Fact: Fasted exercises can be difficult. Not only do you have to fast, but then you have to get through a workout while fasting. No easy feat.
Yes. There are benefits to fasted training sessions. But understanding the context for when your body actually needs them is critical. There are two main reasons to do fasted exercise sessions:
1) Enhance your ability to rely on fat as a fuel during low-to-moderate intensities
2) Induce some specific metabolic adaptations linked to exercising with reduced glycogen stores
(Note: These effects are not mutually exclusive of only carbohydrates restricted diets.)
Preparing to Exercise Fasted
Begin fasting (i.e., no caloric intake) after dinner time. Plan to exercise the following day in a fasted state. The length of your fast is dependent on your goals and experience, but generally an overnight fast is sufficient for analyzing the impact to your glucose levels.
Train as planned without any caloric intake pre- or mid-exercise.
Analysis: What to Expect in Your Glucose Curve
Depending on the various aspects of your training session–intensity, duration, individual factors like current glycogen stores and metabolic adaptations–your glucose data will vary. Apply the information below to your analysis and open discussion in our Facebook Athletes Group if you have any questions.
Rises in Glucose Levels
THE WHAT: If your exercise is high intensity, your body will deploy liver glycogen to aid in fueling. This will cause a rise in glucose even though you haven’t consumed any carbohydrate.
THE WHY:
1) The brain is telling the liver to output some glucose for the muscles to use as fuel.
2) Your muscle glycogen metabolism is signaling the liver to control the muscle glucose burning rate.
This glucose rise may continue as long as your exercise is sufficiently high intensity or as long as you have sufficient liver glucose stores. It will also likely rise any time you drop your intensity a little bit, at least initially.
THE ACTION: At some point your glycogen stores will be exhausted, your RPE will go up, intensity will drop, and you will feel negative physical and mental performance degradations. In other words, consider how long you plan to train at high intensity as your glucose levels continue to rise. This is where fueling to increase or maintain exercise intensity is a good idea. But if your training that day includes a long, slow, medium-intensity workout, you can avoid it. Your body is more reliant on fat as fuel for lower intensity efforts.
Plateaus in Glucose Levels
THE WHAT: When exercising at a relatively consistent and low or medium intensity, your glucose levels will likely be stable. Notice this from the onset of your workout, particularly during low intensity training.
THE WHY: This is an indication that things are stable. You may still exhaust your liver glycogen stores as you would in a high intensity session, but this will be more gradual, less sudden. Your body might be capable of maintaining a relatively stable glucose level for at least 2 hours of moderate exercise intensity. Interestingly, the glucose curve might be used as a sign of metabolic adaptation over time with positive adaptation showing longer maintenance of a stable glucose curve and/or this occurring at a higher intensity.
THE ACTION: Using a standardized intake prior to a session, aim to complete 2 identical training sessions with steady, low zone 3 effort (using the 5 zone model) for the first experiment. Check your glucose levels and look for any drops over time. If you can keep a stable glucose level without needing to intake any fuel to maintain glucose on the second attempt, this might be a good indication that you have improved your ability to oxidize fat and use it as a fuel, sparing glucose for higher outputs.
Drops in Glucose Levels
THE WHAT: Dropping glucose levels can result from a few situations. Commonly there is an initial drop in glucose as activity starts or perhaps if there is a spike in intensity. This occurs as a result of increased demand and usage of glucose, with supply yet to catch up. The reason for this increased demand is that your metabolism is yet to catch up to energy requirements. This might be one of the reasons for a warm up.
THE WHY: More concerning drops in glucose levels occur with longer session durations where both liver and muscle glycogen stores are depleted. These drops may follow sufficiently high intensity exercise to utilize stores of glycogen, the timeline for this is largely dependent on intensity and somewhat individual, based on carbohydrate and fat oxidation rates and pre-training glycogen stores. This can happen quite swiftly if the exercise intensity is high enough and fuel stores are truly depleted in your fasted state. During rapid drops in glucose levels you might experience fatigue, empty legs, dizziness, or worse. This is what leads to that all-too-familiar feeling of the “bonk.”
THE ACTION: Once you start seeing significant drops in glucose levels, exercise intensity and quality might be compromised. If you continue the session for an extended period of time, drop intensity and get some glucose.
Experiments
Decide on a strategy to test your training in a fasted state. Either plan to fast or simply employ minimal fuel intake between two sessions. For example, a morning and afternoon session.
*Note: fasted sessions, especially for those inexperienced in doing them, can be quite difficult and should not be performed in high intensity sessions initially.
Experiments to observe using the 5 Zone Model:
- Complete 60-90 minutes at Zone 2 intensity
- Complete 60-90 minutes at low Zone 3
- Complete 60-90 minutes at Zone 2 intensity with some intermittent 5 minute efforts at Zone 4 or 5
Note how you felt, performed, and compare your pace and power output to your glucose graph.
*Try to keep your post workout meal the exact same as other sessions and note the difference the exercise makes to your glucose response.
REFERENCES
- Coyle, E F et al. “Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate.” Journal of applied physiology (Bethesda, Md. : 1985) vol. 61,1 (1986): 165-72. doi:10.1152/jappl.1986.61.1.165
- Malone, James J et al. “Exogenous carbohydrate and regulation of muscle carbohydrate utilisation during exercise.” European journal of applied physiology vol. 121,5 (2021): 1255-1269. doi:10.1007/s00421-021-04609-4
- Mohebbi, Hamid et al. “Hyperinsulinaemia and hyperglycaemia promote glucose utilization and storage during low- and high-intensity exercise.” European journal of applied physiology vol. 120,1 (2020): 127-135. doi:10.1007/s00421-019-04257-9
- Fell, J Marc et al. “Carbohydrate improves exercise capacity but does not affect subcellular lipid droplet morphology, AMPK and p53 signalling in human skeletal muscle.” The Journal of physiology, 10.1113/JP281127. 26 Mar. 2021, doi:10.1113/JP281127
- Impey, Samuel G et al. “Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis.” Sports medicine (Auckland, N.Z.) vol. 48,5 (2018): 1031-1048. doi:10.1007/s40279-018-0867-7
