What we eat before, during and after training has the potential to influence how well we train and the adaptive responses to endurance training. As highlighted in our article on fat adaptation, endurance training causes physiological adaptations to help you go further and faster, and part of that adaptation is an increase in fat oxidation.
Some recommendations for fasted training are based on either jump-starting the process of fat oxidation in untrained individuals, or enhancing fax oxidation rates in moderate- to well-trained athletes above those already gained from training. In both cases, this is done in hopes of performance improvement during ultra endurance races, big mountain days, ski mountaineering events, etc.
The interest and popularity around fasted training seems to originate from a 2016 review and meta analysis paper which concluded that training in a fasted state increases higher fat oxidation when compared to training in the fed state (1).
Following this study, there were a number of anecdotal reports from elite athletes claiming that the benefits of enhancing fat oxidation through fasting improved their performance.
Keep reading to dig into these claims and the basics of fasted training with high performance sports dietitian Rebecca Dent.
WHAT IS FASTED TRAINING?
Prevalence and Reasons to Carry out Fasted Training
A recent study looked at the reason why endurance athletes chose to do fasted training. Out of the study’s 1,227 participants, 62.9% reported carrying out some or all of their training in an overnight-fasted state. Out of the participants who reported training in a fasted state, 65% were non-professional athletes and only 29.4% were professional athletes. The most common reason given for fasted training was to use fat as a fuel source (n = 526, 42.9%). Other reasons were related to gut comfort (n = 435, 35.5%) and time constraints/convenience (n = 385, 31.4%).
Other reasons why athletes carried out fasted training were due to recommendations seen on social media/online (n = 851, 43.6%), from another athlete (n = 454, 23.3%), by a coach (n = 343, 17.6%), a nutritionist (n = 297, 15.2%), a physiologist (n = 138, 7.1%), or other sources such as personal reading (n = 83, 4.3%) (2).
Of those who stated an avoidance of fasted training, their reasons were that they felt it did not help their training (n = 340, 47.0%), they felt their training was worse (n = 251, 34.7%), and they got too hungry (n = 250, 34.6%) (2).
What does the research say?
Uphill Athlete makes nutritional and coaching recommendations based on general consensus from the current body of research, also known as evidence-based practice. This isn’t to dismiss anecdotal reports, also known as practice-based evidence, as these can lead to hypotheses to be tested for future research. However, we want to focus on the concrete benefits, costs to training, and health outcomes that are demonstrated in the current literature.
In a comprehensive review on the prevalence and determinants of fasted training in endurance athletes (2), the authors highlight the lack of evidence that supports that long-term fasted training leads to enhanced fat oxidation. After reviewing the full body of evidence on the impact of fasted training on endurance performance, researchers concluded that results are mixed and inconclusive. Importantly, none of the studies reviewed showed an improvement in actual endurance performance. There is also a lack of research measuring the impact of fasted training on real-life endurance performance when athletes are consuming carbohydrates (2).
A study by Hespel et al. (2007) (3) examined the impact of fasted training following an endurance training program (6 weeks, 3 days/week, 1–2 hours/day, 75% of peak V̇O2) on moderately active males. The participants were divided into two groups: one group trained in the fasted state, and the other group in the carbohydrate-fed state.
At the end of the 6-week program, neither group showed differences between fat oxidation rates and similar responses to training, indicating no significant differences in training adaptations. Fat oxidation rates were as follows: CHO group 0.421 ± 0.07 and 0.389 ± 0.07, Fasting group 0.444 ± 0.05 and 0.421 ± 0.05.
Several meta analyses highlight gaps in the literature on the long-term effects of fasted training.
A meta analysis reviewing the effect of the fasted or fed state on aerobic training showed aerobic exercise performed in the fasted state leads to a significant increase in fat oxidation during exercise compared to exercise performed in the fed state. The meta-analysis included 27 studies with a total of 273 participants, providing a robust evidence base for the effect of fasted state on fat oxidation during exercise of up to 120 min performed at a VO2 max of 40-65%. Since this meta-analysis only looked at the acute effects (in that specific training session) on fasted training and not the influence of fasted training after a number of weeks, this application needs to be followed with caution (1).
Another systematic review carried out by Carson et al 2018 (4) showed that fasting conditions during exercise increased post-exercise circulating free fatty acids (FFAs) when compared to fed conditions. This suggests that fasted exercise may enhance the release of FFAs for use as fuel. However, this paper also highlights the limitations to studying the short- term effects of fasted training and that large gaps remain in the literature and in our understanding.
Although there appears to be an increase in fat oxidation rates and potentially an enhancement in some physiological adaptations that occur during fasted training (5), overall evidence does not show fasted training improves longevity of sessions or performance.
Overall evidence does not show that fasted training leads to improved performance.
the BENEFITS AND COSTS OF FASTED TRAINING
The extent to which fasted-state training boosts the body’s ability to use fat as a fuel source (fat oxidation) and its direct impact on performance in endurance events remain unclear (2).
The downsides to fasted training include: increased perceived effort, the training session feels harder to do (6), and reduced training quality during that session (7). If you are not eating enough carbohydrates day-to-day to meet the demands of your training, you may end up carrying out this fasted training session with depleted muscle carbohydrate stores. This can lead to low carbohydrate availability, which further increases the stress load of the exercise, reduces immune response (6), and requires a longer recovery period after that training session.
When exercising at higher intensities (>65% maximum oxygen), carbohydrate is the dominant fuel source (8). High-intensity efforts, such as strength sessions, weighted hikes, Uphill Athlete Chamonix Fit Program, or climbing gym sessions, should not be carried out fasted.
Nutrition coaching
SHOULD FEMALE ATHLETES DO FASTED TRAINING?
Research on female athletes and fasted training shows females have an increase in fat oxidation rates when carrying out fasted training (10). However, female athletes are more prone to restrictive eating practices and disordered eating (11). They are also highly susceptible to compromised health following short periods of energy restriction, such as fasted training. A female athlete’s bone health can be compromised following just 5 days of energy restriction (12).
A female athlete's bone health can be compromised following just 5 days of energy restriction.
The degree of energy deficit incurred within a single hour or day can have detrimental effects on the health of female athletes, impacting various physiological functions and potentially compromising overall well-being (13). Overnight fasted training, extends and accentuates the energy deficit and time spent in this extended energy deficit has been shown to impair health. This can lead to low energy availability (LEA) and the ensuing negative impacts such as menstrual dysfunction, injury, illness, poor bone health etc (14).
When active female athletes train fasted, they reduce their daily carbohydrate intake by either missing out on a meal and/or not fueling around the session. This could lead to reduced carbohydrate stores within the muscles. Without consuming sufficient daily carbohydrates across the training week, this will contribute to low carbohydrate availability (LCA). LCA has been shown to have negative effects on health (independent of LEA) such as impaired immune function, hormone dysregulation, and impaired bone health (14).
Take a look at our article on nutrition recommendations for female athletes of menstruating age for more specific guidance.
What about male athletes?
Male athletes are also negatively impacted by this time spent in extended energy deficits. In a study of well-trained male endurance athletes, despite all having similar daily energy intakes, 65% demonstrated a suppressed resting metabolic rate (RMR). Those that experienced reduced RMR were shown to spend an extended amount of time daily in a severe energy deficit exceeding 400 kcal and greater single-hour energy deficits. These conditions were linked to elevated cortisol levels and a reduced testosterone-to-cortisol ratio and impaired bone health (13).
COMMON MISTAKES and Downsides
Below are some of the common mistakes in practicing fasted training:
- Carrying out all weekday training sessions fasted
- Carrying out fasted training because you think it will help you to lose weight
- Carrying out strength sessions fasted (i.e. Chamonix Fit, weighted hikes, strength, and core)
- Carrying out long runs/hikes fasted
- Delaying fueling a few hours into a long run/hike
- Testing how far you can go in your big mountain day or long run/hike before you need to eat carbohydrates
The Downsides of Fasted Training
- Increased stress on the body in addition to that caused by training, which increases recovery time
- Impaired training quality
- Increased perception of effort
- Difficulty meeting daily energy intake goals
- Low energy levels and long-term negative health consequences from chronic underfueling
KEY NUTRITION RECOMMENDATIONS
The Importance of Carbohydrates
The benefit of consuming carbohydrates around training and fueling endurance performance is well-established (17). Rather than thinking that fasted training is a shortcut to improving fitness, fuel your training sessions with carbohydrates to support your training session, gain important training adaptations, and improve your recovery.
For Aerobic training sessions:
- Consume adequate carbohydrates (1-1.5g) in your meal 2-3 hours before your session.
- Consider eating a carbohydrate-rich snack before your training sessions (e.g rice cakes, banana, energy bar) that is easy to digest.
- Drink 500ml (more in warmer weather) of a sports drink during your session (a quick homemade recipe = 250ml water + 250ml fruit juice + pinch of salt).
- Consume a snack containing carbohydrate and protein within 30-60 minutes of finishing your session (e.g. whey/soy/vegan blend powder mixed with water or milk as desired + banana, glass of milk + banana, yogurt + banana, commercial recovery shake powder, etc.).
For strength sessions training sessions:
- Consume adequate carbohydrates in your meal 2-3 hours before your session.
- Consider eating a carbohydrate-rich snack before your training sessions (e.g rice cakes, banana, energy bar) that is easy to digest.
- Drink 500ml (more in warmer weather) of a sports drink during your session (a quick homemade recipe = 250ml water + 250ml fruit juice + pinch of salt).
- Make up a carbohydrate and protein shake and drink half 15-30 minutes before training and sip on the rest during the session or drink as soon as you’ve finished your training (e.g. whey/soy/vegan blend powder mixed with water or milk as desired + banana, glass of milk + banana, yogurt + banana, shop bought recovery shake powder).
For long runs, long or weighted hikes, big mountain days:
- Eat a balanced breakfast prior to all of these sessions.
- Consume a carbohydrate-based snack once in every hour of your session.
- Have a protein and carbohydrate recovery snack ready to eat as soon as you’ve finished your session.
If you are one of those Uphill Athletes that get up early in the morning to get your training session done before breakfast due to work or family commitments, or simply for convenience, we encourage you to:
- Include enough carbohydrate in your evening meal the night before training.
- Eat a small ripe banana or have a glass of fruit juice 15-30 min before training.
- Drink 500ml of a sports drink during your session (a quick home made recipe = 250ml water + 250ml fruit juice + pinch of salt).
- Aim to eat your breakfast within 30-60 minutes of finishing your training.
If you carry out the odd training session fasted (less than 60 minutes in duration at low intensity) due to time constraints, that is okay! But make sure your evening meal the night before contains a source of carbohydrate and you eat breakfast or consume a recovery snack straight after the fasted training session.
WHERE ARE YOUR LIMITATIONS?
For some Uphill Athletes, fasted training may seem like a fast track and an easy option to get fitter, but there are other areas to focus your time and energy that will bring you far greater gains. Ask yourself the following questions:
- Are you missing training sessions week to week?
Instead, aim to train more consistently. - Are you intentionally controlling your food intake to keep your weight down?
Instead, focus on eating enough to support your daily energy demands. - Do you avoid fueling your training sessions?
Instead, focus on fueling each session appropriately to keep you training strong throughout the week. - Do you have an injury?
Recognize that when you are easing back on training to rehab an injury, your body still needs fuel to repair. Make sure you are not reducing your energy intake drastically when your training volume changes. - Are you going to bed late at night and/or not sleeping well?
Focus on going to bed earlier to get at least 8 hours of sleep to support your recovery and prepare for the next day of training.
There are other important factors to consider that surpass fasted training and will benefit your performance and your overall health. Training consistently, meeting your basic nutritional needs, and having a recovery strategy focused around nutrition and balanced rest—these are the real “magic bullets” that will help you reach your full potential.
SUMMARY
While fasted training may increase fat oxidation rates and certain physiological adaptations, there is no conclusive evidence supporting improved endurance performance. Research indicates that carrying out training in a fasted state leads to increased perceived effort, reduced training quality, and potential negative health outcomes.
Uphill athletes may naturally shift to higher fat oxidation rates during longer training sessions due to reduced carbohydrate availability from stored muscle glycogen.
Currently, there is not enough scientific evidence that supports fasted training as an enhancement to aerobic fitness or endurance performance for athletes above training in the fed state.
For the biggest gains to your performance, it’s important to focus on fueling training and eating enough to meet daily needs for improved health and performance.
Reviewed by Chantelle Robitaille, MSc and Alyssa Leib, MSc, RD.
SOURCES AND FURTHER READING
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Vieira, A., Costa, R., Macedo, R., Coconcelli, L., & Kruel, L. (2016). Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: A systematic review and meta-analysis. British Journal of Nutrition, 116(7), 1153-1164. doi:10.1017/S0007114516003160
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Rothschild JA, Kilding AE, Plews DJ. Prevalence and Determinants of Fasted Training in Endurance Athletes: A Survey Analysis. Int J Sport Nutr Exerc Metab. 2020 Sep 1;30(5):345-356. doi: 10.1123/ijsnem.2020-0109. Epub 2020 Jul 23. PMID: 32702658.
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De Bock K, Derave W, Eijnde BO, Hesselink MK, Koninckx E, Rose AJ, Schrauwen P, Bonen A, Richter EA, Hespel P. Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake. J Appl Physiol (1985). 2008 Apr;104(4):1045-55. doi: 10.1152/japplphysiol.01195.2007. Epub 2008 Feb 14. PMID: 18276898
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Aird TP, Davies RW, Carson BP. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 2018 May;28(5):1476-1493. doi: 10.1111/sms.13054. Epub 2018 Feb 23. PMID: 29315892.
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Jeukendrup, A.E. Periodized Nutrition for Athletes. Sports Med 47 (Suppl 1), 51–63 (2017). https://doi.org/10.1007/s40279-017-0694-2
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Jonathan D. Bartlett, John A. Hawley & James P. Morton (2015) Carbohydrate availability and exercise training adaptation: Too much of a good thing?, European Journal of Sport Science, 15:1, 3-12, DOI: 10.1080/17461391.2014.920926
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Burke LM, Hawley JA, Jeukendrup A, Morton JP, Stellingwerff T, Maughan RJ. Toward a Common Understanding of Diet-Exercise Strategies to Manipulate Fuel Availability for Training and Competition Preparation in Endurance Sport. Int J Sport Nutr Exerc Metab. 2018 Sep 1;28(5):451-463. doi: 10.1123/ijsnem.2018-0289. Epub 2018 Sep 25. PMID: 30249148.
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Purdom, T., Kravitz, L., Dokladny, K. et al. Understanding the factors that affect maximal fat oxidation. J Int Soc Sports Nutr 15, 3 (2018). https://doi.org/10.1186/s12970-018-0207-1
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Cermak NM, van Loon LJ. The use of carbohydrates during exercise as an ergogenic aid. Sports Med. 2013 Nov;43(11):1139-55. doi: 10.1007/s40279-013-0079-0. PMID: 23846824.
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Iwayama K, Kawabuchi R, Nabekura Y, Kurihara R, Park I, Kobayashi M, Ogata H, Kayaba M, Omi N, Satoh M, Tokuyama K. Exercise before breakfast increases 24-h fat oxidation in female subjects. PLoS One. 2017 Jul 10;12(7):e0180472. doi: 10.1371/journal.pone.0180472. PMID: 28692687; PMCID: PMC5503250.
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Graybeal AJ, Kreutzer A, Willis JL, Braun-Trocchio R, Moss K, Shah M. The impact of dieting culture is different between sexes in endurance athletes: a cross-sectional analysis. BMC Sports Sci Med Rehabil. 2022 Aug 17;14(1):157. doi: 10.1186/s13102-022-00549-4. PMID: 35978366; PMCID: PMC9382595.
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Heikura IA, Uusitalo ALT, Stellingwerff T, Bergland D, Mero AA, Burke LM. Low Energy Availability Is Difficult to Assess but Outcomes Have Large Impact on Bone Injury Rates in Elite Distance Athletes. Int J Sport Nutr Exerc Metab. 2018 Jul 1;28(4):403-411. doi: 10.1123/ijsnem.2017-0313. Epub 2018 Jun 12. PMID: 29252050.
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Melin AK, Heikura IA, Tenforde A, Mountjoy M. Energy Availability in Athletics: Health, Performance, and Physique. Int J Sport Nutr Exerc Metab. 2019 Mar 1;29(2):152-164. doi: 10.1123/ijsnem.2018-0201. Epub 2019 Feb 26. PMID: 30632422.
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Lodge MT, Ward-Ritacco CL, Melanson KJ. Considerations of Low Carbohydrate Availability (LCA) to Relative Energy Deficiency in Sport (RED-S) in Female Endurance Athletes: A Narrative Review. Nutrients. 2023 Oct 20;15(20):4457. doi: 10.3390/nu15204457. PMID: 37892531; PMCID: PMC10609849.
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Vieira AF, Costa RR, Macedo RC, Coconcelli L, Kruel LF. Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. Br J Nutr. 2016 Oct;116(7):1153-1164. doi: 10.1017/S0007114516003160. Epub 2016 Sep 9. PMID: 27609363.
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Stellingwerff T, Cox GR. Systematic review: Carbohydrate supplementation on exercise performance or capacity of varying durations. Appl Physiol Nutr Metab. 2014 Sep;39(9):998-1011. doi: 10.1139/apnm-2014-0027. Epub 2014 Mar 25. PMID: 24951297.
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De Bock K, Derave W, Eijnde BO, Hesselink MK, Koninckx E, Rose AJ, Schrauwen P, Bonen A, Richter EA, Hespel P. Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake. J Appl Physiol (1985). 2008 Apr;104(4):1045-55. doi: 10.1152/japplphysiol.01195.2007. Epub 2008 Feb 14. PMID: 18276898.
-
Aird TP, Davies RW, Carson BP. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 2018 May;28(5):1476-1493. doi: 10.1111/sms.13054. Epub 2018 Feb 23. PMID: 29315892.
-
Rothschild JA, Kilding AE, Plews DJ. What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions. Nutrients. 2020 Nov 12;12(11):3473. doi: 10.3390/nu12113473. PMID: 33198277; PMCID: PMC7696145.
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Iwayama K, Kawabuchi R, Nabekura Y, Kurihara R, Park I, Kobayashi M, Ogata H, Kayaba M, Omi N, Satoh M, Tokuyama K. Exercise before breakfast increases 24-h fat oxidation in female subjects. PLoS One. 2017 Jul 10;12(7):e0180472. doi: 10.1371/journal.pone.0180472. PMID: 28692687; PMCID: PMC5503250.
-
Cermak NM, van Loon LJ. The use of carbohydrates during exercise as an ergogenic aid. Sports Med. 2013 Nov;43(11):1139-55. doi: 10.1007/s40279-013-0079-0. PMID: 23846824.
-
Vieira AF, Costa RR, Macedo RC, Coconcelli L, Kruel LF. Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. Br J Nutr. 2016 Oct;116(7):1153-1164. doi: 10.1017/S0007114516003160. Epub 2016 Sep 9. PMID: 27609363.
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Stellingwerff T, Heikura IA, Meeusen R, Bermon S, Seiler S, Mountjoy ML, Burke LM. Overtraining Syndrome (OTS) and Relative Energy Deficiency in Sport (RED-S): Shared Pathways, Symptoms and Complexities. Sports Med. 2021 Nov;51(11):2251-2280. doi: 10.1007/s40279-021-01491-0. Epub 2021 Jun 28. PMID: 34181189.