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The Effects of Fasted Training on Exercise Performance and Weight Loss



Fasted training is nothing new— both for the recreational exercise enthusiasts and athletes. Some believe fasted training allows them to get more out of their training because they don’t have to deal with gastrointestinal discomfort, while others cannot fathom having the energy to train efficiently without eating something, beforehand. Let’s talk about Fasted Training!


As I started thinking about this, the first question I had in mind was how does fasted training affect performance? Also, if there is a notable difference, does it apply with all styles of training? How does fasted training affect performance in resistance training? Long, steady-state endurance training? And how about short, high-intensity interval workouts?


Is there an added benefit towards possible weight loss that is seen with fasted training? Also, how will it affect one’s progress in developing muscle mass? Let’s take a deep dive into the world of fasted training so you can make a more informed decision, whether or not you decide to try it out for yourself.


Like I said earlier, what I had in mind was that training performance would be the first to take a hit when fasted. I know that for me, personally, I can’t even concentrate, let alone have a superbly productive workout, if I was trying to train without eating beforehand. In fact, as far as I can remember, during my days playing baseball in high school or rowing in college, I always made sure that I would have at least a banana or eat an apple before practice.


One thing that is for sure is that the human body is capable of going a couple days without eating and still being functional. So, while it may not necessarily be a hugely physiological hindrance, dealing with the hunger can be quite difficult psychologically. When the intensity of the exercise is increased, it is likely that the psychological obstacle is increased for some.


In this case, I would be comfortable with recommending folks that struggle with the psychological hindrance caused by hunger opt out of high-intensity fasted training. As for low-intensity fasted training, this may be more doable for some, but, still, others might not be able to get over the psychological obstacle caused by hunger.



In Aird et. al (2018), out of a meta-analysis of 46 studies that had to do with exercise performance and fasted training, what their findings uncovered was that there was no to minimal amount of difference in performance with short aerobic exercise. In other words, if you were only planning on running a mile or two, first thing in the morning, skipping out on eating something before you head out might not make a giant difference.


According to Aird, as the exercise duration and intensity was increased, the performance levels exhibited by the fasted groups increasingly waned in comparison with the fed groups. So, according to this meta-analysis, if you wanted to run 10 miles in search of a new PR, your time is probably going to be better when you have something to eat before the run.


Interestingly, since most individuals that adopt fasted training are typically those that train in the morning (this is a no-brainer because fasting for a couple hours is way easier to do while sleeping than when a person is awake), Papadakis et al. (2021) found that test subjects who were sleep deprived and fasted, getting 3-3 1/2 hours of sleep, did not have their performance doing high-intensity interval exercises substantially drop compared to the group that got a full night’s rest and fasted.



In fact, according to this study, the sleep deprived, fasted group was found to utilize a higher percentage of fat as an energy source than the fasted group that got adequate sleep. Both groups exhibited similar VO2 max readings, as well. However, this study really has to be taken with a grain of salt for a couple reasons.


First off, I think everyone can agree that being sleep deprived for a long period is not ideal for most people. While these individuals that were sleep deprived did not exhibit any disadvantages, this study only lasted 7 days, therefore, had this study been extended out to 6 months or a year, things would likely have been much different. And yeah, if you’re not very familiar with looking at studies, 7 days is really short.


Another glaring weakness that has to be cited is the fact that they only had 15 participants in this study. Again, this isn’t the most ideal. Nevertheless, what we can gain from this study is that if a person that likes fasted training was to have trouble sleeping for a couple nights, progress to better health might not be hurt too bad. But, if any of you have trouble sleeping for more than a couple weeks, seeing your doctor about it is probably the best thing for you.


In my opinion, people looking to maximize their training performance might be less inclined to adopt doing it while fasted. The more popular reason why fasted training is adopted has more to do with weight loss, so let’s take a look at that.


As mentioned earlier, Papadakis found that the body may use a larger percentage of fat as its energy source, even high intensity, when training while fasted. One thing that we already know is that at rest and very low intensity, fat is the primary energy source. A steady walk also uses more fat than carbohydrates as the main energy source, for this reason, we see bodybuilders promoting walking as the essential form of cardio to lose fat and preserve muscle mass.


In a review by Wallis & Gonzalez (2018), they found that a single bout of fasted training induced a higher percentage of fat used as an energy source during training. As with other studies, fasted training resulted in lower caloric consumption in a 24-hour period. However, when it came to seeking a long-term benefit from fasted training, fat loss and overall weight loss was not much different compared to those that trained while fed.


According to Frampton et al. (2021), where it was a meta-analysis of 23 studies where researchers compared food intake, hunger, gastrointestinal hormone release, and energy expenditure between test subjects that trained while fasted versus those who were fed. The researchers found that fasted training exhibited lower food consumption throughout the day. Since the goal for successful weight loss is caloric deficit, this may sound like a win for fasted training because fewer calories a consumed. However, they also found that the fasted training test subjects exhibited lower energy expenditure.


My initial assumption for the lower energy expenditure was that it might have been due to decreases exercise performance, but according to Frampton’s meta-analysis, there was no indication of this being the case. In fact, the studies they looked at indicated perceived exertion and exercise enjoyment was the same for fasted and fed test groups. Instead, the researchers believe the lower energy expenditure was due to decreased diet-induced thermogenesis.



Schoenfeld et al. (2014) compared body composition changes between fasted and fed aerobic training, and, again, researchers found that the fasted training group consumed fewer calories throughout the day, but the fat loss between both groups was fairly similar. Researchers compared fat mass loss, overall weight loss, changes in fat-free mass, and BMI, but found no significant difference.


Much like Frampton, Schoenfeld theorizes that even though fewer calories are consumed by the fasted group, the thermic effect of food might have contributed to evening out the amount of fat loss between the fasted and fed groups.


Ultimately, while the difference in weight loss between being fed or fasted when working out is relatively the same and performance in more time-consuming training sessions may dip more while fasted, I believe that there is still some value in fasted training. Some of you may be very sensitive to gastrointestinal discomfort, and if you feel you can get a much better workout while fasted, I would be in the wrong to try convincing you to eat before training. But if you would like to train with higher intensity, maybe eating something that is easily digestible a couple hours before training might work. With the majority of the studies about fasted training, one thing that they can agree on is that more research is still very much needed.


I hope this video helped clarify some of the questions you might have had about fasted training, but if you have some more, send me an email!


References


Aird, T. P., Davies, R. W., & Carson, B. P. (2018). Effects of fasted vs fed-state exercise on

performance and post-exercise metabolism: A systematic review and meta-analysis.

Scandinavian Journal of Medicine & Science in Sports, 28(5), 1476–1493.


Frampton, J., Edinburgh, R. M., Ogden, H. B., Gonzalez, J. T., & Chambers, E. S. (2022). The

acute effect of fasted exercise on energy intake, energy expenditure, subjective hunger

and gastrointestinal hormone release compared to fed exercise in healthy individuals: A

systematic review and network meta-analysis. International Journal of Obesity (2005),

46(2), 255–268.


Papadakis, Z., Forsse, J. S., & Stamatis, A. (2021). High-intensity interval exercise performance

and short-term metabolic responses to overnight-fasted acute-partial sleep deprivation.

International Journal of Environmental Research and Public Health, 18(7), 3655.


Schoenfeld, B. J., Aragon, A. A., Wilborn, C. D., Krieger, J. W., & Sonmez, G. T. (2014). Body

composition changes associated with fasted versus non-fasted aerobic exercise.

Journal of the International Society of Sports Nutrition, 11(1), 54.


Wallis, G. A., & Gonzalez, J. T. (2019). Is exercise best served on an empty stomach? The

Proceedings of the Nutrition Society, 78(1), 110–117.

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