Intermittent Fasting: When to Work Out

Intermittent fasting is an increasingly popular eating pattern that involves restricting your eating to predetermined hours per day or days per week. You probably know intermittent fasting as a weight-loss method, and while it works well as such, the benefits of time-restricted eating extend to many aspects of health, aging, and disease.

Exercise is an essential part of any healthy lifestyle. Exercise helps you build muscle, lose fat, improve your cardiovascular fitness, and strengthen mental health. And that’s just a few of the numerous benefits you gain from regular exercise.

But what if you want to combine intermittent fasting with exercise? What are the most important things to consider, and when is the optimal time to work out? In this article, you’ll find out everything you need to know about the best time to exercise while practicing intermittent fasting.

Can You Exercise While Intermittent Fasting?

If you wonder if you can exercise while intermittent fasting and if it is safe to do so, then the short answer is yes to both of those questions as long as you are healthy. That last part is essential to emphasize. If you have diabetes or have any other kind of medical condition, make sure to have your doctor give you the thumbs up first.

Not only can you exercise fasted, but it’s also probably what we are built to do. Think about it: when we were hunters and gatherers, did we hunt and gather with a full stomach? Probably not, at least not during lean seasons. We worked hard, and then we ate.

When to Work Out While Doing Intermittent Fasting

When determining the best time to work out while intermittent fasting, you need to consider your exercise goals. What’s your main goal with the workout? Are you doing cardio or lifting weights? If your goal is fat loss, the optimal time for a workout is probably not the same as for someone looking to pack on the muscle in the gym. If you’re an athlete looking to perform your best, it’s more important to time your workout correctly than it is for someone who exercises for health and general well-being.

The good news is that regardless of your exercise goals, you can reach them while doing intermittent fasting, 

Let’s take a look at the best time to work out while practicing intermittent fasting, depending on your fitness goals and the type of exercise we’re talking about.

Losing Weight and Body Fat

When you haven’t eaten for a long time, like after an overnight fast, your body is primed to burn fat. You use energy from your fat stores instead of relying on the carbs from your last meal. When you haven’t eaten for hours, your insulin levels are low, your glucose stores and blood sugar levels are down, and your blood is full of fatty acids. Combined, that means you oxidize, or “burn,” more fat when exercising in the fasted state than when you’ve eaten.1 2

Burning more fat during exercise sounds great if you want to lose weight, doesn’t it? But hold on, it’s not that easy. Fasted exercise, combined with a calorie deficit, is an excellent option when you want to lose fat, but it’s probably not more effective than exercising at any other time.3 4

Scientific evidence does not support morning workouts or exercising on an empty stomach as a superior way to improve your body composition. Unfortunately, your body is too intelligent to be tricked like that. Undoubtedly, you burn more fat during the exercise when performed during your fasting window. But fat burning must be considered over the course of days, not on an hour-to-hour basis. If you look at the day as a whole, your body compensates by burning less fat or storing more fat during the rest of the day after fasted exercise. The result is no difference in fat loss over time.

However, fasted exercise might give you an indirect advantage if you want to lose weight. Research shows that people tend to eat less after exercising fasted.5 Everyone is unique, so this might not apply to you. But if weight loss is your goal, and you find yourself less hungry following a fasted workout, eating fewer calories without even thinking about it could be a game-changer.

Purely anecdotal evidence says that fasted cardio can help you mobilize fat stores and get rid of the last stubborn fat when you are already lean and looking to get shredded. This method makes sense in theory, and countless fitness enthusiasts and bodybuilders utilize it, but controlled trials haven’t confirmed that you gain an advantage doing so.

But what about your muscle mass?

In general, you don’t have to worry about losing muscle by doing fasted cardio. Much like with fat oxidation, you will likely burn a little more muscle mass during the exercise itself when you perform it in a fasted state, but again, your body is smart enough to compensate over the day as a whole. However, you might want to limit your fasted cardio to 60 minutes to minimize the risk of muscle loss.6

If you’re worried and don’t mind exercising in a semi-fasted state, consider eating some protein or drinking a protein shake before exercising. Doing so does not impair fat oxidation.7 While you’re technically not fasted after chugging a shake, it makes sense if you’re concerned about burning precious muscle along with your body fat, as it minimizes protein breakdown during exercise.8

The bottom line: exercising in the fasted state burns more fat during the training session, but that doesn’t necessarily translate into a more significant fat loss over time. But if you’re looking for maximal fat oxidation, the best time for working out is sometime during your fasting window, possibly towards the end of it for maximum benefits. However, don’t worry if you don’t like exercise on an empty stomach or can’t time your workouts for other reasons. Over time, you likely lose as much weight and body fat if you exercise during your eating window. It’s a win-win situation, so pick a time you enjoy that fits your schedule.

Building Muscle and Strength Training in General

While fat burning is enhanced when you have fasted, your muscles depend on an abundant supply of nutrients to grow and get stronger.

Muscle protein synthesis is the process of building muscle mass. Muscle protein breakdown, on the other hand, is the opposite. When your muscle protein synthesis is greater than your muscle protein breakdown, you build muscle, i.e., your muscle protein balance is positive. However, when you’re fasted, your muscle protein balance is negative.9 You lose a little muscle on an empty stomach. That’s not a problem since you gain that muscle back after eating. That’s why your muscle mass remains constant over time. You alternate periods of fasting (muscle loss) with periods of feeding (muscle gain) over the day or week.

When you lift weights, you stimulate muscle protein synthesis. You tell your muscles that it’s time to grow to meet the demands of lifting. And to grow, your muscles need energy and amino acids. After a workout, you must provide your body with enough protein to create new muscle tissue.

Fasted strength training stimulates muscle growth, but muscle protein breakdown exceeds muscle protein synthesis until you eat. You can think of strength training as the switch that activates muscle protein synthesis, but that activation remains limited without protein.

That means that if you are looking for the best way to stimulate muscle protein synthesis and flip the switch to “growth,” you need to eat before or after training. However, you’re not chasing minutes here. Much like with fat oxidation, your body is smart.

Following a strength-training session, your muscles are more sensitive to protein.10 Each meal you eat during the 24 hours after a lifting session builds more muscle than it would have if you hadn’t worked out. According to research, timing your protein intake around your training sessions offers a small benefit on muscle growth, but only if it leads to an overall greater protein intake.11

An “anabolic window” after a workout where eating protein builds more muscle will probably extend a lot longer than an hour. That being said, no one has studied it in the context of intermittent fasting. If your fasting window is 16–20 hours long, it could very well be suboptimal to work out in the middle of it. You would not flip the switch to start building muscle for many hours, and it’s unknown if your body can compensate for that during your feeding window. You’d likely still get results from your efforts in the gym, but they might not be optimal.12

According to research, you’d do best to time your lifting between meals no more than 3–4 hours apart.13 If we’re talking about hefty meals like lunch or dinner or the kind of meals you might eat if you practice intermittent fasting and squeeze them into a limited time frame, you could extend that interval to 5–6 hours.

You’d have to work out sometime during your feeding window if you take that as gospel. Any other option would mean fasting either before or after the training session. That might be true if you’re looking to maximize your gains, but I think the difference over time would be negligible compared to timing your workout to just one meal, meaning eating before or after training.

A high-protein meal after a fasted training session maximally stimulates muscle protein synthesis.14 As long as you’re able to perform in the gym on an empty stomach, there should be no issue training towards the end of the fasting window and then go eat. You’ll turn a negative muscle protein balance into a positive one and keep it there as you’ll continue to feed your muscles during the following hours.

Eating before training also works fine, as demonstrated in several studies.15 16 If you eat before you hit the weights, your muscles still have all the amino acids they need to grow after the workout, even if you don’t eat again that day. That means that working out during the hours following your feeding window should be fine.

You might have heard about fasting boosting your growth hormone levels, and yes, that is true.17 Don’t expect it to boost your muscle growth as well, though. There is no evidence that human growth hormone is anabolic in adults, despite what some people claim.18 Not if we’re talking levels you reach through natural means, that is. Increased GH secretion while fasting is not bad; just don’t think of it as a magic bullet to muscle growth.

The bottom line: lifting weights while doing intermittent fasting is no problem. The optimal time to work out may be sometime during your feeding window. Doing so will maximize the number of nutrients your muscles have access to, giving them the best possible growth potential. However, lifting either before or after your feeding window likely works just fine as well. If you prefer to train during your fasting window, consider doing so during the hours closest to feeding. Working out in the middle of your fasting period with no food before or after the workout is likely the least beneficial option and could hinder your muscle growth.

Exercise Performance

There are surprisingly few studies comparing performance during fasted exercise and fed exercise. A relatively recent review found no differences between exercising on an empty stomach and exercising after one or more meals, as long as you keep your workout to an hour or less.19 That goes for both a low-intensity workout and a high-intensity workout. HIIT, or high-intensity interval training, doesn’t seem to be affected either. However, if you’re looking for peak performance during aerobic exercise lasting more than 60 minutes, you’d do best to eat first, then exercise. Very few studies observe any performance benefits from fasted training, regardless of duration or intensity.

An interesting topic for long-term performance benefits is the “train high, compete low”-concept. That means performing most of your training fasted or at least with low muscle glycogen levels. By doing so, you train your body to rely more on fat as a fuel source during exercise. Then, when it’s competition time, you load up on carbs and get the benefit of both improved fat burning and filled glycogen stores. It’s an interesting theory, but so far, research has not shown any consistent benefits from fasted training, and more research is needed.20 21

As for strength training, there is even less research available. A couple of studies found minimal adverse effects of intermittent fasting on resistance exercise performance.22 A new meta-analysis concluded that completely fasted training might reduce strength-training performance somewhat and recommended that you consume at least 15 grams of carbs within three hours before training.23 Keep in mind that most of these were very short-term studies. If you regularly engage in fasted strength training, your body might adapt over time, leading to no difference in performance whatsoever.

The bottom line: if you’re doing endurance exercise lasting more than an hour, you’ll probably perform better after having eaten. Working out during your feeding window or the first half of your fasting window is probably a good idea. If your feeding window is short, and you cram a lot of eating into a few hours, waiting until it’s over might be the best option. Exercising with a full stomach can be unpleasant. If your workout is 60 minutes or shorter, feel free to perform it whenever you prefer and when your schedule allows it. Strength-training performance might not be affected by fasting, but you might want to time your workout so that you provide your muscles with the nutrients they need, as described earlier.

Summary and Conclusions

Intermittent fasting and exercise are a winning combination, and for the most part, it doesn’t make a huge difference when you work out. Exercise is always beneficial.

However, there are some key points to keep in mind:

  • Exercising on an empty stomach increases fat oxidation during the workout. If that’s what you’re looking for, perform your training session during your fasting window. Keep in mind that there is little evidence that fasted cardio leads to greater long-term weight or fat loss.
  • Strength training requires nutrients to build muscle. It’s likely not a matter of a minute here or an hour there making a huge difference, though. As long as you eat within a few hours before or after hitting the weights, you should be good. If you’re looking for maximum growth potential, consider working out sometime during your feeding window. Also, you might want to avoid consistently scheduling a strength-training session mid-fasting, as it could compromise your results. Doing so now and then probably doesn’t matter at all, though.
  • Train when you prefer if performance is your goal! According to research, your performance during training sessions is not significantly affected by fasting or feeding. The exception is aerobic exercise lasting more than 60 minutes. Most studies show improved performance during long training sessions after eating compared to fasting. If you have a long workout planned, consider scheduling it either during your feeding window or during the hours following it. When your eating is time-restricted, it might also be necessary to wait until a few hours into your fasting window for high-intensity exercise, simply because a full stomach and exercise don’t always go well together.
  • If you’re exercising for health reasons, performing your workouts in the fasted state might offer some benefits. For example, doing so improves insulin sensitivity, a cornerstone of preventing and managing diabetes.24 Exercise is always beneficial for your health, but exercising during your fasting windows could boost those benefits even more.

Tips and Tricks When Exercising Fasted

  • Stay hydrated! This is especially important if you exercise soon after getting out of bed. After a night of sleeping, you’ll be slightly dehydrated, and dehydration is the enemy of both performance and fat oxidation.25
  • A cup or two of black coffee boosts performance and helps you burn fat during fasted cardio.26 27 A great pick-me-up if you’re feeling sluggish.
  • Listen to your body. Fasted exercise isn’t for everyone. At least not high-intensity exercise. Lower your intensity or take a break if you’re not feeling well. If you find yourself unable to get used to fasted exercise, don’t do it. There is nothing magical about exercising on an empty stomach, and you get most if not all benefits if you schedule your workouts to your feeding window or shortly after.
  • If you’re doing longer fasts than 24 hours, consider sticking to low- to moderate-intensity cardio workouts. Lifting weights during a longer fast is still beneficial, though. Your muscles won’t grow from it, but it will curtail muscle loss.
  • After intense workouts on an empty stomach, it’s probably a good idea to break your fast sooner than later, at least if you exercise many days of the week and look to replenish the energy stores in your muscles in time for the next workout.

There you have it! The essentials of when to work out while practicing intermittent fasting. If you’re looking for more evidence-based and up-to-date info about intermittent fasting, check out out comprehensive guide:

References

  1. Br J Nutr. 2016 Oct;116(7):1153-1164. Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis.
  2. Int J Exerc Sci. 2018; 11(2): 827–833. Effects of Prior Fasting on Fat Oxidation during Resistance Exercise.
  3. Strength and Conditioning Journal: October 2020 – Volume 42 – Issue 5 – p 71-78. Fasted Versus Nonfasted Aerobic Exercise on Body Composition: Considerations for Physique Athletes.
  4. J Int Soc Sports Nutr. 2014; 11: 54. Body composition changes associated with fasted versus non-fasted aerobic exercise.
  5. J Nutr Metab. 2016:1984198. Exercising in the Fasted State Reduced 24-Hour Energy Intake in Active Male Adults.
  6. Strength and Conditioning Journal: October 2020 – Volume 42 – Issue 5 – p 71-78. Fasted Versus Nonfasted Aerobic Exercise on Body Composition: Considerations for Physique Athletes.
  7. Journal of the International Society of Sports Nutrition volume 15, Article number: 56 (2018). Metabolic impact of protein feeding prior to moderate-intensity treadmill exercise in a fasted state: a pilot study.
  8. Journal of the International Society of Sports Nutrition volume 14, Article number: 20 (2017). International Society of Sports Nutrition Position Stand: protein and exercise.
  9. J Am Coll Nutr. 2005 Apr;24(2):134S-139S. Dietary protein to support anabolism with resistance exercise in young men.
  10. The Journal of Nutrition, Volume 141, Issue 4, April 2011, Pages 568–573. Enhanced Amino Acid Sensitivity of Myofibrillar Protein Synthesis Persists for up to 24 h after Resistance Exercise in Young Men.
  11. J Int Soc Sports Nutr. 2013; 10: 53. The effect of protein timing on muscle strength and hypertrophy: a meta-analysis.
  12. Front. Nutr., 09 June 2021. A Muscle-Centric Perspective on Intermittent Fasting: A Suboptimal Dietary Strategy for Supporting Muscle Protein Remodeling and Muscle Mass?
  13. Journal of the International Society of Sports Nutrition volume 10, Article number: 5 (2013). Nutrient timing revisited: is there a post-exercise anabolic window?
  14. The American Journal of Clinical Nutrition, Volume 89, Issue 1, January 2009, Pages 161–168. Ingested protein dose response of muscle and albumin protein synthesis after resistance exercise in young men.
  15. Medicine & Science in Sports & Exercise: October 2012 – Volume 44 – Issue 10 – p 1968-1977. Preexercise Aminoacidemia and Muscle Protein Synthesis after Resistance Exercise.
  16. PeerJ. 2017; 5: e2825. Pre- versus post-exercise protein intake has similar effects on muscular adaptations.
  17. J Clin Invest. 1988 Apr; 81(4): 968–975. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man.
  18. Br J Sports Med. 2003 Apr; 37(2): 100–105. Claims for the anabolic effects of growth hormone: a case of the Emperor’s new clothes?
  19. Scand J Med Sci Sports. 2018 May;28(5):1476-1493. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis.
  20. Sports Medicine volume 48, pages 1031–1048 (2018). Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis.
  21. Open Access J Sports Med. 2020; 11: 1–28. Exercise Training and Fasting: Current Insights.
  22. Current Sports Medicine Reports: July 2019 – Volume 18 – Issue 7 – p 266-269. Intermittent Fasting and Its Effects on Athletic Performance: A Review.
  23. Nutrients 2022, 14(4), 856. The Effect of Carbohydrate Intake on Strength and Resistance Training Performance: A Systematic Review.
  24. Sports Medicine volume 47, pages 415–428 (2017). Impact of Endurance Exercise Training in the Fasted State on Muscle Biochemistry and Metabolism in Healthy Subjects: Can These Effects be of Particular Clinical Benefit to Type 2 Diabetes Mellitus and Insulin-Resistant Patients?
  25. Physiol Rep. 2015 Aug;3(8):e12483. The effect of dehydration on muscle metabolism and time trial performance during prolonged cycling in males.
  26. Journal of the International Society of Sports Nutrition volume 18, Article number: 5 (2021). Caffeine increases maximal fat oxidation during a graded exercise test: is there a diurnal variation?
  27. Front Sports Act Living. 2020; 2: 574854. Caffeine and Exercise Performance: Possible Directions for Definitive Findings.
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Andreas Abelsson

Andreas has over 30 years of training experience and is a highly appreciated writer and educator on exercise, fitness, and nutrition. Few people stay more up to date and have a better grasp of the field of exercise science than Andreas.