Fasting workouts are practiced by many endurance athletes in attempt to increase their body’s ability to use “fat as fuel” and spare carbohydrates. This seems like a beneficial training technique since your body has much greater stores of fat (about 72,000 kcal) compared to carbohydrate (about 2,500 kcal).  In theory, if you can efficiently tap into your fat stores, you should have access to more energy. Some athletes will also use this technique to increase fat metabolism with a goal of weight loss.


The most common practice to “tap into the fat stores” is by completing low intensity exercise after an overnight fast in order to train with low carbohydrate (CHO) availability (i.e. do a morning run without eating). The thought here is that your overnight fast will deplete your body’s CHO stores and therefore you will force your body to use fat for fuel and improve performance and or/ lose weight.


Your body has 2 main CHO (glycogen) storage centres – MUSCLE and LIVER. When we sleep, our body prefers to use up the liver glycogen and not the muscle glycogen. Therefore in the morning, our muscles are actually not “starved” of energy, but rather it is our liver that is “empty”. When you run or work out after an overnight fast, it is true that you will likely be burning more fat but this fast is actually just “free fatty acids” in your blood as opposed to the “subcutaneous” fat (i.e belly fat) that would help with weight loss. Further, according to a 2018 review of fasting for endurance athletes (Hearris et. al 2018) this strategy of overnight fasting wasn’t shown to really have any outstanding performance benefits either. Granted, many of these studies were very short and certain cellular changes (such as increased mitochondria function) was improved, but actual performance was not affected.

There have been more consistent findings that training with low MUSCLE glycogen seems to have a greater performance effect as opposed to low liver glycogen. This strategy also seems to be more effective at increasing your body’s reliance on fat as a fuel source. To do this, athletes complete a high intensity workout in the evening to deplete muscle glycogen, avoid carbs (CHO) post workout and complete a low intensity training session the next morning before finally ingesting CHO (like this high protein energy bar recipe). This method seems to yield not only increases in important cellular pathways associated with endurance performance, but also performance itself (i.e. (Marquet LA et al. 2016).


It is suggested that CHO plays a role in maintaining a healthy immune system. When you don’t have enough CHO in your body, you are at risk for increased levels of stress hormones such as cortisol, which can impair your immune system. One way which our immune system is compromised is by suppressing antibody production (Gleeson et al. 2004). Therefore, carrying out this strategy long term could have a negative impact on performance (p.s. check my recipe for an immune boosting turmeric/ ginger tonic). In addition, a 2016 interview with Dr. Louise Burke, PhD and Head of the Discipline in Sports Nutrition for the Australian Institute of Sport outlined some of the research her lab has been carrying out and found evidence to suggest that chronic training with a lack of CHO availability can actually DECREASE your ability for your muscles to use CHO as a fuel source. This is quite impactful for endurance athletes, since CHO is the main fuel for anything powerful such as climbing a hill or a sprint to the finish line. Anytime you need energy FAST or need to complete a hard effort (i.e. hill climb), you will be relying on CHO as your primary fuel source, so a decrease in your ability to access CHO can be detrimental to performance.


Best practice in terms of fasting protocol seems to be a combination of workouts with both low and high CHO availability. In order to achieve high intensity training adaptations, it is ideal to have high CHO availability (i.e. don’t fast!) but for low intensity workouts (long slow runs, aerobic recovery runs), doing them fasted could potentially have some benefits. One thing to keep in mind is that training fasted can take some time to feel comfortable, and you may feel sluggish and tired in the beginning. Often with continued practice, you will find it easier and easier to train in a fasted state. There are options to mitigate these feelings by drinking black coffee (no cream or sugar) prior to your fasted workout, however the research is still quite young in this department and therefore it is unknown if coffee would inhibit any of those positive performance cellular changes. It is best to work with a run coach to design a strategy that compliments your current training program.

Amanda Regnier, MSc. Strength and Conditioning
Run Coach