Fasting has become a popular diet and fitness trend, supported by research indicating that fasting can provide benefits such as increasing health span (years of life that one is “healthy”)1 and autophagy (the body cleaning out damaged cells and replacing them with new cells)2.
Many argue that training in a fasted state has its own benefits. A quick Google search will return claims such as training fasted being “more effective” or even “the best choice” when it comes to workouts.
However, there is not enough research to support such claims.
Training fasted usually describes training after 8 or more hours without eating—this is not an uncommon practice, as many prefer to get their workout out of the way as soon as they wake up.
If you find yourself skipping breakfast due to early morning workouts or because you follow Intermittent Fasting (IF), here are some things to keep in mind:
1. Your Performance May Be Hindered
Research on fasting and athletic performance is not as clear as many advocates of training fasted make it seem. Advocates like to cite one 1988 study on fasting and growth hormone secretion as support for fasting before exercise3, but it is only fair to also consider that there is at least preliminary evidence that fasting can negatively impact both aerobic and anaerobic activities.
Eating before prolonged aerobic workouts can improve performance4, and there is evidence that overnight fasting decreases handgrip strength (a common test of muscular function)5. However, fasting may not have a negative impact on shorter aerobic workouts4.
It is difficult to make generalized statements since many studies on fasting and athletic performance have tested the impact of intermittent fasting, which is a long-term protocol rather than simply fasting overnight.
Taking a step away from the research, if we take a common-sense approach, there is obvious logic to eating before training. Since exercise expends energy, it is good practice to consume energy before partaking in any exercise activity. Otherwise, your body will have to use energy that it might have otherwise used for critical tasks, such as brain function and heart rate.
For prolonged and/or high-intensity workouts, training in a fasted state may lead to fatigue, hunger, inability to focus, and/or preoccupation with thoughts about food.
2. There Is No Benefit for Weight Loss
A meta-analysis of five distinct studies concluded that exercising in a fasted state does not influence weight loss or body composition changes6. If you train after eating, your body can use calories from your last meal to fuel your workout, but if you train fasted, your body will use stored energy to fuel the workout.
Thus, it might sound like training fasted will help you lose weight faster. However, this line of reasoning is basically saying that eating nothing is a good weight loss strategy. In reality, we eat at some point during the day.
Fasting before a workout does nothing for weight loss efforts if you eat the food you would have eaten before a workout sometime later in the day. You might even overcompensate later and eat more than you normally would have if training fasted makes you especially hungry.
Each time we eat, the food goes to fuel something, so why shouldn’t we eat to fuel our training? It doesn’t make sense to train fasted only to later eat a big meal to fuel while sitting at your desk. Training fasted will not improve weight loss unless overall caloric intake is decreased. Training fasted increases fat oxidation, or the use of fat or energy, but fat oxidation is always happening.
3. Your Hormonal Health May Be Affected
Research on the benefits of fasting has also yielded findings on altered hormonal balance in response to fasting. This is especially significant in females, as normal functions such as ovulation rely on the hypothalamus secreting gonadotropin releasing hormone (GnRH) as a first step in a chain of commands, and fasting can modify estrogen and GnRH receptors7.
Many studies conducted in animals have indicated that fasting can impact other aspects of reproductive health as well8.
4. It May Take Some Trial & Error to Figure Out What Works for You
Despite limited evidence of any benefits to training fasted, many individuals prefer to work out without eating beforehand. Advantages or disadvantages to training fasted may depend on the individual.
For example, in individuals with hyperglycemia, or high blood sugar levels, training fasted does not reduce blood sugar levels in the same way that training after eating does9. Unstable blood sugar levels can lead to fatigue, shakiness, sweating, and anxiety.
It is important to take into account your own fitness and medical history and consult with your doctor if you decide that you want to implement any sort of fasting into your routine—before training or otherwise.
Although some people may prefer to train fasted, it is wise to eat something before a workout, no matter what time of day, to protect overall health and wellness. If you do not have time to eat a balanced meal before a workout, then you may still benefit from a quick snack.
Easy sources of carbohydrates include bananas, rice cakes, or juice. If you have more time, you can pair with some protein for some essential amino acids. A protein shake with fruit or yogurt with granola can be just as easy.
If you work out early in the morning and prefer not to eat anything before, then you should prioritize your bedtime meal and be sure to get adequate amounts of all macronutrients, especially carbohydrates before bed.
One example of a balanced meal before bed would be chicken or tofu with pasta, asparagus, and olive oil. If you have a history of hormonal imbalances, then this method is not ideal, and you should make time for a balanced meal first thing in the morning.
However, overall dietary patterns are much more important than any one meal. Fasting or eating a pre-workout meal might not have a huge impact on your training, but your overall nutrition will. Adequate intake of all nutrients is critical to making progress in the gym, as well as sustaining overall health and well-being.
When it comes to training fasted vs. fed, it ultimately depends on your preference and how you feel. Regardless of what the research shows, fasting might have a positive or negative impact on your subjective feeling.
Feeling energized or exhausted might be enough to influence your decision on whether to train fasted. As long as you are getting the nutrients you need at some point during the day, your pre-workout meal (or lack of a meal) won’t have a big impact on your training progress or health.
Because there are so many proponents of training fasted, it can be difficult to find accurate information on the advantages and disadvantages. Research on outcomes of training fasted is still rather sparse, so any claims that training fasted is absolutely positive or negative are unsubstantiated. Positive and negative effects of training fasted may vary depending on the individual.
While there may be little evidence that training fasted is a bad idea, there is certainly no evidence to back up claims that training fasted is a shortcut for fat loss or a performance enhancer.
- Longo VD, Panda S. Fasting, Circadian Rhythms, and Time-Restricted Feeding in Healthy Lifespan. Cell Metab. 2016;23(6):1048-1059. doi:10.1016/j.cmet.2016.06.001
- Golbidi S, Daiber A, Korac B, Li H, Essop MF, Laher I. Health Benefits of Fasting and Caloric Restriction. Curr Diab Rep. 2017;17(12):123. doi:10.1007/s11892-017-0951-7
- Ho KY, Veldhuis JD, Johnson ML, et al. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988;81(4):968-975. doi:10.1172/JCI113450
- 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;28(5):1476-1493. doi:10.1111/sms.13054
- Correa-Arruda WS, Vaez IDA, Aguilar-Nascimento JE, Dock-Nascimento DB. Effects of overnight fasting on handgrip strength in inpatients. Einstein (Sao Paulo). 2019;17(1):eAO4418. doi:10.31744/einstein_journal/2019AO4418
- Hackett D, Hagstrom AD. Effect of Overnight Fasted Exercise on Weight Loss and Body Composition: A Systematic Review and Meta-Analysis. Journal of Functional Morphology and Kinesiology. 2017;2(4):43. doi:10.3390/jfmk2040043
- Parillo F, Zerani M, Maranesi M, et al. Ovarian hormones and fasting differentially regulate pituitary receptors for estrogen and gonadotropin-releasing hormone in rabbit female. Microsc Res Tech. 2014;77(3):201-210. doi:10.1002/jemt.22328
- Kumar S, Kaur G. Intermittent Fasting Dietary Restriction Regimen Negatively Influences Reproduction in Young Rats: A Study of Hypothalamo-Hypophysial-Gonadal Axis. PLOS ONE. 2013;8(1):e52416. doi:10.1371/journal.pone.0052416
- Nygaard H, Rønnestad BR, Hammarström D, Holmboe-Ottesen G, Høstmark AT. Effects of Exercise in the Fasted and Postprandial State on Interstitial Glucose in Hyperglycemic Individuals. J Sports Sci Med. 2017;16(2):254-263.