The last few weeks I have been doing a fair amount of fasted training first thing in the morning.
Nothing builds a little mental resiliency like trying to produce 150% of your 2k wattage for 100m before 6AM!
Outside of the potential mental benefits of fasted training sessions, doing fasted endurance exercise can help improve your metabolic flexibility.
Metabolic flexibility is all about your ability to use the correct fuel based on the demands placed on your body.
Watching Netflix? You want to be using primarily fat.
Running a 5k as hard as you can? You want to be using primarily carbs.
The problem is most people are using carbs when watching Netflix.
When it comes to fasted training, researchers have found that a specific protocol called “sleep low” has been shown to improve fat oxidation (your ability to burn fat as a fuel) which is exactly what most people struggle with.
The idea is quite simple.
On a day you are going to do a particularly hard workout, something like intervals, heavy strength training or other high intensity workout, you finish your workout but consume very little carbohydrates for the rest of the day. The next day, before eating a meal, you perform some type of endurance training in a fasted state. It could be an easy row, run, bike, hike or swim for example.
The combination of low muscle glycogen, depleted by the high intensity workout the day before, and fasted aerobic exercise appears to elicit a better training adaptation to endurance exercise as well as improve your ability to burn fat.
Here is a simple diagram illustrating the sleep low protocol.
In one study on cyclists, a sleep low strategy improved overall cycling efficiency by 3.1%, 20-km cycling time trial performance by 3.2% and 10-km running performance while also improving fat oxidation . We should keep in mind though that these improvements were made in the sleep low group from the start of the experiment to the end. It is not an improvement when compared to the carbohydrate consuming group in the experiment. Both the carbohydrate consuming group and the sleep low group saw similar improvements in performance over the course of the trial. In other words a sleep low approach did not seem to prevent adaptations from training to occur. The only difference was that the sleep low group was utilizing more fat.
Another study looked at sleep and immune system impacts of a sleep low strategy and found it to have no impact on the sleep or immune system participants using a sleep low strategy for 3 weeks. Again this is over the course of just three weeks. The impacts of a sleep low approach have not been studied long term. Anecdotally, for myself and other athletes I have worked with, not replenishing carbohydrates post workout can have a negative impact on sleep quality over the course of time.
It is important not to confuse the enhanced fat oxidation benefits of a sleep low approach to training with enhanced performance when compared to an athlete consuming carbohydrates. A diagram from the study done on cyclists clearly demonstrates that the athletes consuming carbohydrates post exercise are able to produce more power.
Should you utilize a sleep low approach in your training?
If you are an athlete who is highly dependent on carbohydrate to fuel even the easiest of efforts, incorporating a “sleep low” strategy can help you become more metabolically flexible while at the same time continuing to improve aerobic performance. Since aerobic metabolism and fat oxidation are contributing to your performance even at high intensities, a sleep low approach can help even on race day when it is time to drop the hammer.
How should I incorporate it?
When it comes to incorporating a sleep low approach into the programming for the athletes I work with I rarely formally program it into their training. I like to incorporate it as a “happy accident” that occurs randomly and naturally. I find that opportunities to sleep low happen naturally as a side effect of eating a healthy diet and having a well defined training routine.
For example, a client might do a strength training session or some intervals and then decide to have a salad for dinner post training and just go to bed eating nothing else. The next morning they get up and do their usual aerobic exercise like an easy 5k on the rower or a 3 mile walk, or 20 minutes on the bike and then have a breakfast containing carbohydrates.
I find that when people get caught up trying to maximize their ability to burn fat they often end up sacrificing performance and recovery with too much fasted training at too high of an intensity. A more intuitive, relaxed approach works best and produces just as good of results in practice. If you are looking to enhance your metabolic flexibility it is fine to try some fasted training, but make sure it is easy and not too rigid. Even a simple walk in the morning can help up-regulate your ability to burn fat. No reason to do intervals, lift heavy weights, or do extremely long drawn out aerobic work. Can you do that from time to time? Sure. But if you want to improve your metabolic flexibility in a safe sustainable manor keep it intuitive and relaxed
Looking to increase your metabolic flexibility to better utilize fats or carbs to make you a better athlete? Sign up to my newsletter then! . By joining you will get practical information and advice on how to become a better athlete by following an ancestral approach to life and training. Metabolic flexibility was built into the lives of our ancestors and by looking to them to help guide our approach to life and training you can naturally build the metabolic flexibility to help overcome your greatest obstacle.
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(n.d.). Training with low muscle glycogen enhances fat metabolism …. Retrieved September 8, 2020, from https://pubmed.ncbi.nlm.nih.gov/20351596/ ↑
(2016, August 4). The impact of sleeping with reduced glycogen stores … – NCBI. Retrieved September 8, 2020, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5020129/ ↑
(n.d.). Training with low muscle glycogen enhances fat metabolism …. Retrieved September 9, 2020, from https://pubmed.ncbi.nlm.nih.gov/20351596/ ↑