Exercising in a fasted state has some pros and cons over exercising in a fed state. Fasted training can help to burn more fat. But, on the other hand, it can lead to the dreaded “hitting the wall”. In the end, you have to find out yourself what works best for you.
You’ve probably heard or seen exercise gurus or influencers on the internet or social media discussing intermittent fasting and how they’ve been able to incorporate this dietary pattern into their daily routine.
But is intermittent fasting safe to do while exercising? If so, should you be exercising while fasted or after you’ve eaten?
In this article, hopefully, we can provide you with some information so that you can make a decision that’ll work best for you!
Let’s start by explaining what intermittent fasting is. intermittent fasting is when an individual incorporates small bouts of fasting into their everyday life. There are many forms of intermittent fasting such as alternate day fasting, one, or more, day(s) a week fasting, 16/8 (fasting for 16 hours of the day and eating for the remaining 8 hours), one meal a day (OMAD), and more.
This type of diet pattern has become very popular because of its ability to help people lose body fat while improving their overall health.
Now you’re probably wondering if incorporating an exercise program into your daily routine while practicing intermittent fasting is healthy. The answer is yes! Intermittent fasting while exercising is healthy, safe, and can help you reach some of your physical goals faster.
The health benefits of normal physical activity are well known and include helping to build/maintain muscle mass, accelerating fat loss, decreasing the risk of various diseases, and it has the ability to improve your overall mood and mental health (1, 2).
Unfortunately, there aren’t many studies that have investigated exercise and intermittent fasting together. But out of the few that have been conducted, it appears that when exercise and intermittent fasting are used together, people can expect to lose more body fat and show greater signs of improvement in overall health when compared to a more normal eating pattern where people are eating for 12+ hours a day (3, 4).
The intermittent fasting participants in these studies were also able to maintain their muscle mass and the amount of calories they burned each day. It’s also important to note that these studies made sure their participants maintained their calories while participating in intermittent fasting.
There are some other studies that investigated exercise and intermittent fasting but their participants decreased their calories while fasting. These studies did not report the same improvements in markers related to overall health or a more substantial loss in body fat (5, 6).
Based on these studies, this means that in order to maximize your physical and health goals while on intermittent fasting, you may want to make sure you maintain your calorie intake.
The next big question is should you exercise while fasted? The easy answer is that you should exercise in a way that makes you feel best and where you can keep this habit throughout your life. What’s most important with exercise is that you keep doing it.
But let’s dive a bit deeper.
There are essentially two different types of exercise: endurance (AKA cardio) and resistance-training. These two different types of exercise use different metabolic pathways and therefore, may result in different physical outcomes if you do them while fasted.
Unfortunately, there isn’t much research on intermittent fasting and different types of training, but we’ll do our best to summarize what is known.
Endurance training typically refers to any type of physical activity that increases your heart rate and keeps it elevated for a certain period of time. Some examples include running, biking, swimming, or even walking.
People generally use this form of exercise because they enjoy it and because it’s a great way of burning calories and shedding bodily fat.
What are some of the pros and cons to doing some endurance training while fasted?
A benefit to engaging in fasted cardio is that you may be able to burn more fat than exercising in the fed state. In fact, a review that evaluated 405 different articles concluded that not only is fasted cardio better at burning fat, it also helps decrease blood glucose and insulin (7).
This means that fasted cardio may help you lose fat faster and improve other aspects of your overall health.
You should be cautious before doing fasted cardio for a long period of time. Have you ever heard of an athlete “hitting the wall?” When doing cardio, your body uses some fat and some stored glucose (called glycogen) for energy. When you run out of your glycogen, your body is suddenly very fatigued, and you’ll have very little energy (8).
This, however, usually only happens to very trained athletes who are engaging in very vigorous exercising for a long period of time, like running for over an hour, without eating before and/or during their exercise. If you plan on doing 30 or so minutes of fasted cardio, you’ll be fine. But if at any point you feel unusually tired or dizzy, stop exercising. If you plan on doing more than an hour of intense cardio, make sure you either eat before or snack on something half-way through to be safe.
Another con is that you may feel hungrier during your workout. This may lead to you stopping your workout earlier than you had planned. Remember, the important part of exercise is being able to maintain it.
Resistance training usually refers to any type of weight-bearing exercise or lifting weights. People will engage in resistance training because they enjoy it and because it’s a great way of building muscle and strength.
What are some pros and cons to fasted resistance training?
Unfortunately, there isn’t much research on fasted resistance training. However, one study found that resistance training while fasted caused the subjects to burn more fat during their workouts than participants who exercised after eating (9). This means that fasted resistance training may help you lose more fat than if you were doing it after eating a meal.
A con to fasted resistance training is that it may be harder for you to build muscle. Eating upregulates certain pathways that help with muscle growth and development, meaning that not eating prior to working out, you may be accidentally limiting the full capacity of these pathways (10, 11, 12).
On the other hand, fasting also stimulates growth hormones that help to build and maintain muscles (13, 14). Unfortunately, at this point, it’s not possible to say which of these two effects is stronger and whether fasted resistance training is good or bad for muscle gain.
Similar to endurance training, engaging in fasted resistance exercise might cause you to feel hungrier, causing you to stop working out earlier than planned.
Using both intermittent fasting and exercise is a great way to meet your physical and health goals.
You can probably manipulate when you exercise relative to when you eat in order to accelerate reaching your goals but what is most important is listening to your body and doing what feels best for you.
Find a routine that makes you happy and make sure it is something that you can instill as a habit for the rest of your life!
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2. Warburton DER, Nicol CW, Bredin SSD. Health benefits of physical activity: The evidence [Internet]. CMAJ. Canadian Medical Association; 2006 [cited 2021 May 13]. p. 801–9. Available from: /pmc/articles/PMC1402378/
3. Moro T, Tinsley G, Bianco A, Marcolin G, Pacelli QF, Battaglia G, Palma A, Gentil P, Neri M, Paoli A. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. BioMed Central Ltd.; 2016;14.
4. Tinsley GM, Moore ML, Graybeal AJ, Paoli A, Kim Y, Gonzales JU, Harry JR, Vandusseldorp TA, Kennedy DN, Cruz MR. Time-restricted feeding plus resistance training in active females: a randomized trial. Am J Clin Nutr [Internet]. 2019 [cited 2020 May 21];110:628–40. Available from: https://academic.oup.c
5. Brady AJ, Langton HM, Mulligan M, Egan B. Effects of 8 wk of 16:8 Time-restricted Eating in Male Middle- and Long-Distance Runners. Med Sci Sports Exerc [Internet]. NLM (Medline); 2021 [cited 2021 May 13];53:633–42. Available from: https://pubmed.ncbi.nlm.nih.gov/32796255/
6. Tinsley GM, Forsse JS, Butler NK, Paoli A, Bane AA, La Bounty PM, Morgan GB, Grandjean PW. Time-restricted feeding in young men performing resistance training: A randomized controlled trial. Eur J Sport Sci [Internet]. Routledge; 2017 [cited 2018 Mar 6];17:200–7. Available from: https://www.tandfonline.com/doi/full/10.1080/17461391.2016.1223173
7. Vieira AF, Costa RR, Macedo RCO, Coconcelli L, Kruel LFM. 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 [Internet]. Cambridge University Press; 2016 [cited 2021 May 13];116:1153–64. Available from: https://pubmed.ncbi.nlm.nih.gov/27609363/
8. Rapoport BI. Metabolic factors limiting performance in marathon runners. PLoS Comput Biol [Internet]. Public Library of Science; 2010 [cited 2021 May 13];6:1000960. Available from: /pmc/articles/PMC2958805/
9. Frawley K, Greenwald G, Rogers RR, Petrella JK, Marshall MR. Effects of Prior Fasting on Fat Oxidation during Resistance Exercise. Int J Exerc Sci [Internet]. Western Kentucky University; 2018 [cited 2021 May 13];11:827–33. Available from: http://www.ncbi.nlm.nih.gov/pubmed/29997729
10. Yoon MS. mTOR as a key regulator in maintaining skeletal muscle mass [Internet]. Frontiers in Physiology. Frontiers Media S.A.; 2017 [cited 2021 May 13]. p. 788. Available from: /pmc/articles/PMC5650960/
11. Bodine SC, Stitt TN, Gonzalez M, Kline WO, Stover GL, Bauerlein R, Zlotchenko E, Scrimgeour A, Lawrence JC, Glass DJ, et al. Akt/mTOR pathway is a crucial regulator of skeletal muscle hypertrophy and can prevent muscle atrophy in vivo. Nat Cell Biol [Internet]. Nat Cell Biol; 2001 [cited 2021 May 13];3:1014–9. Available from: https://pubmed.ncbi.nlm.nih.gov/11715023/
12. Saxton RA, Sabatini DM. mTOR Signaling in Growth, Metabolism, and Disease. Cell. Cell Press; 2017. p. 960–76.
13. Hartman ML, Veldhuis JD, Johnson ML, et al. Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men. J Clin Endocrinol Metab. Apr 1992;74(4):757-65. doi:10.1210/jcem.74.4.1548337
14. Welle S, Thornton C, Statt M, McHenry B. Growth hormone increases muscle mass and strength but does not rejuvenate myofibrillar protein synthesis in healthy subjects over 60 years old. J Clin Endocrinol Metab. Sep 1996;81(9):3239-43. doi:10.1210/jcem.81.9.8784075
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