We often talk about concepts like aerobic threshold, anaerobic threshold, lactate threshold, etc. But have you heard of glycogen threshold? In a nutshell, low carbohydrate availability in the muscles can serve as strong signals to increase the aerobic capacity of the muscles.
Hello everyone, welcome to fall! I hope you had a great summer riding and that you are 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 ‘Glycogen Threshold’. I’ll briefly discuss the cellular mechanisms behind this, as well as discuss how and why it might help your workout.
The role of carbohydrates in cell signaling
As cyclists, we tend to love our carbs: cakes, pastas, and pizzas, oh my! And we’re all familiar with the principle of carb-loading before a big ride or race. With that in mind, the theory of deliberate training with reduced Carbohydrate availability (CHO) is a hotly debated topic in sports nutrition.
Fasted/no-carbohydrate training can significantly impact fat oxidation during the steady-state cycle (Hulston, et al., 2010), as well as stimulate mitochondrial biogenesis, i.e., the creation of more mitochondria, commonly known as “the powers of the cell” (Bartlett, et al, 2013). In fact, I have already developed the concept of “low training”, or training with reduced carbohydrate availability previously.
Increasing mitochondrial biogenesis and fat oxidation is highly sought after in the endurance community, as our bodies are only capable of containing a limited amount of carbohydrates. The feeling of hitting the wall/hitting on a ride when you run out of carbs can be miserable.
What happens during carbohydrate depletion?
There are a few ways to intentionally cause carb depletion, but they can be summarized below:
- Two-day training: The first training session of the day is followed by a reduced CHO intake, so that the second training session is performed with reduced muscle glycogen.
- Fasted training: We’ve covered this topic before, but the idea is to wait until after your training session for breakfast
- Sleep light, train light: Like twice a day, the idea here is to have a training session in the evening followed by a reduced intake of CHO overnight before a fasted training session in the morning.
Regardless of how carbohydrate depletion is achieved, the result is similar: the depletion of muscle glycogen serves as a powerful signal to our cells. Our cells respond to these low levels of carbohydrates by employing two vital cellular messengers, PGC-1α and AMPK, which are vital energy regulators for your cells. These messengers signal that the cell is critically low on CHO, and in response, the cell must be primed to effectively 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 note that key training sessions and competitions (HIIT) must be performed with a high availability of carbohydrates!
glycogen threshold
Researchers have proposed a “glycogen threshold,” where a critical absolute level of glycogen depletion during (or after) training is especially potent for skeletal muscle adaptations, primarily increasing mitochondrial biogenesis and increasing fat oxidation. To make it simpler, the low availability of carbohydrates in your muscles can serve as strong signals to increase the aerobic capacity of your muscles.
It is interesting to note that glycogen levels can be depleted by 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 across a wide variety of training sessions, from 9 min of HIIT to 4 hours of endurance cycling. Muscle glycogen concentration is shown on the vertical axis, while exercise duration is shown on the horizontal axis. Image taken from Impey, et al., 2018.
future studies
The existence of a glycogen threshold does not No it means that you will not make adaptations for resistance training if you train with higher levels of muscle glycogen. Rather, this threshold suggests that improved the adaptations associated with low muscle glycogen levels are especially prominent once a certain amount of depletion has been reached. However, more 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 populations of athletes? More research is also needed to understand the carbohydrate cost of essential training sessions (think workouts like 4×8, 5×5, etc.) for athletes to ensure they have enough carbohydrates to successfully complete those hard sessions. No one wants to be fucking around during a tough workout!
How can you apply this?
In the real world, you’ll likely practice one (or several) of the methods listed above to reduce your carbohydrate availability. I recommend trying a fasted walk maybe once or twice a week. Make sure it’s for one of your easier/aerobic walks and not for a high intensity workout/workout, or Zwift run!
Try it on a Zwift run
conclusion
In today’s article, we explore a little more about what makes fasted training and low carb availability such a potent pick-me-up. In my next 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, drive 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 Medicine 2018 May;48(5):1031-1048. doi: 10.1007/s40279-018-0867-7. PMID: 29453741; PMID: 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). Low muscle glycogen training improves fat metabolism in well-trained cyclists. Medicine and Science in Sport and Exercise, 42(11), 2046-2055. https://pubmed.ncbi.nlm.nih.gov/23364526/
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