This is an interesting question because there are no hard and fast answers. It is quite dependent upon what type of exercise you are doing, what sort of diet you are eating and what your goals are.
There have been studies that have indicated no difference between working out fasted or fed during aerobic workouts, however the participants were eating fairly high carb diets and may not have been sufficiently fasted to lower insulin enough to get lipolysis (fat breakdown) working efficiently.
A benefit to being fasted when you workout is that Human Growth Hormone levels are increased, which helps to burn fat, build new muscle tissue, improve bone density and generally improve overall physical functions. Being in a fasted state also means that your insulin levels are low and glucagon is high, which is the hormone that releases fat from your body’s stores to be utilised for energy.
There is a good argument, however, to suggest that a really high intensity workout which lasts longer than 20 minutes, may require more glucose and therefore being fasted might mean that your energy levels just cannot get ramped up enough. One study showed that if your exercise bout lasted less than one hour then performance was not likely to be affected, whereas over one hour suggested that there may be a need for fuelling prior to the workout. But this energy crisis could also occur because the body is not truly in a fat burning state, such as you would be if you were following a ketogenic diet, and therefore it may struggle to provide the energy required for such high intensity work, especially in a fasted state.
The other aspect to look at is strength workouts. Whilst it is possible to perform perfectly good strength workouts in a fasted state, if the goal is to really pack on some muscle then it may be beneficial to eat before a heavy lifting session. Some studies suggest that eating protein before a heavy strength session will increase mTOR, which promotes muscle protein synthesis and the amino acids in circulation in the bloodstream will protect muscle breakdown and encourage hypertrophy. There is much speculation about the timing of nutrients before or after such a workout and there has been no real definitive answer to that. This study suggests that fasted resistance training shows a heavy reliance on using fat for fuel.
What About Fat Loss?
For fat loss, it may be very beneficial to perform your workout in a fasted state – and probably later in the fast to ensure that glucose and glycogen stores have been depleted, which means that fat rather than glucose is being used for energy. Exercising later in the fast also means that you can replenish well with food soon after your workout – particularly if you have included strength work in the session, as this will help prevent muscle protein breakdown. This study found that aerobic exercise performed in the fasted state resulted in more fat oxidation, however a meta-analysis found that, whilst there may be more fat oxidation in the short term during a bout of fasted exercise, this did not necessarily lead to weight loss and that it would be more likely that an individual would experience more effective weight loss if controlling food intake in the long-term. Interestingly, another study indicated that there was a greater loss of protein during acute fasted exercise, which may contribute to the short term weight loss – and is not something to aim for as loss of muscle mass is not what you should be hoping to achieve! But it is important to note that the study was done with a carbohydrate loading protocol, and this may be very different to fasted training under a low carbohydrate or ketogenic protocol, because ketones are muscle sparing. And if you are strength training then this increases both muscle breakdown and muscle synthesis, so once you consume protein after your workout you should be back in a positive protein balance.
High intensity workouts are certainly helpful (and can target visceral fat – the fat that surrounds the organs, which can result in conditions such as NAFLD – Non-Alcoholic Fatty Liver Disease), and appear to be more favourable in this regard compared to Low Intensity Steady State exercise (LISS). HIIT workouts appear to target visceral fat whether you are in a fasted or fed state, but there is a limit to how much HIIT you are able to perform. Short bursts between 15-30 minutes are really the maximum as any longer will be much more difficult to do whilst fasted. Also it is not ideal to perform HIIT workouts more than a couple of times per week as they are very draining on the nervous system.
It is important to remember that EPOC (excess post-exercise oxygen consumption) after a high intensity workout burns fat for hours (and sometimes days) AFTER the workout is finished. This means that more fat is burned post workout than during the actual workout. This is true whether fasted or fed, however it may be that the amount burned AFTER exercise may actually end up the same as the amount burned DURING exercise when comparing HIIT to LISS.
LISS workouts utilise fat more than high intensity workouts DURING the exercise period. Low and slow activities such as jogging (keeping the heart rate within a certain range), cycling, walking briskly, swimming etc are all good cardio exercises that we are able to do for longer whilst in the fasted state. And although there is far less EPOC after this type of exercise, perhaps that is not an issue if the benefits are pretty much the same as those where EPOC is raised for longer with HIIT. The main benefit probably comes from the fact that you can achieve the same effect in far less time with HIIT compared to long bouts of cardio training.
And, of course, it might simply come down to the fact that you simply feel better training fasted. Some people find that once they have eaten they feel less like working out as the body has gone into digestive mode and they start to feel a bit sluggish, finding it harder to summon the energy required to push through a tough workout. Conversely, others find that eating before a workout supplies them with more energy and they perform better.
Personally, I find that something small such as a bone broth concentrate drink with about 10 grams of beef fat will help give me a boost for strength training sessions, without making me feel full or putting my body into full blown digestion. However, at other times, such as a short HIIT session, or a run, I feel great doing it fasted and really feel energetic. This may come down to what you ate the evening before and how long since you ate. If your body still has glycogen stored in the muscles and liver then it is likely that you will feel more able to work out without eating anything else. Remember, though, that until you have fully depleted your glycogen stores you will not yet be burning fat. The longer you are fasted, the more you have depleted those stores, and the more effectively you will then start to use body fat.
At the end of the day, it is a totally personal thing. There is no right or wrong way. It depends on your goals and how you feel when working out, whether you feel energised or need something to give you that oomph. What type of activity are you doing? Are you trying to build muscle or lose fat?
Experiment and see what works for you.
I’d love to hear your comments about whether you find a benefit in training fasted.