Whether you train fueled or fasted, knowing what you should eat before a workout is critical to achieving your performance goals.
In this article, we’ll cover:
- Fueling for different types of training sessions: duration, intensity, and goal
- Nutrition planning: fueling for the phase of the season, time between training sessions, and post-training
- Nutritional status: fueling up based on the duration of your fast and intake prior to that fast
- Glucose intake during a training session based on the above factors
Remember that the ultimate goal of nutritional intake prior to or during training is to ensure training quality. Of course you want to make sure your total caloric needs are met, but your primary focus should be on not compromising training quality. Many athletes think fueling or fasting for training drives adaptation, but training quality is the ultimate driver of such adaptations.
Let’s dive into the factors that influence how you should fuel, focusing on the timing and amount of carbs.
1. Fueling for Different Types of Training Sessions
One of, if not the biggest, factors impacting your glucose intake is the type of training session. Specifically, we’re referring to duration and intensity.
For the purposes of this piece, anything less than 60 minutes is considered short duration.
In short duration training sessions, the body often has enough available glucose and glycogen stores to ensure that training quality can be maintained without swings in energy. Even when training at a high intensity (assuming appropriate nutrition the day before) the body should be able to cope with all energy demands via two metabolic processes: glycolysis (breakdown of glucose) and gluconeogenesis (making new glucose).
As training duration increases, the need for some carbohydrate intake prior to and/or during the training session becomes more apparent.
That said, this is somewhat intensity-dependent. In truly aerobic training (Zone 1 or 2 in a 5-zone model, Zone 1 in a 3-zone model, or below VT1/ Aerobic Threshold) athletes should be able to train fasted without much intake for a significant amount of time given the relative contribution of different substrates. That’s because a majority of energy comes from fat oxidation at low intensities. Given a rough estimate of fat stores in the human body, it is said that a human could run over 400 miles at this sort of intensity if energy alone was the rate limiter.
Generally, as intensity increases, so too does your body’s reliance on glucose as fuel. To optimize performance at higher intensities (especially for long duration training), optimal glucose levels are critical.
But with over 40 factors impacting your glucose levels (from food to stress to altitude), maintaining optimal glucose levels to meet your desired performance at high intensity or long duration becomes increasingly difficult, especially without real-time data. This is made easier with your own glucose biosensors, available at supersapiens.com.
2. Nutrition Planning
Something that is often undervalued is where any one training session fits into a broader week of training, or the phase of training, or even the annual training calendar.
How you fuel for training sessions is critical depending on the specific performance or adaptations you are seeking.
How to Fuel for Easy Training Sessions
In earlier phases of the training calendar, when lower intensity training is more common and competition is quite far away, carbohydrate loading and in-training consumption may be of less importance. Especially if one of the training objectives is fat adaptation, or to increase the capacity to mobilize and utilize fat as a fuel.
Similarly, fasted training may be more reasonable and desirable during these earlier phases of the season for physical, mental, and metabolic training. Certainly these practices should be aligned with carbohydrate periodization.
How to Fuel for High Intensity Sessions Closer to Race Day
When higher intensity training becomes more voluminous and more frequent, typically closer to the competitive season, carbohydrate availability is especially important to maintain training intensity and quality. Timing carbohydrate intake before, during, and after training helps mitigate accumulating fatigue and inflammation.
Additionally, this phase of training should be a time to “train the gut” and get into race-day routine–practicing competition nutrition.
How to Fuel with Short Recovery Times
When training multiple times a day or with fairly short recovery periods, nutrition becomes more complex and more important. With limited time between training sessions, eating before training may be the best way to ensure appropriate carbohydrate availability and meeting total caloric requirements. Additionally, fueling with carbs during and immediately after training (much more than you’ve previously eaten) can support glycogen replenishment and muscle protein synthesis to ensure you are better prepared for the next session.
3. Nutritional Status
Whether to eat or not before a training session depends on when and what you last ate, or your nutritional status.
The nutritional status of an endurance athlete can be boiled down to: “How full are your glycogen stores?”
Remember, glycogen is the storage form of glucose.
Ensuring appropriate glucose intake allows glycogen stores to be topped off and provide you the extra high-intensity fuel needed to crush training sessions of higher intensity and longer duration. Generally, that’s what “carb loading” means.
Different glycogen stores play a role in metabolic adaptations during and after training. This research paper by Impey in 2018 dives deep into the topic.
For the sake of keeping things simple in this article, that means glycogen stores in your muscles and glycogen stores in your liver are used differently. Muscle glycogen can only be used locally. Liver glycogen can be partially depleted even during sleep.
Higher intensity and longer duration training sessions will deplete glycogen stores, so refueling with carbohydrates is crucial immediately after these types of training sessions. This is particularly true if you have multiple training sessions in a day.
While still early, elite athletes like Lisa Norden have been noticing this pattern: If your glucose levels are lower than normal, or less responsive (flatter) than normal, especially after food or activity that would normally increase glucose, your glucose data could be telling you that your are seriously under-fueled and need to eat more carbohydrates.
Scenarios:
1. Low Glycogen
Scenario: low fuel intake the previous day (specifically carbohydrates) and/or high training load with low glucose intake, limited post-training glucose intake, and resultant caloric deficit.
Strategy: more carbohydrates at breakfast (ideally at least 3 hours prior to training), more carbohydrates during training if breakfast isn’t an option, or a combination of these.
2. Normal to High Glycogen
Scenario: rest day with normal or large fuel intake (specifically carbohydrates) or an easy training day with sufficient glucose intake. Given these situations, it is likely that your glycogen stores are full enough for most (shorter) training sessions, but probably still not optimal for racing.
Strategy: carbohydrate intake at breakfast at least 3 hours prior to training or supplemental glucose intake during training. If your nutritional status is sufficient from a glucose/glycogen perspective, you will likely be ready for any training session not excessively long or sustained high intensity.
4. Glucose Intake During the Training Session
If you don’t fuel up before a training session, you’re going to want to increase your carbohydrate intake during the workout.
This is not a perfect strategy and is definitely not a direct substitute for carbohydrate loading, but serves as a way to help maintain training quality at intensity when you otherwise are not able or willing to eat something before. For example, very early training sessions due to climate or life constraints (work, family, etc). In these scenarios, it’s recommended to fuel up and carbohydrate load the night before.
The goal of a pre-training meal is to top up liver glycogen stores (though there is probably some topping up of muscle glycogen stores that occurs too). The point of increasing intake during training can support your body’s fuel requirements when your glycogen stores are depleted.
This is illustrated below in Figures 4a and 4b from a study where cyclists performed a time to exhaustion test using a placebo or carbohydrate solution. The contribution of both liver glycogen stores, gluconeogenesis (making of new glucose), and carbohydrate supplementation can be seen in purple. The difference in the two diagrams (with and without carbohydrate supplementation) gives an indication that carbohydrate supplementation can help when glycogen stores are depleted.
Similarly, the figures give an indication of the role of glycogen (muscle and liver) and its contribution to exercise, which should be considered if, for instance, when trying to complete a high quality training session at a higher intensity when you have had a low carbohydrate intake over the previous few days.
All things considered, topping off your glycogen stores prior to a long duration or high intensity training session is preferred over not eating and trying to keep up with your body’s fuel demands during the workout.
Another thing to think about: logistics. For cyclists, increasing fuel intake during a workout is probably not a big concern. But runners, swimmers, and team sports athletes may have a challenge to significantly increase glucose intake during training or competition–thus the pre-exercise meal is even more important to help stay fueled to perform.
Example Training Weeks
Below are a few examples for athletes training once a day at an amateur, but serious level.
Endurance Athlete
Team Sport Athlete
*Note: “Not necessary” does not mean to not fuel. As mentioned, individual situations will vary based on lifestyle, athlete goals, and other factors.
Takeaways:
- Both fueled and fasted training can lead to metabolic, physical, and mental adaptations.
- Fasted training may impair quality of training in some cases, including long duration and high intensity.
- Fueling should be considered with the bigger picture in mind rather than just one training session in isolation.
Deciding to train fasted or fueled is individual and depends on numerous factors. We all know the pain and suffering of trying to push through a hard workout on an empty stomach–listen to your body and know that not all pain is leading to positive adaptations! In fact, you can achieve those same results by instead optimizing the timing of your glucose intake.
The biggest challenge in fueling is understanding and quantifying your current nutritional status. Supersapiens gives you the opportunity to visualize your glucose levels minute-by-minute to help dial in your nutrition plan.
References
- Coyle EF. Substrate utilization during exercise in active people. Am J Clin Nutr. 1995 Apr;61(4 Suppl):968S-979S. doi: 10.1093/ajcn/61.4.968S. PMID: 7900696. https://pubmed.ncbi.nlm.nih.gov/7900696/
- Ørtenblad N, Westerblad H, Nielsen J. Muscle glycogen stores and fatigue. J Physiol. 2013 Sep 15;591(18):4405-13. doi: 10.1113/jphysiol.2013.251629. Epub 2013 May 7. PMID: 23652590; PMCID: PMC3784189. https://pubmed.ncbi.nlm.nih.gov/23652590/
- Baur DA, Saunders MJ. Carbohydrate supplementation: a critical review of recent innovations. Eur J Appl Physiol. 2021 Jan;121(1):23-66. doi: 10.1007/s00421-020-04534-y. Epub 2020 Oct 27. PMID: 33106933. https://pubmed.ncbi.nlm.nih.gov/33106933/
- 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 Med. 2018 May;48(5):1031-1048. doi: 10.1007/s40279-018-0867-7. PMID: 29453741; PMCID: PMC5889771. https://pubmed.ncbi.nlm.nih.gov/29453741/
- Rothschild, J.A.; Kilding, A.E.; Plews, D.J. What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions. Nutrients2020, 12, 3473. https://doi.org/10.3390/nu12113473 https://www.mdpi.com/2072-6643/12/11/3473
