‘Glycogen Manipulation Training (GMT)’
The human body primarily stores the carbohydrates (CHO) we eat as muscle and liver glycogen. During exercise these two ‘fuel tanks’, along with fat, supply the working muscles with energy. Before a race we do everything we can to ensure these ‘fuel tanks’ are topped up by carbo-loading and tapering. During a race we regularly take in CHO to ensure these ‘fuel tanks’ do not run dry. It is common knowledge then, that CHO intake improves endurance performance. However, when trying to develop and improve endurance capacity, are these practices optimal? Most athletes, nutritionists and coaches would say: “of course - you should be well fuelled before and during training to allow you to train longer and harder”. In this article I will explore whether this is in fact the case.
If we strip ‘training’ back to basics, we see that it is a ‘stress’ that we place upon the body. The human body is an amazing thing. When it is placed under stress it adapts so that the next time it experiences a similar stress it is better prepared to deal with it:
Training (stress) + Recovery = Increased Performance
With this in mind, when we head out the door to go training we are doing so to add a ‘stress’ to our body (different training sessions will induce different forms of stress). If we look at the development of endurance capacity (being able to exercise for a long duration), we find that two of the main changes occurring in the body are up-regulation of fat being utilized as fuel, and an increase in the number and size of mitochondria (the powerhouses of muscles).
At a genetic level, the signalling pathways that tell the body to use more fat and make mitochondria are activated when our body experiences the stress of low glycogen stores caused by sustained exercise. Consequently, when the body next experiences these low glycogen levels it is better prepared to cope. These adaptations help improve an athlete’s ability to perform while low on glycogen, leading to improved endurance capacity. If we can improve our endurance capacity by instigating this glycogen depletion, then consuming CHO during training can actually be counterproductive. You might argue that failing to take on CHO during exercise might hinder your ability to train for as long. However I would suggest that by achieving a state of energy depletion faster by withdrawing CHO intake, you will stimulate the endurance adaptations described above sooner and effectively negate the need to train for as long.
So what might GMT look like?
After an eight hour overnight fast (i.e. sleeping) your liver glycogen levels are depleted. Any training done in the morning will therefore take approximately one hour at moderate intensity to achieve a state of energy depletion.
Training twice a day can manipulate glycogen stores. The morning session will cause low muscle glycogen meaning if you train again in the evening you will be training with low muscle glycogen stores and therefore stimulating the low energy adaptations that improve endurance capacity (fat utilization and mitochondrial biogenesis).
Preceding long steady sessions with a few high intensity intervals can cause muscle glycogen depletion and allow the remainder of the session to be performed in a low energy state.
During GMT sessions you may feel flat and your absolute training intensity will be compromised. However these sessions will contribute significantly towards endurance development.
When should GMT be used?
During your base training phase: when you are developing your endurance capacity and actual training intensity is not of primary importance
When training time is short: for example a 45 minute session on an empty stomach before breakfast may instigate similar training adaptations to a 90 minute ‘fuelled’ session.
When GMT should NOT be used?
If you have diabetes or have other conditions that impair your ability to regulate your blood glucose.
During high intensity training sessions: the focus during speed work sessions is the intensity. Low glycogen levels will compromise performance during these sessions and should thus be avoided.
During race simulations: the aim of race simulations is to trial gear, nutrition and pacing strategies specific to the upcoming race.
If you are feeling ill: avoid the added stress of GMT when feeling unwell. Instead ensure you are well nourished and rested.
If you struggle to maintain weight: GMT can compromise your ability to maintain a healthy weight and should be avoided if this issue exists.
Precautions for GMT:
If you feel shaky, light-headed, faint or empty then STOP and refuel.
Always carry spare gels, sports drink and/or money etc, so you can refuel yourself if needed.
Post-training consumption of quality CHO and protein food is critical following these sessions.
Finally this type of training should not to be performed year-round. Just like other training focuses (e.g. strength, speed intervals), GMT should be inserted thoughtfully within a periodised plan with the goal of extending your performance limits on race day.
If you have any questions about GMT or if you would like help intergrating it into your training please let me know.
Sport Scientist and Performance Coach