aerobic threshold, anaerobic threshold, lactate threshold, etc. But have you heard of the glycogen threshold? To put it simply, low availability of carbohydrates in your muscles can serve as strong signals to increase the aerobic capacity of your muscles.

Hello everyone, welcome to autumn! Hope you had a great summer riding and enjoying your hard earned fitness as the days continue to get shorter. 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 how and why it might help your training.

The role of carbohydrates in cell signaling

As cyclists, we tend to love our carbs: pastries, pasta and pizza, oh my! And the principle of carb loading before a big ride or race is well known to all of us. With this in mind, the theory of deliberate training with reduced Carbohydrate availability (CHO) is a hotly debated topic in sports nutrition.

Fasted/reduced carbohydrate training can significantly impact fat oxidation during the steady-state cycle (Hulston, et al., 2010), as well as stimulate mitochondrial biogenesis, for example, the creation of more mitochondria, commonly referred to as “the powerhouse of the cell” (Bartlett, et al, 2013). In fact, I have already developed 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 because our bodies can only hold so many carbohydrates. The feeling of bumping into the wall or banging on a commute when you’re low on carbs can be miserable.

What happens during carb depletion?

There are several ways to intentionally induce carb depletion, but they can be summarized below:

  1. Alternate day training – the first training session of the day is followed by reduced carbohydrate intake, so the second training session is performed with reduced muscle glycogen.
  2. Fasted training – we have already covered this topic, but the idea is to wait after your workout to have breakfast
  3. Sleep low, train low – like two days a day, the idea here is to perform an evening workout followed by reduced CHO intake overnight before a morning fasted workout

Regardless of how carbohydrate depletion is achieved, the result is similar: the reduction of muscle glycogen serves as a powerful signal to our cells. Our cells respond to these low carbohydrate levels using 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 extremely low in CHO and in response, the cell must be primed to efficiently use fat as a fuel source. These types of adaptations that increase fat utilization are commonly sought after by athletes and their coaches.

Note: It is important to emphasize that key workouts and competitions (HIIT) should be performed with high carbohydrate availability!

The glycogen threshold

The researchers proposed a “glycogen threshold,” where a critical absolute level of glycogen depletion during (or after) training is particularly potent for skeletal muscle adaptations – primarily for increased mitochondrial biogenesis and l increased fat oxidation. For simplicity, low availability of carbohydrates in your muscles can serve as strong signals to increase the aerobic capacity of your muscles.


Interestingly, glycogen levels can be depleted from a wide range of exercise, from short, high-intensity efforts to longer aerobic exercises.

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

Future studies

The existence of a glycogen threshold does not This means that you won’t get adaptations for endurance training if you train with higher levels of muscle glycogen. This threshold suggests rather that improved the adaptations associated with low levels of muscle glycogen are particularly important once a certain degree of exhaustion has been reached. However, further research is needed 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 carb cost of core workouts – think workouts like 4×8, 5×5, etc. – so that athletes ensure they have sufficient carbohydrates to carry out these difficult sessions. No one wants to have a blast during a hard workout!

How can you apply this?

In the real world, chances are you practice one (or more) of the previously mentioned methods to reduce your carb availability. I recommend trying a fasted ride maybe once or twice a week – make sure it’s for one of your easier/aerobic rides and not a high intensity ride/workout, or a Zwift race!

fast
Try it on a Zwift run

Conclusion

In today’s article, we explored a bit more about what makes fasted training and low carb availability such a powerful stimulus. In my future articles, we will continue to explore some of the key cellular regulators that help you adapt and 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. Med. athletic. 2018 May;48(5):1031-1048. doi: 10.1007/s40279-018-0867-7. PMID: 29453741; PMCID: 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 improves fat metabolism in well-trained cyclists. Sport and Exercise Medicine and Science, 42(11), 2046-2055. https://pubmed.ncbi.nlm.nih.gov/23364526/

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