Ah the age old debate of exercising fasted or not…one of the oldest debates on the interwebz.
It’s been a long held belief that eating before exercise is a requirement….you need to fuel the work you’re about to do bro and if you don’t you won’t make any progress towards your goal! A lot of this is not really driven by science though and it is primarily driven (at least in today’s world) via marketing…there is a lot of money to be made by telling people that not only need to eat something before they exercise but also during and immediately after.
However more recently there has been more and more people advocating exercising completely fasted.
Why?
You burn more fat bro!
And with 88% of US adults metabolically unhealthy [1], it seems like the right thing to do if you want to lose a few extra LBs!
So which approach is right?
Should you fuel your exercise by making sure you eat a meal, or should you train fasted to burn as much fat as possible?
Is there a universal answer?
There is no doubt that some people can see benefits from eating before training. For me personally, I have noticed I have better performance when strength training and anaerobic performance when I train fed.
Research tends to back up my personal experience as well.
In a study done on men who are resistance training, researchers found that they performed better when the men ate before lifting versus just having water.
As you can see from the graphs above, the participants that ate a meal before training (BC) tended to be able to increase the number of reps they could do in the bench press and squat.
What did the fed participants eat in this study?
Here is the macro breakdown…..
When it comes to aerobic performance the answer as to whether fed vs fasted exercise might depend on the intensity and duration of the exercise being done.
In a meta-analysis (a study of several studies) done on fasting and exercise performance [2], 54% of the studies (there were 13 total studies) showed a performance improvement when eating before exercise. However, when the duration was less than 60 minutes, 57% of the studies (there were 7 total studies) showed no difference in performance between the fasted and fed conditions. There was only one study that tested anaerobic performance in a fasted and fed state and that study found that a carbohydrate meal before exercise improved performance. At the same time when HIIT training was assessed in the fasted or fed state 3 studies found no difference between the fasted and fed state.
So as you can see, it is not a cut and dry answer as to whether fed vs fasted aerobic exercise is better in terms of performance.
You might be wondering why in some cases eating before exercise results in better performance than fasted exercise.
The obvious answer is that the carbohydrates from the food are the reason for the increased performance[3]. However there is also evidence that the performance impacts of eating something before exercise is all in your head and has nothing to do with the carbohydrate of your food!
Crazy right?
In a recent study[4] researchers fed 22 male participants a carbohydrate based breakfast, a placebo breakfast (the same breakfast minus the carbohydrates), or water. Two hours after eating researchers had the participants perform max rep bench press and squats. They found that both the carbohydrate and placebo based breakfasts saw a 15% gain in squat reps over the water only condition!
Yup the placebo breakfast which has 0 carbohydrates and basically 0 calories saw the same performance improvements as the carbohydrate breakfast.
As a side note, if you are wondering what the breakfast was they were consuming…here you go…
“These meals consisted of a small amount of orange juice, flavoring, xanthan gum, and water mixed with or without 1.5g/kg body weight of maltodextrin”
YUM!!!!
Anyways, it is clear that the benefits of being “fed” before exercise do not all have to do with the performance benefits of carbohydrates, but instead there is some kind of psychological impact of thinking you have had carbohydrates. This is actually not terribly surprising as there has been lots of research on the effects of the performance impacts of just purely rinsing your mouth with a carbohydrate solution without ingesting it.
You might also be wondering if you could also gain performance adaptations by training fasted?
For sure!
Training in a fasted state, regardless of the modality or intensity, will inherently cause you to utilize more fat than carbs. This is primarily due to the fact that you will have depleted some amount of liver glycogen from an overnight fast (or longer depending on the protocol) and insulin should be low (if the person is metabolically healthy). Think of insulin as a fuel selector switch, if it’s low you will prioritize fat as a fuel source, if it’s higher you will prioritize carbohydrates. The combination of depleted liver glycogen and low insulin after an overnight fast is the perfect situation to cause your body to utilize the maximum amount of fat.
If you train in this manner over several weeks and months your body can adapt to more easily use fat for fuel. If you are an athlete participating in an event where more fat utilization is a benefit, for example in endurance sports, training in a fasted state for a period of time could result in adaptations that result in increased performance come race day.
As a side note the above adaptation due to training fasted can also be useful for improving metabolic health if that is a top concern for you!
There are also times where someone may want to do a form of distress training in order to improve performance. Distress training is training where they purposely put the body in situations where there may be a large amount of stress due to the conditions under which you are exercising.
Doing a set of heavy back squats or deadlifts for a higher amount of reps fasted is one such situation is one example of distress training. Another example would be doing tabata intervals on your favorite cardio device in a fasted state.
What kind of training adaptation are you gaining by doing distress training?
You are making yourself “harder to kill”!
Why do this?
If you are an athlete there will be undoubtedly times when things are not ideal, something will go wrong during competition. When this happens it can cause athletes to panic and break down mentally. Once you are broken mentally all help is lost and it’s hard to come back after you have been broken mentally.
Training under less than ideal conditions in a distress situation is more mental training than physical training. It teaches your body and your mind that it’s OK, that you can get through the challenge and come out the other side just fine.
I should note that distress training can take a toll on you. So if you decide to do some kind of exercise fasted, especially high intensity cardio or high rep resistance training, that is putting added stress on your body then it should be done acutely, not chronically. When done chronically you can start to see signs of overtraining very quickly, even within a few days.
The other major situation I see where people benefit from fasted training is if they have “tummy troubles”. Some people just don’t do well when exercising with any kind of food in their stomach[5] and they actually perform better both during and post training if they train fasted. In this case their overall health dictates the necessity to train fasted AND THAT’S OK!!! It’s far more important they actually do the training and feel good after, than train in a fed state and feel like garbage.
There is one final situation where fasted training shines, and that is when people are short on time. Time is a major constraint for many people, including myself. While my preference is to train mid-afternoon if I can after having one solid meal in me, sometimes that is not possible. In fact as I write this blog post I am on a plane to Florida and I trained fasted this morning to make sure I had enough time to get to the airport for my flight. Sometimes life will make the decision to train fasted for you. Again this is perfectly fine. If the only time of day you can train is first thing in the morning fasted, then do it! Don’t sweat it if it’s not optimal for the training you plan to do, it is more important you do the training, than not be able to do it at all.
Now on to the question of body composition. Many people will advocate that fasted training is better if your goal is fat loss. I mentioned above that when you train in a fasted state you will burn more fat to fuel that exercise due to low liver glycogen and lower insulin levels. Based on that you are probably guessing that it means your body fat is just pouring off of you, right?
Not necessarily.
The single predictor of weight loss is whether you ate less than you burned. Obviously there are many other factors that play into that equation, none of those factors have to do with the fuel being used to provide energy to the body or the fuel being consumed to provide that energy. Eat less than you burn and you will lose weight, regardless of whether you are burning 100% fat or 100% carbs, and regardless of whether you are eating only protein, only plants, a mixed diet, a high fat low carb diet, or a low fat high carb diet, keto, paleo, standard american, twinkies, potatoes, it doesn’t matter at the end of the day.
This means that if two people eat a diet with the same caloric deficit except one does fasted exercise and the other trains after eating that at the end of the day they will both lose the same amount of weight. In fact there was a study that did exactly this[6]! Researchers split a group of young women and put both groups in a 500 calorie deficit and ate the same amount of fats, carbs, and protein. The only difference was one group trained fasted and the other trained fed. Researchers even picked an exercise protocol that should have benefited the fasted group. Both groups did 1 hour of aerobic running on the treadmill 3 times a week for 4 weeks. In theory the low intensity nature of the exercise should make sure the fasted participants were burning maximal fat.
Despite setting everything up perfectly for maximum fat burn for the fasted participants, there was no significant difference in body weight, fat loss, muscle gain, or waist circumference between the fasted and fed groups.
So while fasted training may enhance your body’s ability to burn fat as a fuel, which is certainly a good thing from a metabolic flexibility and metabolic health point of view, it provides no additional benefit in terms of body composition.
So who’s the winner in the fasted vs. fed training debate?
In my opinion there is no winner, it all depends on you, your goals, and your preference.
The answer comes down to which option works best for you.
Have tummy troubles, or cannot train any other time than first thing in the morning?
Train fasted!
Need to improve your metabolic health or improve your ability to utilize fat as a fuel?
Train fasted!
Want to inject a bit of tough mental training into your exercise programming?
Train fasted!
Does training fasted make you ravenously hungry and cause you to overeat?
Train fed!
Looking to make sure you are eeking out every last bit of performance from your training?
Train fed!
Ready to do a nasty 2k on the rower?
Train fed!
Looking to lose fat?
Choose the modality that makes it easier for you to maintain a caloric deficit.
You get the picture, context is key, the answer depends on the person and their goals. The answer may also shift throughout the year as life changes and/or you enter different phases of training.
These types of nuanced questions are exactly the type of information I try to decipher for you in my weekly newsletter. Nothing is black and white when it comes to health, wellness and performance optimization. Each week I try to provide you with a balanced approach to decipher these thorny topics in the content I put out in my newsletter. To get all this information and jump on my newsletter list enter your email address in the form below to start getting actionable balanced information delivered directly to your inbox on a weekly basis.
- (n.d.). Prevalence of Optimal Metabolic Health in American Adults. Retrieved March 5, 2022, from https://www.liebertpub.com/doi/10.1089/met.2018.0105 ↑
- (2016, September 9). Effects of aerobic exercise performed in fasted v. fed state on fat and …. Retrieved March 5, 2022, from https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/effects-of-aerobic-exercise-performed-in-fasted-v-fed-state-on-fat-and-carbohydrate-metabolism-in-adults-a-systematic-review-and-metaanalysis/0EA2328A0FF91703C95FD39A38716811 ↑
- (2019, May 16). Carbohydrate Availability and Physical Performance – NCBI. Retrieved March 5, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6566225/ ↑
- (2020, March 16). Viscous placebo and carbohydrate breakfasts similarly decrease …. Retrieved March 2, 2022, from https://pubmed.ncbi.nlm.nih.gov/32174286/ ↑
- (n.d.). The impact of physical exercise on the gastrointestinal tract – PubMed. Retrieved March 5, 2022, from https://pubmed.ncbi.nlm.nih.gov/19535976/ ↑
- (2014, November 18). Body composition changes associated with fasted versus non-fasted …. Retrieved March 5, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4242477/ ↑