How low-carb can increase aerobic capacity: the glycogen threshold

We often talk about concepts like aerobic threshold, anaerobic threshold, lactate threshold, etc. But have you heard of the glycogen threshold? Simply put, the low availability of carbohydrates in the muscles can serve as strong signals to increase the aerobic capacity of the muscle.

Hello everyone, welcome to fall! I hope you had a great summer riding and are enjoying your hard earned fitness as the days continue to grow shorter. In this article, I would like to introduce a relatively new concept known as “Glycogen Threshold”. I will briefly discuss the cellular mechanisms behind it, as well as how and why it can help your training.
The role of carbohydrates in cell signaling
As cyclists, we tend to love our carbs – pastas, pastas and pizzas, oh my! And the principle of carb loading before a big ride or race is well known to all. With this in mind, the theory of deliberately training with reduced The availability of carbohydrates (CHO) is a much debated topic in sports nutrition.
Training in a fasted/carbohydrate-depleted state can significantly affect fat oxidation during the steady-state cycle (Hulston, et al., 2010), as well as stimulate mitochondrial biogenesis, e.g., the creation of more mitochondria, commonly known as “the powerhouse of the cell” (Bartlett, et al, 2013). In fact, I have previously developed the concept of “low training”, or training with reduced carbohydrate availability.
Increasing mitochondrial biogenesis and fat oxidation is highly sought after in the endurance community, as our bodies can only handle so many carbohydrates. The feeling of hitting the wall/stuck 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-day training: The first training session of the day is followed by reduced CHO intake, so the second training session is performed on reduced muscle glycogen.
  2. Fasted Training: We covered this topic earlier, but the idea is to wait until after your breakfast workout
  3. Sleep Low, Train Low – Like two a day, the idea here is to do a training session in the evening followed by a reduced CHO intake overnight before a fasted training session in the morning

Regardless of how carbohydrate depletion is achieved, the result is similar: the reduction in muscle glycogen serves as a powerful signal to our cells. Our cells respond to these low carbohydrate levels by employing two vital cellular messengers, PGC-1α and AMPK, both vital energy regulators for cells. These messengers signal that the cell has a critical level of CHO, and in response the cell must be ready 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 and competition (HIIT) sessions should be performed with high carbohydrate availability!
The glycogen threshold
Researchers have proposed a “glycogen threshold,” where a critical absolute level of glycogen depletion during (or after) exercise is particularly potent for skeletal muscle adaptations, primarily to increase mitochondrial biogenesis and increase fat oxidation. To put it simply, Low availability of carbohydrates to muscles can serve as strong signals to increase aerobic muscle capacity.

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 by a wide variety of training sessions: from 9 minutes of HIIT to 4 hours of endurance riding. Muscle glycogen concentration is shown on the vertical axis, while exercise duration is shown along the horizontal axis. Image taken from Impey, et al., 2018.
Future studies
The existence of a glycogen threshold yes no means that you will not achieve resistance training adaptations if you train with higher levels of muscle glycogen. Rather, this threshold suggests it get well the adaptations associated with low muscle glycogen levels are particularly prominent once a certain amount of exhaustion has been reached. However, more 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 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.) so that athletes can ensure they have enough carbs to successfully complete these tough sessions. No one wants to hurt themselves during a hard workout!
How can you apply this?
In the real world, you likely practice one (or more) of the methods listed above to reduce your carbohydrate availability. I recommend trying a fasting trip 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 Zwift race!
Try it in a Zwift race
In today’s article, we’ve explored a little more what makes fasted training and low carbohydrate availability such a powerful stimulus. 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 it for this month. Be safe, drive fast and see you next month!
Impey SG, Hearris MA, Hammond KM, Bartlett JD, Louis J, Close GL, Morton JP. Fuel for required work: a theoretical framework for carbohydrate periodization and the glycogen threshold hypothesis. Sports Med. May 2018;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., … and 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.
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. Medicine and Science in Sports and Exercise, 42(11), 2046-2055.

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