We have previously discussed timing of your pre-exercise meal and whether or not you should train fasted. We have also discussed the role that eating before training may play in training the gut. Despite all of this, we are still regularly asked about what to do if eating is not possible before exercise.
The answer to this question hinges largely on the context of the exercise itself, specifically;
- Overall dietary pattern
- Experience level
Let’s dissect this a little further and then discuss some answers.
The question of not being able to eat before exercise is much less of a factor for exercise taking place later in the day. That said, there is still the concern of proximity to meals, for example having to exercise on your lunch break or after work.
Generally this is not a significant issue, as it is unlikely that you have depleted your glycogen stores within the time between breakfast and lunch or even lunch and dinner. In these cases, you need not worry about the timing of exercise, which becomes more of a factor when discussing exercise in the morning before breakfast.
In short, this is all about glycogen. So if you are doing a training session that is high enough intensity and long enough duration, dictating a need for high glycogen store level(see below) make sure you have had some carbohydrates in the previous 6 hours or so, preferably within the last 4.
Duration of Exercise:
To a degree, the duration of exercise is very tightly connected to the intensity of exercise. It is very difficult to exercise at truly high intensities for prolonged durations.
Generally though, exercise of less than 90 mins does not require a great deal of fueling (there is some gray here, experienced athletes may not need anything whereas beginners may). Once there is some intensity added or significant duration beyond this, carbohydrate availability can become important and glycogen stores may start to become limiting for the quality of training. Experience level becomes a factor in this, with more experienced athletes perhaps dealing with long low intensity training sessions more easily given their higher fat oxidation capacities whereas beginners may struggle.
In short, shorter sessions, even if intense, are probably fine fasted.
Intensity is the primary driver of carbohydrate oxidation rates, with higher intensities burning proportionately and ultimately more carbohydrates than lower intensities. As mentioned above the mixture of intensity and duration is key, because shorter bouts of intensity are well tolerated even with an overnight fast, as muscle glycogen content is still preserved overnight even though liver glycogen may be somewhat depleted.
In short, intensity is what drives carbohydrate needs. If fasted make sure your glycogen stores are high enough from the previous days’ intake.
The below is from a 2022 paper by Podlogar and Wallis, and provides a proposed framework for carbohydrate periodization (strategically limiting and providing carbohydrates for the requirements of the upcoming session or session just completed). Of note, the exercise intensity domain in this case refers to the highest intensity achieved during the session and uses the equivalent of a 3 zone model of intensity where moderate, heavy and severe refer to zones 1, 2 and 3 respectively.
*Abbreviations: CHO carbohydrates, CP critical power, LT1 lactate threshold 1, LT2 lactate threshold 2, MLSS maximal lactate steady state
The goal of exercise is extremely important for the context of dietary patterns surrounding the exercise session. Those training for athletic performance, specifically with the goal of performance output in the upcoming session should ensure carbohydrate availability is optimized. Whereas those who are exercising for more health purposes or are completing a training session that is less important may find carbohydrate availability is less important. This may also be impacted by the phase of the season as mentioned in this article.
In short, understand the carbohydrate needs of the specific exercise session planned and make sure you plan ahead for this.
As discussed in this article both habitual diet and recent dietary intake impact oxidation rates of fat and carbohydrates - you will burn more of what you have eaten more of. More importantly, for this specific question, is that glycogen stores will vary based on a more sub-acute intake (think 2-4 meals) and exercise. It can be difficult to really understand glycogen levels, even for researchers, but you can get a gauge on these using a few factors:
- Carbohydrate intake from the previous 24-48hrs
- Training load in the previous 24hrs, especially high intensity training
- Trailing average glucose - this is not a direct relationship as discussed in the above article but there are some signals that you may be helpful (significantly higher or lower trailing average glucose over the previous 24-48 hrs).
In short, following days with lower carbohydrate intakes or more activity that many have depleted glycogen, stores may be lower, making carbohydrate intake more important to sustain output.
As discussed above, experience level or more specifically training history in endurance sports plays a major role in fat oxidation rates. Higher fat oxidation rates generally mean more sparing of glycogen stores and such less need for supplementation of carbohydrate to ensure carbohydrate availability.
In short, as a beginner or someone stepping up to newer durations of activity, you may require more carbohydrate intake to ensure appropriate carbohydrate availability.
It should now be clear that you may not always need to eat before exercise, in which case, you can safely have your coffee (surely you can’t ‘human’ let alone exercise without this) and get going.
If, however, you have a session that may require some intake and you can’t eat before training, then the best thing to do is fuel during your training session.
Some interesting facts about fasted training when fueling during training:
- When restricting carbohydrate intake, the rate of oxidation of endogenous (stored) carbohydrates is reduced.
- Fat oxidation rates will be higher when training fasted - even if fueling during training.
- It is important to ensure appropriate caloric intake during the preceding days - there is some suggestive research that indicates caloric needs may impact ability to oxidize carbohydrates when fueling during these sessions.
Take Home Message:
- If you can’t eat before training, don’t sweat it.
- If you haven’t eaten before training, consider whether the session needs fuel based on the factors mentioned above.
- If you need to fuel your session and can’t eat ahead of time, do so during the session - likely split evenly across it.
- Make sure you are eating enough calories and carbohydrates (unless intentionally trying to train in a low glycogen state) in the 24-48 hrs prior to your key training sessions.
- Margolis LM, Wilson MA, Whitney CC, Carrigan CT, Murphy NE, Hatch AM, Montain SJ, Pasiakos SM. Exercising with low muscle glycogen content increases fat oxidation and decreases endogenous, but not exogenous carbohydrate oxidation. Metabolism. 2019;97:1–8. https://pubmed.ncbi.nlm.nih.gov/31095946/
- Stephanie D Small, Lee M Margolis, Impact of Dietary Carbohydrate Restriction versus Energy Restriction on Exogenous Carbohydrate Oxidation during Aerobic Exercise, Advances in Nutrition, Volume 13, Issue 2, March 2022, Pages 559–567, https://doi.org/10.1093/advances/nmab124
- Podlogar T, Wallis GA. New Horizons in Carbohydrate Research and Application for Endurance Athletes. Sports Med. 2022 Dec;52(Suppl 1):5-23. doi: 10.1007/s40279-022-01757-1. Epub 2022 Sep 29. PMID: 36173597; PMCID: PMC9734239. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9734239/