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Fasted Exercise & Fat Loss


I got asked the question “Do you think it’s worth doing fasted cardio?” So I though I would so a short post on the topic…

There has been a surge of research carried out in order to establish the most effective methods and strategies to aid body fat loss and improve body composition. Something perhaps many of us have thought about, or are currently striving to reach that goal. Therefore, will exercising in a fasted-state, compared to non-fasted, help us lose body fat quicker?

So what is ‘Fasted Exercise’ (also known as ‘Train Low)’? This is when exercise is performed with low muscle glycogen stores (carbohydrate restriction), resulting in reduced energy availability. This is most commonly, and easily, achieved when exercise is performed in the morning before breakfast (overnight fast; ~8-12hr with no food or fluid [except water]), and usually at a low to moderate exercise intensity.

Other ways to achieve fasted exercise / ‘Train Low’ –

Train twice a day – Performing exercise two times per day. The first can be fuelled with some carbohydrate, followed by low or no carbohydrate consumption before the second workout.

Sleep Low – Following a late night workout, carbohydrate stores will be depleted. Carbohydrate intake is then restricted until after the next exercise workout the following morning.

Prolonged training with no carbohydrate intake – Exercising >90min with no carbohydrate consumption, increasing the use of fat as the energy source.

So is fasted exercise better for fat loss? Well, during exercise, when performed in fasted-state, the body increases its reliance on fat, therefore reducing its reliance on carbohydrate, as its primary fuel source. Fat oxidation can increase by ~20-30% during fasted exercise. Therefore, it seems logical that, for fat loss, this strategy should be implemented.

However, the picture is so much greater than the 45-90 minutes you spend exercising. The day is much longer, and also has a greater bearing on total fat loss and body composition. Research has shown that, although fasted exercise burns more fat during exercise, fat burning tends to be higher in the 2-3 hours following non-fasted exercise. So it seems these two methods ultimately reach the same effect over time.

Recent research carried out in Japan (Iwayama et al. 2015a, Iwayama et al. 2015b, Shimada et al. 2013) compared fasted versus non-fasted exercise on daily fat utilisation. Findings suggest, consistently, that daily fat oxidation (burn) increased ONLY when exercise is performed in a fasted state. In theory, regular fasted exercise could be more efficient at burning fat, therefore accelerating fat loss. To date, only a few studies have looked at this (Marquet et al. 2016; Schoenfield et al. 2014; Gillen et al. 2013; Van Proeyen et al. 2010). In Summary, these findings remain mixed as to whether fasted exercise is more effective, compared to non-fasted exercise, at reducing body fat when performed on a regular basis. In theory, fasted-state should be more effective. However, more longitudinal studies are required, including the comparison of body composition between fasted and non-fasted exercise, to prove this theory.

With all this said, lets not forget the role energy balance plays in fat loss -

Fat Oxidation is NOT the same as Fat Loss

Body fat loss will ONLY happen in the presence of a negative fat balance -

Fat Oxidation (used) > Fat Synthesis (created)

A negative energy balance (burning more calories than we consume) is key to creating a negative fat balance; and a negative fat balance is key to reducing body fat over a sustained period of time. Therefore undertaking diet and/or exercise strategies to promote a sustained negative fat balance is the key to losing weight. Fasted exercise is not a top priority, but could be implemented into your training program to help increase daily fat oxidation which may help promote a negative fat balance over time and accelerate body fat loss.


In a fasted state, liver and muscle glycogen stores are likely to be low, resulting in low blood glucose levels. This could make tough, high intensity workouts, more difficult as the body will struggle to work at high intensities with the absence of sustained glucose availability. Fasted exercise should target low – moderate intensity workouts.

Additionally, fasted exercise (and also negative energy balance) may make it more difficult to maintain muscle mass. Muscle mass is important in supporting good health and wellness, along with its aesthetic benefits. Therefore it is crucial that protein is consumed frequently throughout the day, and particularly around exercise.

Furthermore, remaining in a prolonged energy deficit, or repeatedly training in a fasted state may have negative effect on you immune function. Therefore nutrition and training periodisation are an important consideration to avoid immunosuppression.


There is a clear need for further research in this area, and I hope to add to this and share any new scientific findings with you as they become known. What we currently know is that fasted exercise is NOT the holy grail for body fat loss. However, it is a useful tool in combination with a well-designed exercise program and nutrition strategy.

References -

Gillen et al. (2013). Interval training in the fed or fasted state improves body composition and muscle oxidative capacity in overweight women. Obesity.

Iwayama et al. (2015). Transient energy deficit induced by exercise increases 24-h fat oxidation in young trained men J Appl Physiol.

Iwayama et al. (2015). Exercise Increases 24-h Fat Oxidation Only When It Is Performed Before Breakfast. EBioMedicine.

Marquet et al. (2016). Enhanced endurance performance by periodisation of CHO intake: “sleep low” strategy. Med Sci Sports Exerc.

Schoenfield et al. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. JISSN.

Shimada et al. (2013). Effect of exercise performed before or after breakfast on 24-hour fat oxidation. Metabolism.

Van Proeyen et al. (2010). Training in the fasted state improves glucose tolerance during fat rich diet. J Physiol.

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