Written by Garett Reid | NSCA, CSCS, CISSN, M.S.E.S.S
Fact checked by Sam Coleman, Co-Founder
FACT CHECKED
Working out fasted: should you do it? There’s a lot of misinformation out there concerning fasting and its effects on working out. Some swear it unlocked the secret to muscle growth while others say it’s a sure way to lose mass.
Who’s right? Well, neither; but there’s plenty of nuance to go over. In this article we’ll explore working out fasted and what happens to your body.
Working Out Fasted
Working out fast is one of the fads entering the fitness world. Going to the gym without eating is suggested to have many benefits, such as:
- Increased muscle mass
- Increased fat loss
- Improved metabolic function
- Improved hormone profile
There are many others and while some of them are true, others aren’t. As always, there's a lot of nuance to unpack to determine if you should workout on an empty stomach or fill up on protein.
Before we go further, note that this article will primarily concern lifting weights while fasting.
What Does "Fasted" Mean?
In its simplest definition, "fasted" simply means the absence of eating. This is where breakfast comes from as you "break" "fast" after a night of not eating. Regardless, it simply means you haven't supplied your body with nutrients for some time, and it now must depend on your fuel stores.
That said, saying you "work out fasted" generally suggests you haven't eaten for a long time. The exact time will depend on who you ask, as some will say 8-12 hours. However, some people suggest even longer is needed, such as 18.
The main idea is that you abstained from eating for an extended duration. At this point, your body has metabolized the food from your last meal.
Under normal circumstances, assuming you eat carbs, your body has stores of glycogen within the muscles and liver. However, the longer you go without eating, the more these stores deplete.
As you continue your fast, these stores will further deplete, resulting in an increase in lipolysis1, or the breakdown of fat for fuel. This is generally the primary mechanism people use to determine if they’re fasting, eventually resulting in the production of ketones.
We’re not going to make a distinct point in this article but most people who work out fasted, fast for 12-16 hours. This is the average time of fasting from someone who skips breakfast or follows intermittent fasting using a 16:8 schedule.
However, this may affect you even if you go without eating for 6 hours. We’ll get to that below when we talk about post-workout meals.
What Happens When You Fast?
When your body enters a fasting state, a lot of changes occur physiologically. However, this article isn’t necessarily concerned with exploring all of these as we’re primarily concerned with how fasting affects your workout.
However, there are three physiological processes that are especially important to working out.
- Increased growth hormone
- Improved insulin sensitivity
- Increased ability to utilize fats
Now, when it comes to HGH, there’s actually mixed results on its ability to increase muscle mass and strength2. While some studies do show increases in muscle mass, others don’t. There are numerous factors involved but it definitely does not have the same effect as testosterone.
On the other hand, improved insulin sensitivity and increased fat utilization are meaningful benefits. However, you don’t need to work out while fasting for this to occur, at least in the gym.
Perhaps a better option to get these benefits would be to perform low-intensity exercise. This would get these benefits while mitigating any possible negative effects.
Should You Workout While Fasting?
So, the big question is, should you work out while fasting? The answer is a bit nuanced and depends on your goals.
We'll now examine some claims made and how they hold up. In addition, we'll consider some things you need to consider.
Energy Levels While Fasted
One of the first things you need to consider is how much energy you will need to work out. While working out fast could help increase fat utilization, it will likely result in less work. There's always give-and-take.
For example, a large meta-analysis published in Sports Medicine found that compared to fasting for 8+ hours, working out in a fed state resulted in much more volume when performing anaerobic resistance training3.
Interestingly, this seemed to be magnified depending on how many maximal effort sets were performed. Performing a set with maximum effort to fail made it much harder to recover from while training fasted. This is likely due to the lack of glycogen stored.
In addition, RPE (Rate Of Perceived Exertion) is usually higher during the workout. This can occur when using the same work volume or even less, and it can be mentally challenging to deal with.
This may improve once a person becomes used to training quickly, but even that suggestion is undetermined. While RPE may not be as high, the total work volume could still be affected.
Does Working Out Fasted Affect Muscle Growth?
One of the biggest concerns with working out fast is whether it will affect muscle growth. Well, it definitely won't help it.
However, it might not necessarily hurt you as well.
This next part might come as a surprise, but the difficulty with answering this question is that there's surprisingly a lack of direct studies. You would think that due to its popularity, there would be a plethora of studies. However, this is not the case.
Many studies have been done on Muslim bodybuilders during Ramadan. One thing to consider is that these individuals have likely been doing this for a while, so their bodies have become accustomed to working out while fasting. Additionally, Ramadan only lasts 29-30 days, which may or may not reflect long-term results.
With that said, let's look at some of the research.
The Research On Working Out Fasted
1. One study had two matched groups working out fast following a similar training program4. The only difference is that one group trained during the fasting window while the other trained during the feeding window.
At the end of Ramadan, both groups saw an improvement in 1RM strength for bench press and deadlift, with the fed group improving more. In terms of muscle hypertrophy, neither group saw significant improvements.
This study shows that training in the fed state can result in more strength gains. Unfortunately, there is no control group, so we can't comment on whether fasting affects muscle hypertrophy.
Now, there is one meta that examines the effect of intermittent fasting on lean body mass.
2. Another meta-analysis examined 8 different studies5. Of these 8, 3 were done with those following Ramadan. In addition, only one study used a control, so it's hard to draw concrete conclusions.
That said, only one study demonstrated a significant increase in lean body mass. The other 7 did show the ability to maintain muscle mass, but there was no significant increase.
This shows that working out fasted may not be a great choice if building muscle is your goal, at least according to studies. There are a lot of anecdotal reports that demonstrate the opposite, so that's something to consider.
Does Working Out Fasted Help Burn Fat?
A common belief about training fast is that it will help you lose weight. Proponents suggest burning more fat since your body must draw from fat stores! This is true. However, it doesn't mean you'll lose fat.
Burning fat does not equal losing fat or even losing weight. In fact, your body is always burning fat, as it supplies a large percentage of fuel. However, it doesn't mean you'll lose weight since you're always replacing the calories with your meals.
While there's some nuance to “calories in, calories out” (the sum of total calories consumed and burned), it’s still the basic rule to fat loss. Therefore, burning more fat doesn't matter if you replace it later with calories as you won’t lose fat.
If anything, training fasted will increase your ability to burn fat. This can aid in your metabolic profile while also possibly improving your performance output as your body will be more effective in producing energy.
But still, this doesn’t necessarily mean you’ll lose fat.
Post Workout Meal After Working Out Fasted
Now that we've discussed what happens during a fasted workout, let's discuss what happens afterward—specifically, the post-workout meal. The post-workout meal has always been a major point of interest within the building community.
The general consensus is that post-workout, your body is metabolically primed for muscle growth. During this time, you should eat your protein and carbs to maximize growth. However, people used to believe that this window was only open for about 30 minutes, and if you missed it, you threw away the chance for gains!
This was termed the anabolic window, and basically, it suggested that you had to eat your post-workout meal within 30 minutes, or you would waste your workout. Luckily, we now know that isn't true.
While it is true that your body is metabolically primed and you should take advantage of your post-workout meal, the time frame is much longer; some even suggest up to three hours!
However, this is only true for those who had eaten before their workout. Their bodies have a certain glycogen and amino acids circulating through them. While it's still probably a good idea to eat sooner rather than later, this supply of nutrients mitigates its importance.
So what does that mean when you workout fasted?
If you work out fast, the idea that you need to eat after your training holds a lot more substance. As your body is depleted of nutrients, muscle protein synthesis will be blunted, increasing the risk of muscle breakdown post workout.
Research suggests that meals with proper calories and macros will keep muscle protein synthesis levels elevated for 6 hours6. Therefore, if you train after not eating for 4-5 hours, you should probably eat within an hour post-workout, realizing that sooner is better.
Tips For Working Out Fasted
If you still want to train while fasting, here are some tips to optimize your training.
- Eat a meal within 30 minutes post-workout: As we mentioned above, be sure to get some protein and carbs in you shortly after working out.
- Keep your fasting window as short as possible. Training while fasting for 36 hours is vastly different from training for 10 hours: The longer you fast before you workout increases the possibility and magnitude of risks. Therefore, unless you have a specific reason, keep your fasted window as short as possible. There’s a big difference between working out after fasting 8 hours and 36 hours.
- Consider utilizing EAAs during your fasted workout. EAAs could be a good option for an intra workout. Technically, EAAs do have calories (about 4-5 per gram), but it would be minimal. More importantly, it will fuel your body with amino acids to mitigate muscle breakdown
- Eat a meal rich in protein and carbs before your fast: Eating a meal high in protein and carbs as your last meal could mitigate the risks.
What Should You Eat Before A Workout?
Now, some people simply prefer not eating before workout because they feel more comfortable with an empty stomach. Or perhaps they just want something little; what should these people eat?
Well, the good thing is you don’t need to eat a ton to mitigate potential risk. Of course you could always do a simple protein shake. This provides plenty of protein as well as some carbs to fuel your body and support muscle protein synthesis.
In addition, here are some other simple ideas:
- Bowl of Greek yogurt (our favorite!)
- Glass of milk or chocolate milk
- Banana with peanut butter
- Proats (oatmeal and protein powder)
Final Verdict On Working Out Fasted
Now that we've reviewed all the evidence for working out fast, we can draw a pretty good conclusion. That is, working out fasted will either hurt your overall outcome, or have no effect.
That means if one is better, it would be working out in a fed state.
Yes, working out fasted can improve fat oxidation and growth hormone. However, none of these necessarily mean you’ll burn more fat or build more muscle. More importantly, you need to also consider the possible risks such as less workout volume or decreased muscle protein synthesis.
Then there’s also the studies that have actually looked at working out while fasted which showed negative results. This seems to suggest you would be better off eating before you work out.
At the same time, there are plenty of anecdotal reports from people who swear they work out better while fasted. Therefore, if you simply miss breakfast, you’ll probably be ok as long as you eat right after and know the risk. On the other hand, if you work out fasted because you think it has some sort of superior effect on building your muscle, go ahead and eat.
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References:
- Edwards, M., & Mohiuddin, S. S. (2021). Biochemistry, Lipolysis. PubMed; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK560564/
- Rennie, M. J. (2003). Claims for the anabolic effects of growth hormone: a case of the Emperor’s new clothes? British Journal of Sports Medicine, 37(2), 100–105. https://doi.org/10.1136/bjsm.37.2.100
- King, A., Helms, E., Zinn, C., & Jukic, I. (2022). The Ergogenic Effects of Acute Carbohydrate Feeding on Resistance Exercise Performance: A Systematic Review and Meta-analysis. Sports Medicine, 52(11). https://doi.org/10.1007/s40279-022-01716-w
- Triki, R., Zouhal, H., Chtourou, H., Salhi, I., Jebabli, N., Saeidi, A., Laher, I., Hackney, A. C., Granacher, U., & Ben Abderrahman, A. (2023). Timing of Resistance Training During Ramadan Fasting and Its Effects on Muscle Strength and Hypertrophy. International Journal of Sports Physiology and Performance, 1–11. https://doi.org/10.1123/ijspp.2022-0268
- Keenan, S., Cooke, M. B., & Belski, R. (2020). The Effects of Intermittent Fasting Combined with Resistance Training on Lean Body Mass: A Systematic Review of Human Studies. Nutrients, 12(8), 2349. https://doi.org/10.3390/nu12082349
- Schoenfeld, B. J., & Aragon, A. A. (2018). Is There a Postworkout Anabolic Window of Opportunity for Nutrient Consumption? Clearing up Controversies. Journal of Orthopaedic & Sports Physical Therapy, 48(12), 911–914. https://doi.org/10.2519/jospt.2018.0615
Garett Reid
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